Chapter 5. Implantation and Placental Development
BS. Nguyễn Hồng Anh
All obstetricians should understand the basic biological steps
required or women to achieve pregnancy. Moreover, abnormalities aecting these steps can lead to inertility or pregnancy
loss. Te biological and molecular changes involved in human
zygote implantation and subsequent etal and placental development are intricate. In the past 50 years, researchers have
delineated many o these molecular and physiological events.
Yet, much work remains in the continual challenge to improve
clinical outcomes.
OVARIAN-ENDOMETRIAL CYCLE
In most women, cyclical ovulation continues during the almost
40 years between menarche and menopause. Tus, without
contraception, approximately 400 opportunities or pregnancy exist, and these are tightly regulated by complex interactions o
the hypothalamic-pituitary-ovarian axis. Concurrently, endometrium undergoes aithully reproduced cyclical changes to
prepare or pregnancy (Fig. 5-1). Essential contributors in this
process include gonadotropin-releasing hormone (GnRH), the
gonadotropin hormones ollicle-stimulating hormone (FSH)
and luteinizing hormone (LH), and the ovarian sex steroid hormones estrogen and progesterone. For a detailed description o
menstrual cycle physiology, the reader is reerred to Chapter 16
in Williams Gynecology, 4th edition (Halvorson, 2020).
■ Ovulation
Tis dening event separates the ollicular and luteal phases o
the menstrual cycle. Following ovulation, the corpus luteum
develops rom the remains o the graaan ollicle in a process
reerred to as luteinization. Te basement membrane separating
the granulosa-lutein and theca-lutein cells breaks down, and
by day 2 postovulation, blood vessels and capillaries invade the
granulosa cell layer. During luteinization, these cells undergo
hypertrophy and increase their capacity to synthesize hormones. LH is the primary luteotropic actor responsible or
corpus luteum maintenance (Vande Wiele, 1970).
Te hormone secretion pattern o the corpus luteum diers
rom that o the ollicle. First, the greater capacity o granulosa-lutein cells to produce progesterone results rom enhanced
access to blood-borne, low-density lipoprotein (LDL)-derived
cholesterol, which is a steroidogenic precursor (Carr, 1981).
Ovarian progesterone production peaks at 25 to 50 mg/d during the midluteal phase. With pregnancy, the corpus luteum
continues progesterone production in response to placental
human chorionic gonadotropin (hCG). LH and hCG both act
via the same LH-hCG receptor.
Te human corpus luteum is a transient endocrine organ. In
the absence o pregnancy, it rapidly undergoes apoptosis 9 to
11 days ater ovulation (Vaskivuo, 2002). Te dramatic drop in
FIGURE 5-1 Gonadotropin control of the ovarian and endometrial cycles. The ovarian-endometrial cycle is structured as a 28-day cycle.
The follicular phase (days 1 to 14) is characterized by rising estrogen levels, endometrial thickening, and selection of the dominant “ovulatory” follicle. During the luteal phase (days 14 to 21), the corpus luteum (CL) produces estrogen and progesterone, which prepare the endometrium for implantation. If implantation occurs, the developing blastocyst begins to produce human chorionic gonadotropin (hCG) and
rescues the corpus luteum, thus maintaining progesterone production. FSH = follicle-stimulating hormone; LH = luteinizing hormone.
circulating estradiol and progesterone levels initiate molecular
events that lead to menstruation.
Between days 22 and 25 ater ovulation, the secretory-phase
endometrium undergoes striking changes associated with predecidual transormation o the upper two thirds o the unctionalis layer. Te glands exhibit extensive coiling, and luminal
secretions become visible. Changes within the endometrium
can also mark the window o implantation seen on days 20 to
24. Epithelial surace cells show ewer microvilli and cilia, but
luminal protrusions appear on the apical cell surace (Nikas,
2003). Tese pinopodes help prepare or blastocyst implantation. Tey also coincide with changes in the surace glycocalyx
that allow acceptance o a blastocyst (Aplin, 2003).
Another highlight o the secretory phase is the continuing
growth and development o the spiral arteries. Tese vessels arise
rom the radial arteries, which are myometrial branches o the
arcuate and, ultimately, uterine vessels. Te morphological and
unctional properties o the spiral arteries are unique and essential to blood ow changes seen during menstruation or implantation. During endometrial growth, spiral arteries lengthen at84 Placentation, Embryogenesis, and Fetal Development
Section 3
a rate appreciably greater than the rate o endometrial tissue
thickening. Tis growth discordance obliges even greater coiling. Spiral artery development reects a marked induction
o angiogenesis, reected by widespread vessel sprouting and
extension.
At this juncture with hormonal withdrawal, menstruation
ollows. With blastocyst implantation, however, the endometrium is converted to the decidua.
DECIDUA
Tis is a specialized, highly modied endometrium o pregnancy. It is essential or hemochorial placentation, that is, one
in which maternal blood contacts trophoblast. Tis relationship requires trophoblast invasion, and considerable research
has ocused on the interaction between decidual cells and
invading trophoblasts. Decidualization transorms prolierating
endometrial stromal cells into specialized secretory cells. Tis
process depends on estrogen, progesterone, androgens, and actors secreted by the implanting blastocyst (Gibson, 2016). Te
decidua produces actors that regulate endometrial receptivity
and that modulate immune and vascular cell unctions within
the maternal–etal microenvironment. Te special immunomodulatory relationship between the decidua and invading
trophoblasts is largely mediated by decidual natural killer (NK)
cells and ensures success o the pregnancy semiallograt.
■ Decidual Structure
Te decidua is classied into three parts based on anatomical
location. Decidua directly beneath the implanted blastocyst
is modied by trophoblast invasion and becomes the decidua
basalis. Te decidua capsularis overlies the enlarging blastocyst
and initially separates the conceptus rom the rest o the uterine
cavity (Fig. 5-2). Tis portion is most prominent during the
second month o pregnancy and consists o stromal decidual
cells covered by a single layer o attened epithelial cells. Te
other side o the capsularis contacts the avascular, extraembryonic etal membrane—the chorion laeve. Te remainder o the
uterus is lined by decidua parietalis. During early pregnancy, a
space lies between the decidua capsularis and parietalis because
the gestational sac does not ll the entire uterine cavity. Te
gestational sac is the extraembryonic coelom and also called the
chorionic cavity. By 14 to 16 weeks’ gestation, the expanding sac has enlarged to completely ll the uterine cavity. Te
resulting apposition o the decidua capsularis and parietalis
creates the decidua vera, and the uterine cavity is unctionally
obliterated.
In early pregnancy, the decidua begins to thicken, eventually attaining a depth o 5 to 10 mm. With magnication, urrows and numerous small openings, representing the mouths o
uterine glands, can be detected. Later in pregnancy, the decidua
becomes thinner, presumably because o pressure exerted by the
expanding uterine contents.
Te decidua parietalis and basalis are composed o three
layers. Tere is a surace or compact zone—zona compacta; a
middle portion or spongy zone—zona spongiosa—that has remnants o glands and numerous small blood vessels; and a basal
zone—zona basalis. Te zona compacta and spongiosa together
orm the zona unctionalis. Te basal zone remains ater delivery
and gives rise to new endometrium.
Te decidual reaction is completed only with blastocyst
implantation. Predecidual changes, however, commence rst
during the midluteal phase in endometrial stromal cells adjacent to the spiral arteries and arterioles. Tereater, these alterations spread in waves throughout the uterine endometrium.
Te endometrial stromal cells enlarge to orm polygonal or
round decidual cells. Te nuclei become vesicular, and the cytoplasm becomes clear, slightly basophilic, and surrounded by a
translucent membrane.
As the embryo–etus grows, the blood supply to the decidua
capsularis is lost. However, spiral arteries persist to supply the
decidua parietalis. Tese arteries retain smooth muscle and
endothelium and thereby remain responsive to vasoactive
agents.
In contrast, the spiral arteries that supply the decidua basalis and ultimately the placental intervillous space are altered
remarkably. rophoblastic cells invade the spiral arterioles
and arteries and replace their endothelial cells. Te vessel wall
smooth muscle is destroyed, and the resulting uteroplacental
vessels become unresponsive to vasoactive agents. Deective trophoblastic invasion o spiral arteries is thought to be one underlying cause o preeclampsia (Chap. 40, p. 692). Conversely,
the etal chorionic vessels, which transport blood between the
placenta and the etus, contain smooth muscle and thus do
respond to vasoactive agents.
■ Decidual Histology
Early in pregnancy, the decidual zona spongiosa consists o
large distended glands, oten exhibiting marked hyperplasia
and separated by minimal stroma. At rst, the glands are lined
by typical cylindrical uterine epithelium with abundant secretory activity that contributes to blastocyst nourishment. With
advanced pregnancy, the glandular elements largely disappear,
and the spongy zone o the decidua basalis consists mainly o
arteries and widely dilated veins.
Cervical
canal
Decidua basalis
Chorionic
cavity
Uterine cavity
Decidua
capsularis
Decidua
parietalis
Chorionic villi
Chorionic
villi
Embryo
Yolk sac
Amnion
FIGURE 5-2 Three portions of the decidua—the basalis, capsularis, and parietalis—are illustrated.Implantation and Placental Development 85
CHAPTER 5
Te decidua basalis contributes to ormation o the placental
basal plate (Fig. 5-3). As such, it is invaded by many interstitial trophoblasts. Te Nitabuch layer is a zone o brinoid
degeneration in which invading trophoblasts meet the decidua
basalis. I the decidua is deective, as in placenta accreta, the
Nitabuch layer is usually absent (Chap. 43, p. 759). Normally,
there is also a more supercial, but inconsistent, deposition o
brin—Rohr stria—at the bottom o the intervillous space and
surrounding the anchoring villi. Decidual necrosis is a common phenomenon in the rst and probably second trimesters
(McCombs, 1964). Tus, necrotic decidua obtained through
curettage ater spontaneous abortion should not necessarily be
interpreted as either a cause or an eect o the pregnancy loss.
Both decidual layers contain numerous cell groups whose
composition varies with gestational stage (Loke, 1995). Te
primary cellular components are the true decidual cells, which
dierentiated rom the endometrial stromal cells, and numerous maternal bone marrow-derived cells. O the latter, lymphocytes with unique properties accumulate at the maternal–etal
interace and are essential to evoke immune tolerance between
mother and etus. Tese include regulatory cells, decidual
macrophages, and decidual NK cells. Collectively, these cells
not only provide immunotolerance but also play an important
role in trophoblast invasion and vasculogenesis (PrabhuDas,
2015).
■ Decidual Prolactin
Te decidua produces prolactin, which is present in enormous
amounts in amnionic uid (Golander, 1978; Riddick, 1979).
Decidual prolactin is a product o the same gene that encodes
or anterior pituitary prolactin, but the exact physiological role
o decidual prolactin is unknown. Signaling by prolactin can
lead to the production o proangiogenic actors. Prolactin can
also be cleaved by proteases to orm vasoinhibins (Nakajima,
2015; riebel, 2015). Tese compounds have antiangiogenic
properties and may contribute to peripartum cardiomyopathy,
whose pathogenesis and treatment are discussed in Chapter 52
(p. 933).
Decidual prolactin preerentially enters amnionic uid and
may reach extraordinarily high levels o 10,000 ng/mL at 20 to
24 weeks’ gestation (yson, 1972). In contrast, maternal serum
levels are relatively low at 150 to 200 ng/mL. As discussed in
Chapter 4 (p. 57), prolactin inhibits maternal insulin action
and results in increased glucose levels or etal growth.
IMPLANTATION AND EARLY TROPHOBLAST
FORMATION
■ Fertilization
With ovulation, the ovary releases the secondary oocyte and
surrounding cells o the cumulus–oocyte complex. Te oocyte
complex is quickly enguled by the allopian tube inundibulum. Directional movement o cilia and tubal peristalsis moves
the ovum within the tubal lumen. Fertilization takes place in
the lumen within a ew hours, and no more than a day ater
ovulation. Because o this narrow window, spermatozoa must
be present in the allopian tube at the time o oocyte arrival.
Almost all pregnancies result when intercourse occurs during
the 2 days preceding or on the day o ovulation.
Fertilization is highly complex. Molecular mechanisms allow
spermatozoa to pass between cumulus cells; through the zona
pellucida, which is a thick glycoprotein layer surrounding the
oocyte cell membrane; and into the oocyte cytoplasm. Fusion
o the two nuclei and intermingling o maternal and paternal
chromosomes creates the zygote.
Early human development is described by days or weeks
postertilization, that is, postconceptional (Chap. 7, p. 121).
By contrast, in most chapters o this book, clinical pregnancy
dating is calculated rom the rst day o the last menstrual
period (LMP). Tus, 1 week postertilization corresponds to
approximately 3 weeks rom the LMP in women with regular
28-day cycles. Tis convention is clinically important or dating
pregnancies conceived by in vitro ertilization (IVF), in which
the gestational age is 14 days greater than the day o ertilization. As an example, 8 weeks’ gestation reers to 8 completed
weeks ollowing the LMP but corresponds to 6 weeks postertilization.
Ater ertilization, the zygote—a diploid cell with 46 chromosomes—undergoes cleavage, and zygote cells produced by
this division are blastomeres (Fig. 5-4). In the two-cell zygote, the
blastomeres and polar body continue to be surrounded by the
zona pellucida. Te zygote undergoes slow cleavage or 3 days
while still in the allopian tube. As the blastomeres continue
to divide, a solid mulberry-like ball o cells—the morula—is
produced. Te morula enters the uterine cavity approximately
3 days ater ertilization. Gradual accumulation o uid between
the morula cells leads to ormation o the early blastocyst.
FIGURE 5-3 Section through a junction of chorion, villi, and
decidua basalis in early first-trimester pregnancy. (Reproduced with
permission from Dr. Kurt Benirschke.)86 Placentation, Embryogenesis, and Fetal Development
Section 3
Blastocyst
As early as 4 to 5 days ater ertilization, the 58-cell blastocyst
dierentiates into ve embryo-producing cells—the inner cell
mass (see Fig. 5-4). Te remaining 53 outer cells, called the
trophectoderm, are destined to orm trophoblasts (Hertig, 1962).
In the 107-cell blastocyst, the eight ormative, embryoproducing cells are surrounded by 99 trophoblastic cells. Te
blastocyst is released rom the zona pellucida secondary to secretion o specic proteases rom the secretory-phase endometrial
glands (O’Sullivan, 2002). Release rom the zona pellucida
allows blastocyst-produced cytokines and hormones to directly
inuence endometrial receptivity (Lindhard, 2002). Te blastocyst secretes interleukin-1α (IL-1α) and IL-1β, which are cytokines that likely directly inuence the endometrium. Embryos
also secrete hCG, which may inuence endometrial receptivity
(Licht, 2001; Lobo, 2001).
Te receptive endometrium is thought to respond by producing leukemia inhibitory actor (LIF), ollistatin, and colonystimulating actor-1 (CSF-1). LIF and ollistatin activate signaling
pathways that collectively inhibit prolieration and promote di-
erentiation o the endometrial epithelia and stroma to enable
uterine receptivity (Rosario, 2016b). At the maternal–etal inter-
ace, CSF-1 has proposed immunomodulatory and proangiogenic
actions that are required or implantation (Rahmati, 2015).
■ Implantation
Te blastocyst implants into the uterine wall 6 or 7 days ater
ertilization. Tis process can be divided into three phases: (1)
apposition—initial contact o the blastocyst to the uterine wall;
(2) adhesion—increased physical contact between the blastocyst and decidua; and (3) invasion—penetration and invasion
o syncytiotrophoblast and cytotrophoblasts into the decidua,
inner third o the myometrium, and uterine vasculature.
Successul implantation requires a receptive endometrium
appropriately primed with estrogen and progesterone by the
corpus luteum. Such uterine receptivity is limited to days 20
to 24 o the endometrial cycle. Adherence is mediated by cellsurace receptors at the implantation site that interact with blastocyst receptors (Carson, 2002; Lessey, 2002). I the blastocyst
approaches the endometrium ater cycle day 24, the potential
or adhesion declines because antiadhesive glycoprotein synthesis prevents receptor interactions (Navot, 1991). A mismatch
between uterine receptivity and timing o embryo transer can
lead to repeated implantation ailure in some IVF patients. Tis
has stimulated eorts to dene receptivity by gene expression
proles to improve clinical outcomes (Ruiz-Alonso, 2013).
At the time o its interaction with the endometrium, the
blastocyst is composed o 100 to 250 cells. Te blastocyst
trophectoderm loosely adheres to the decidua by apposition.
Tis appears to be closely regulated by paracrine interactions
between these two tissues and most commonly occurs on the
upper posterior uterine wall.
Successul endometrial blastocyst adhesion involves modi-
ed expression o cellular adhesion molecules (CAMs). Te
integrins—one o our amilies o CAMs—are cell-surace
receptors that mediate cell adhesion to extracellular matrix
proteins (Lessey, 2002). Endometrial integrins are hormonally regulated, and a specic set o integrins is expressed at
implantation (Lessey, 1995). Recognition-site blockade o the
integrins needed or binding will prevent blastocyst attachment
(Kaneko, 2013).
■ Trophoblast Development
Te etus depends on the placenta or pulmonary, hepatic, and
renal unctions. Tese are accomplished through the anatomical relationship o the placenta and its uterine interace. In
overview, maternal blood ows rom uteroplacental vessels into
the placental intervillous space and bathes the syncytiotrophoblast, which surround villi. Here, gases, nutrients, and other
substances are exchanged with etal capillary blood within the
core o each placental villus. Tus, etal and maternal blood do
not normally mix in this hemochorial placenta.
Human placental ormation begins with the trophectoderm,
which gives rise to a trophoblast cell layer encircling the blastocyst. rophoblast exhibits the most variable structure, unction, and developmental pattern o all placental components.
Its invasiveness promotes implantation, its nutritional role or
the conceptus is reected in their name, and its endocrine unction is essential to maternal physiological adaptations and to
pregnancy maintenance.
By the eighth day postertilization, ater initial implantation,
the trophoblast has dierentiated into an outer multinucleated
syncytium—the primitive syncytiotrophoblast, and an inner layer
o primitive mononuclear cells—cytotrophoblasts. Te latter are
solely germinal cells or the syncytium. As cytotrophoblasts di-
erentiate, they express an endogenous envelope protein called
syncytin, which aids their cell usion with the expanding outer
layer o syncytiotrophoblast. Each cytotrophoblast has a welldemarcated cell border, a single nucleus, and ability to undergo
DNA synthesis and mitosis (Arnholdt, 1991). Tese are lacking
Polar body Blastomeres
2-cell stage 4-cell stage 8-cell stage
16-cell stage (Morula) Blastocyst Inner cell
mass
Blastocyst
cavity
Trophoblast
FIGURE 5-4 Zygote cleavage and blastocyst formation. The
morula period begins at the 12- to 16-cell stage and ends when
the blastocyst forms, which occurs when there are 50 to 60 blastomeres present. The polar bodies, shown in the 2-cell stage, are
small nonfunctional cells that degenerate.Implantation and Placental Development 87
CHAPTER 5
in the syncytiotrophoblast, which provides transport unctions
o the placenta. It is so named because instead o individual
cells, it has an amorphous cytoplasm without cell borders,
nuclei that are multiple and diverse in size and shape, and a
continuous syncytial lining.
Following implantation, trophoblasts dierentiate along
two main pathways that give rise to either villous or extravillous trophoblasts. As shown in Figure 5-5, both have distinct
unctions (Loke, 1995). Villous trophoblasts generate chorionic
villi, which primarily transport oxygen, nutrients, and other
compounds between the etus and mother. Extravillous trophoblasts migrate into the decidua and myometrium (Fig. 5-6).
Tey also penetrate maternal vasculature and thus directly contact various maternal cell types (Pijnenborg, 1994). Extravillous
trophoblasts are urther classied as interstitial trophoblasts and
endovascular trophoblasts. Te interstitial trophoblasts invade
the decidua and eventually penetrate the myometrium to orm
placental-bed giant cells. Tese trophoblasts also surround spiral arteries. Te endovascular trophoblasts penetrate the spiral
artery lumens (Pijnenborg, 1983).
Recently, single-cell RNA sequencing technology has
enabled inerence o trophoblast dierentiation lineages (Liu,
2018; sang, 2017). Tis has also yielded insights into the communication between trophoblasts and maternal decidual and
immune cell populations (Vento-ormo, 2018). Furthermore,
human trophoblast stem cells, long hypothesized to exist, have
been isolated rom early gestations (Okae, 2018). Culture o
these cells will likely aid greater understanding o early trophoblast dierentiation and invasion (Haider, 2018; urco, 2018).
■ Early Invasion
Ater gentle erosion between epithelial cells o the surace endometrium, invading trophoblasts burrow deeper. At 9 days o
development, the blastocyst wall acing the uterine lumen is a
single layer o attened cells. By the 10th day, the blastocyst
becomes totally encased within the endometrium (Fig. 5-7).
Te blastocyst wall opposite the uterine lumen is thicker and
comprises two zones—the trophoblasts and the embryo-orming
inner cell mass. As early as 7.5 days postertilization, the inner
cell mass or embryonic disc dierentiates into a thick plate o
primitive ectoderm and an underlying layer o endoderm. Some
small cells appear between the embryonic disc and the trophoblasts and enclose a space that will become the amnionic cavity.
Extraembryonic mesenchyme rst appears as groups o isolated cells within the blastocyst cavity, and later this mesoderm
completely lines the cavity. Within this mesoderm, spaces orm
and then use to orm the chorionic cavity, that is, the extraembryonic coelom. Te chorion is composed o trophoblasts
and mesenchyme. Some mesenchymal cells eventually will condense to orm the body stalk. Tis stalk joins the embryo to the
nutrient chorion and later develops into the umbilical cord.
Te body stalk can be recognized at an early stage at the caudal
end o the embryonic disc (Fig. 7-3, p. 123).
Early
pregnancy
End of 1st
trimester
Midgestation 3rd trimester
Myometrium
Decidua
basalis
Interstitial extravillous trophoblast
Endovascular extravillous trophoblast
Anchoring villus
Syncytiotrophoblast
Cytotrophoblast
Extravillous trophoblasts
Endothelium
Spiral artery
FIGURE 5-5 Endovascular and interstitial extravillous trophoblasts are found outside the villus. Endovascular trophoblasts invade and
transform spiral arteries during pregnancy to create low-resistance blood flow that is characteristic of the placenta. Interstitial trophoblasts
invade the decidua and surround spiral arteries.
FIGURE 5-6 Photomicrograph of extravillous trophoblast invasion
of the decidua basalis.88 Placentation, Embryogenesis, and Fetal Development
Section 3
As the embryo enlarges, more maternal decidua basalis is
invaded by syncytiotrophoblast. Beginning approximately
12 days ater conception, the syncytiotrophoblast is permeated
by a system o intercommunicating channels called trophoblastic lacunae. Ater invasion o supercial decidual capillary walls,
lacunae become lled with maternal blood. At the same time,
the decidual reaction, characterized by decidual stromal cell
enlargement and glycogen storage, intensies (p. 84).
■ Chorionic Villi
With deeper invasion into the decidua, solid primary villi arise
rom buds o cytotrophoblasts that protrude into the primitive syncytium beore 12 days postertilization. Primary villi are
composed o a cytotrophoblast core covered by syncytiotrophoblast. As the lacunae merge, a complicated labyrinth is ormed
that is partitioned by these solid cytotrophoblastic columns.
Te trophoblast-lined channels orm the intervillous space,
and the solid cellular columns orm the primary villous stalks.
Beginning on the 12th day ater ertilization, mesenchymal
cords derived rom extraembryonic mesoderm invade the solid
trophoblast columns. Tese then orm secondary villi. Once
angiogenesis begins in the mesenchymal cords, tertiary villi are
created. Although maternal venous sinuses are tapped early in
implantation, maternal arterial blood does not enter the intervillous space until around day 15. By the 17th day, however,
etal blood vessels are unctional, and a placental circulation is
established. Te etal–placental circulation is completed when
the blood vessels o the embryo are connected with chorionic
vessels. In some villi, angiogenesis ails rom lack o circulation.
Te most striking exaggeration o this is seen with hydatidiorm
mole (Fig. 13-1, p. 236).
Villi are covered by an outer layer o syncytiotrophoblast and an inner layer o cytotrophoblasts. Prolieration o
cytotrophoblast at the villous tips produces the trophoblastic
cell columns that orm anchoring villi. Tey are not invaded by
etal mesenchyme, and they are anchored to the decidua at the
basal plate, which is the maternal side o the intervillous space.
Te chorionic plate orms the roo o the intervillous space. It
consists o the two layers o trophoblasts, which line the intervillous space, and brous mesoderm on the opposite side. Te
nal chorionic plate is ormed by 8 to 10 weeks as the amnionic
and primary chorionic plate mesenchyme use together. Tis
ormation is accomplished by expansion o the amnionic sac.
Early electron microscopic studies demonstrate prominent
microvilli on the syncytial surace (Fig. 5-8) (Wislocki, 1955).
Syncytiotrophoblast
Endometrial gland Spiral artery
Lacunar network
Amnion
Cytotrophoblast
Extraembryonic
mesoderm
Primitive
A yolk sac B
Eroded gland
Maternal blood
Lacunar network
Amnionic
cavity
Extraembryonic
endoderm
Cytotrophoblast
Embryonic disc
FIGURE 5-7 Drawing of sections through implanted blastocysts. A. At 10 days. B. At 12 days after fertilization. This stage is characterized
by the intercommunication of the lacunae filled with maternal blood. Note in (B) that large cavities have appeared in the extraembryonic
mesoderm, forming the beginning of the extraembryonic coelom. Also, extraembryonic endodermal cells have begun to form on the inside
of the primitive yolk sac. (Redrawn from Moore KL, Persaud, TV, Torchia, MG (eds): The Developing Human. Clinically Oriented Embryology,
9th ed. Philadelphia, PA: Saunders; 2013.)
FIGURE 5-8 Electron micrograph of term human placenta villus.
A villus capillary filled with fetal red blood cells (asterisks) is seen in
close proximity to the microvilli border. (Reproduced with permission from Boyd JD, Hamilton WJ: The Human Placenta. Cambridge,
Heffer, 1970.)Implantation and Placental Development 89
CHAPTER 5
Associated pinocytotic vacuoles and vesicles are involved in
both absorptive and secretory placental unctions. Microvilli increase trophoblast surace area in direct contact with
maternal blood, the dening characteristic o a hemochorial
placenta.
PLACENTA AND CHORION
■ Chorion Development
In early pregnancy, the villi are distributed over the entire
periphery o the chorionic membrane (Fig. 5-9). As the blastocyst with its surrounding trophoblasts grows and expands into
the decidua, one pole aces the endometrial cavity. Te opposite pole will orm the placenta. Here, chorionic villi in contact
with the decidua basalis prolierate to orm the chorion rondosum—or leay chorion. As growth o embryonic and extraembryonic tissues continues, the blood supply to the chorion
acing the endometrial cavity is restricted. Because o this, villi
A B
FIGURE 5-9 Complete abortion specimens. A. Initially, the entire
chorionic sac is covered with villi, and the embryo within is not
visible B. Stretch and pressure from further growth prompt partial
regression of the villi. The remaining villi form the future placenta,
whereas the smooth portion is the chorion.
in contact with the decidua capsularis cease to grow and then
degenerate. Tis portion o the chorion becomes the avascular
etal membrane that abuts the decidua parietalis and is called
the chorion laeve—or smooth chorion. Tis smooth chorion is
composed o cytotrophoblasts and etal mesodermal mesenchyme. A paracrine system between the decidua and chorion
also links mother and etus. Tis is an extraordinarily important
arrangement or maternal–etal communication and or maternal immunological acceptance o the conceptus (GuzelogluKayisli, 2009).
Until near the end o the third month, the chorion laeve is
separated rom the amnion by the exocoelomic cavity. Terea-
ter, they are in intimate contact to orm the avascular amniochorion. Tese two structures are important sites o molecular
transer and metabolic activity. Moreover, they constitute an
important paracrine arm o the etal–maternal communication
system.
■ Regulators of Trophoblast Invasion
Implantation and endometrial decidualization activate a unique
population o maternal immune cells that play critical unctions
in trophoblast invasion, angiogenesis, spiral artery remodeling,
and maternal tolerance to etal alloantigens. Decidual natural
killer cells (dNK) make up 70 percent o decidual leukocytes in
the rst trimester and directly contact trophoblasts. In contrast to
NK cells in peripheral blood, dNK cells lack cytotoxic unctions. Tey produce specic cytokines and angiogenic actors
to regulate trophoblast invasion and spiral artery remodeling
(Hanna, 2006). dNK cells promote phagocytosis o cell debris
(Faas, 2017). Tese and other unique properties distinguish
dNK cells rom circulating NK cells and rom NK cells in the
endometrium beore pregnancy (Fu, 2013; Winger, 2013).
dNK cells express both IL-8 and intereron-inducible protein
10, which bind to receptors on invasive trophoblastic cells to
promote their decidual invasion toward the spiral arteries. dNK
cells also produce proangiogenic actors, including VEGF and
placental growth actor (PlGF), which both promote vascular
growth in the decidua.
rophoblasts also secrete specic chemokines that attract
the dNK cells to the maternal–etal interace. Tus, both cell
types simultaneously attract each other. Decidual macrophages
account or approximately 20 percent o leukocytes in the
rst trimester and elicit an M2-immunomodulatory phenotype (Williams, 2009). Recall that while M1 macrophages are
proinammatory, M2 macrophages counter proinammatory
responses and promote tissue repair.
Concurrently, cell subsets aid tolerance toward the allogenic etus. Regulatory cells (regs) are essential or promoting immune tolerance. Other cell subsets are present, such
as T1, T2, and T17, although their unctions are tightly
regulated (Ruocco, 2014).
Endometrial Invasion
Extravillous trophoblasts o the rst-trimester placenta are
highly invasive. Tis process occurs under low-oxygen conditions, and regulatory actors that are induced under hypoxic conditions are contributory (Soares, 2012). Invasive trophoblasts90 Placentation, Embryogenesis, and Fetal Development
Section 3
secrete numerous proteolytic enzymes that digest extracellular
matrix and activate proteinases already present in the decidua.
rophoblasts produce urokinase-type plasminogen activator,
which converts plasminogen into the broadly acting serine protease, plasmin. Tis in turn both degrades matrix proteins and
activates MMPs. One member o the MMP amily, MMP-9,
appears to be critical. Te timing and extent o trophoblast
invasion is regulated by a balanced interplay between pro- and
anti-invasive actors.
Low estradiol levels in the rst trimester are critical or trophoblast invasion and spiral artery remodeling. Animal studies
suggest that the rise in second-trimester estradiol levels suppresses and limits vessel remodeling by reducing trophoblast
expression o VEGF and specic integrin receptors (Bonagura,
2012). Namely, extravillous trophoblasts express integrin receptors that recognize the extracellular matrix proteins collagen IV,
laminin, and bronectin. Binding o these matrix proteins and
integrin receptors initiates signals to promote trophoblast cell
migration and dierentiation. However, as pregnancy advances,
rising estradiol levels downregulate VEGF and integrin receptor expression. Tis represses and controls the extent o uterine
vessel transormation.
■ Spiral Artery Invasion
One o the most remarkable eatures o human placental development is the extensive modication o maternal vasculature by
trophoblasts, which are etal in origin. Tese events occur in the
rst hal o pregnancy and are essential to uteroplacental blood
ow. Tey are also integral to conditions such as preeclampsia, etal-growth restriction, and preterm birth. Spiral artery
modications are carried out by two populations o extravillous trophoblasts—endovascular trophoblasts, which penetrate
the spiral-artery lumen, and interstitial trophoblasts, which surround the arteries (see Fig. 5-5).
Interstitial trophoblasts constitute a major portion o the
placental bed. Tey penetrate the decidua and adjacent myometrium and aggregate around spiral arteries, where they may
aid endovascular trophoblast invasion.
Endovascular trophoblasts rst enter the spiral artery lumens
and initially orm cellular plugs. Tey then promote apoptosis
o the native endothelium and replace the vessel lumen with
cells o etal origin. Fibrinoid material replaces smooth muscle
and connective tissue o the vessel media. Tis decreases resistance o blood ow into the intervillous space. Invading endovascular trophoblasts can extend several centimeters along the
vessel lumen, and they must migrate against arterial ow. O
note, invasion by trophoblasts involves only the decidual spiral
arteries and not decidual veins.
Uteroplacental vessel development proceeds in two waves
or stages (Ramsey, 1980). First, beore 12 weeks’ postertilization, spiral arteries are invaded and modied up to the
border between the decidua and myometrium. Te second
wave, between 12 and 16 weeks, involves some invasion o
the intramyometrial segments o spiral arteries. Remodeling
converts narrow-lumen, muscular spiral arteries into dilated,
low-resistance uteroplacental vessels. Molecular mechanisms o
these crucial events and their signicance or preeclampsia and
etal-growth restriction have been reviewed (Pereira de Sousa,
2017; Xie, 2016).
Although early placenta development occurs in a lowoxygen-tension environment (2 to 3 percent O2), this vascular
remodeling increases blood ow and oxygenation such that the
oxygen concentration more than doubles around the end o the
rst trimester (Chang, 2018).
■ Villus Branching
Although certain villi o the chorion rondosum extend rom
the chorionic plate to the decidua to serve as anchoring villi,
most villi arborize and end reely within the intervillous space.
As gestation proceeds, the short, thick, early stem villi branch
to orm progressively ner subdivisions and greater numbers o increasingly smaller villi (Fig. 5-10). Tis increasing
villous surace area correlates with gestational age and aids
etal growth. In cases o restricted blood ow, more highly
branched villi than expected or gestational age is a compensatory mechanism.
Each truncal or main stem villus and its ramications constitutes a placental lobule or cotyledon. Each lobule has a single
chorionic artery and vein, such that lobules constitute the unctional units o placental architecture.
■ Placental Growth and Maturation
In the rst trimester, placental growth is more rapid than
that o the etus. However, by 17 weeks’ gestation, placental
and etal weights are nearly equal. By term, placental weight
approximates one sixth o etal weight.
Te mature placenta and its variant orms are discussed in
detail in Chapter 6 (p. 108). Briey, viewed rom the maternal surace, which attaches to the uterine wall, the number o
slightly elevated convex areas, called lobes, varies rom 10 to
38. Lobes are incompletely separated by grooves o variable
depth. Tese tissue grooves orm in response to placental septa,
which rise up as projections o decidua. Te total number o
placental lobes remains the same throughout gestation, and
individual lobes continue to grow—although less actively in
late pregnancy (Craword, 1959). Although grossly visible lobes
are commonly reerred to as cotyledons, this is inaccurate. Correctly used, lobules or cotyledons are the unctional units supplied by each main stem villus.
As villi continue to branch and the terminal ramications
become more numerous and smaller, the volume o cytotrophoblasts decline. Tis layer attenuates, and by 16 weeks’ gestation, the apparent continuity o the cytotrophoblasts is lost. As
the syncytium also thins, the etal vessels become more prominent within the villus and lie closer to the intervillous space
(see Fig. 5-8). At term, villi may be ocally reduced to a thin
layer o syncytium covering minimal villous connective tissue
in which thin-walled etal capillaries abut the trophoblast and
dominate the villi.
Te villous stroma also exhibits changes as gestation progresses. In early pregnancy, the branching connective-tissueImplantation and Placental Development 91
CHAPTER 5
cells are separated by an abundant loose intercellular matrix.
Later, the villous stroma becomes denser, and the cells are more
spindly and closely packed. Another stromal change involves
inltration o Hobauer cells, which are etal macrophages.
Tese are round with vesicular, oten eccentric nuclei and with
very granular or vacuolated cytoplasm. Tey grow in number
and maturational state throughout pregnancy and appear to be
important mediators o protection at the maternal–etal inter-
ace (Johnson, 2012). Tese macrophages are phagocytic, have
an immunosuppressive phenotype, produce various cytokines,
and provide paracrine regulation o trophoblastic unctions
(Cervar, 1999; Reyes, 2017).
Some architectural changes, i substantive, can lower placental exchange efciency. Tese include thickening o the basal
lamina o trophoblast or capillaries, obliteration o certain etal
vessels, greater villous stroma, and brin deposition on the villous surace (Chap. 6, p. 109).
■ Placental Circulation
Te gross anatomy o the placenta reects the intimate
approximation o the etal capillary bed to maternal blood.
Te etal surace is covered by the transparent amnion,
beneath which chorionic vessels course. A section through
the placenta includes amnion, chorion, chorionic villi and
intervillous space, decidual (basal) plate, and myometrium
(Figs. 5-11 and 5-12).
A B
C D
E F
FIGURE 5-10 Illustration (A, B), histology (C, D), and electron microscopy (E, F) of early (top panel) and late (bottom panel) human placenta villi. Limited branching of villi is seen in the early placenta. With maturation, increasing villous arborization is seen, and villous capillaries lie closer to the surface of each villus. (Electron micrographs reproduced with permission from King BF, Menton DN: Scanning electron
microscopy of human placental villi from early and late in gestation. Am J Obstet Gynecol 122:824, 1975.)
Myometrium
Decidua vera
Chorion
Amnion
Chorion
Amnion
Decidua
vera
Decidua
basalis
Placenta
FIGURE 5-11 Uterus showing a normal placenta and its membranes in situ.92 Placentation, Embryogenesis, and Fetal Development
Section 3
Fetal Circulation
Deoxygenated venous-like etal blood ows to the placenta
through the two umbilical arteries. As the cord joins the placenta, these umbilical vessels branch repeatedly beneath the
amnion as they run across the chorionic plate. Branching
continues within the villi to ultimately orm capillary networks
in the terminal villous branches. Blood with signicantly higher
oxygen content returns rom the placenta via a single umbilical
vein to the etus.
Te branches o the umbilical vessels that traverse along the
chorionic plate are called placental surace vessels or chorionic
vessels. Tey respond to vasoactive substances, but anatomically, morphologically, histologically, and unctionally, they are
unique. Chorionic arteries always cross over chorionic veins.
Vessels are most readily recognized by this anatomical relationship, but they are difcult to distinguish by histological criteria.
runcal arteries are perorating branches o the surace arteries and pass through the chorionic plate. Each truncal artery
supplies one main stem villus and thus one cotyledon. As the
artery penetrates the chorionic plate, its wall loses smooth muscle, and its caliber increases. Te loss o muscle continues as the
truncal arteries and veins branch into their smaller rami.
Beore 10 weeks’ gestation, end-diastolic ow is not detected
within the umbilical artery at the end o the etal cardiac cycle
(Fisk, 1988; Loquet, 1988). Ater 10 weeks, however, end-diastolic ow appears and is maintained throughout normal pregnancy. Clinically, these ow patterns are studied with Doppler
sonography to assess etal well-being (Chap. 4, p. 262).
Maternal Circulation
Mechanisms o placental blood ow must allow blood to leave
maternal circulation; ow into an amorphous space lined by
syncytiotrophoblast; and return through maternal veins without
producing arteriovenous-like shunts that would prevent adequate
exchange between maternal blood and etal villi. For this, maternal blood enters through the basal
plate and is driven high up toward
the chorionic plate by arterial pressure beore laterally dispersing
(Fig. 5-13). Ater bathing the external microvillous surace, maternal
blood drains back through venous
orices in the basal plate and enters
uterine veins. Tus, maternal blood
traverses the placenta randomly
without preormed channels. rophoblast invasion o the spiral
arteries creates low-resistance vessels that can accommodate massive
increase in uterine perusion during
gestation. Generally, spiral arteries
are perpendicular to, but veins are
parallel to, the uterine wall. Tis
arrangement aids closure o veins
during a uterine contraction and
prevents the exit o maternal blood
rom the intervillous space. Te
number o arterial openings into
the intervillous space is gradually
reduced by cytotrophoblastic invasion to approximately 120 entry
sites at term (Brosens, 1963). Tese
discharge blood in spurts to bath
FIGURE 5-12 Photomicrograph of implantation site with partial
section of an early embryo. Anchoring villi are seen with extravillous trophoblasts invading the decidua basalis. CNS = central
nervous system. (Reproduced with permission from Dr. Kurt
Benirschke.)
Chorionic plate
Umbilical Umbilical artery
vein
Umbilical cord
Spiral
artery
Basal
plate
Chorionic
villus
Decidual septum
FIGURE 5-13 Schematic drawing of a section through a full-term placenta. Maternal blood
flows into the intervillous spaces in funnel-shaped spurts, and umbilical arteries carry deoxygenated fetal blood to the placenta. Exchanges between the maternal and fetal systems occur as
maternal blood flows around the villi. The umbilical vein carries oxygenated blood back to the
fetus. Inflowing arterial blood pushes maternal venous blood into the endometrial veins, which
are scattered over the entire surface of the decidua basalis. Placental lobes are separated from
each other by placental (decidual) septa.Implantation and Placental Development 93
CHAPTER 5
the adjacent villi (Borell, 1958). Ater the 30th week, a prominent venous plexus lies between the decidua basalis and myometrium and helps develop the cleavage plane needed or placental
separation ater delivery.
Both inow and outow are curtailed during uterine contractions. Bleker and associates (1975) used serial sonography during normal labor and ound that placental length,
thickness, and surace area grew during contractions. Tey
attributed this to distention o the intervillous space by
impairment o venous outow compared with arterial inow.
During contractions, thereore, a somewhat larger volume o
blood is available or exchange even though the rate o ow
is decreased. Similarly, Doppler velocimetry has shown that
diastolic ow velocity in spiral arteries is diminished during
uterine contractions. Tus, principal actors regulating intervillous space blood ow are arterial blood pressure, intrauterine pressure, uterine contraction pattern, and actors that act
specically on arterial walls.
■ Breaks in the Placental “Barrier”
Te placenta does not maintain absolute integrity o the etal
and maternal circulations, and cells trafc between mother and
etus in both directions. Tis situation is best exemplied clinically by erythrocyte D-antigen alloimmunization (Chap. 18,
p. 353). Fetal cell transer is small in most cases, although rarely
the etus exsanguinates into the maternal circulation.
Fetal cells can also engrat in the mother during pregnancy
and still be detected decades later. Fetal lymphocytes, mesenchymal stem cells, and endothelial colony-orming cells all reside
in maternal blood, bone marrow, or uterine vasculature (Huu,
2006; Piper, 2007; Sipos, 2013). Tis requent phenomenon,
termed microchimerism, is implicated in the disparate emale:
male ratio o autoimmune disorders (Greer, 2011; Stevens,
2006). As discussed in Chapter 62 (p. 1109), etal cell engratment has been associated with the pathogenesis o lymphocytic
thyroiditis, scleroderma, and systemic lupus erythematosus.
■ MaternalFetal Interface
Tis maternal–etal interace is an active hub o immunological interactions that allows implantation, appropriate placental
development, and immunotolerance o the etus. At the same
time, a unctional immune system must be maintained to protect the mother.
Immunogenicity of the Trophoblasts
rophoblastic cells are the only etus-derived cells in direct
contact with maternal tissues and blood. Fetal syncytiotrophoblast synthesizes and secretes numerous actors that regulate the
immune responses o maternal cells both at the implantation
site and systemically.
Human leukocyte antigens (HLAs) are the human analogue
o the major histocompatibility complex (MHC) (Hunt, 1992).
Tere are 17 HLA class I genes and include three classic genes,
HLA-A, -B, and -C, that encode the major class I (class Ia)
transplantation antigens. Tree other class I genes, designated
HLA-E, -F, and -G, encode class Ib HLA antigens. MHC class
I and II antigens are absent rom villous trophoblasts, which
appear to be immunologically inert at all gestational stages
(Weetman, 1999). Invasive extravillous trophoblasts do express
MHC class I molecules but avoid rejection by the maternal
immune system.
Moett-King (2002) reasoned that normal implantation
depends on controlled trophoblastic invasion o maternal
decidua and spiral arteries. Such invasion must proceed ar
enough to provide or normal etal growth and development
but avoid the pathogenic invasion seen in placenta accreta spectrum disorders (Chap. 43, p. 759). She suggests that dNK cells
combined with extravillous trophoblasts’ unique expression o
three specic HLA class I genes act in concert to permit and
subsequently limit trophoblast invasion.
Extravillous trophoblasts express the class Ia antigen HLA-C
and nonclassic class Ib molecules HLA-E and HLA-G. HLA-G
antigen is expressed only in humans and is restricted to extravillous trophoblasts in contact with maternal tissues. Expressed
in both membrane-bound and soluble isoorms detectable in
maternal circulation, HLA-G is thought to protect extravillous
trophoblasts rom immune rejection by modulating the unction o both decidual and circulating NK populations (Apps,
2011; Rajagopalan, 2012). Te importance o this molecule is
highlighted by the observation that IVF embryos ail to implant
i they do not express a soluble HLA-G isoorm (Fuzzi, 2002).
Tus, HLA-G may act through multiple mechanisms to aid
tolerance o the maternal–etal antigen mismatch (LeBouteiller,
1999). Abnormal HLA-G expression in extravillous trophoblasts rom women with preeclampsia suggests immune dysregulation as one etiology (Goldman-Wohl, 2000).
Decidual Immune Cells
O leukocytes, NK cells predominate in midluteal phase endometrium and in rst-trimester decidua, but numbers decline
by term (Johnson, 1999). In rst-trimester decidua, dNK cells
lie close to extravillous trophoblasts and purportedly regulate
invasion. Teir inltration is increased by progesterone and by
stromal cell production o IL-15 and decidual prolactin (Dunn,
2002; Gubbay, 2002). Although dNK cells have the capacity
or cytotoxicity, they are not cytotoxic toward etal trophoblasts. Teir cytotoxic potential is prevented by molecular cues
rom decidual macrophages. As noted, specic HLA molecule
expression protects against dNK cells’ damaging actions.
Decidual macrophages are another decidual immune cell type
and are distinct rom proinammatory M1 or antiinammatory
M2 macrophages. Tese cells regulate adaptive cell responses;
control dNK dierentiation, activation, and cytotoxicity; and
produce antiinammatory cytokines such as IL-10.
Dendritic cells are antigen-presenting cells that educate
maternal cells. Tey aect development o a receptive endometrium or implantation.
Maternal T cells, as part o the adaptive immune response,
increase in number and unction ater encounter with a specic
antigen. Tese cells subsequently retain the ability to respond
rapidly in a subsequent encounter with the same antigen. In
contrast, regs are immunosuppressive, and during pregnancy
systemic maternal populations expand. Specic reg cell populations persist and protect against aberrant immune responses.94 Placentation, Embryogenesis, and Fetal Development
Section 3
AMNION
At term, the amnion is a tough and tenacious but pliable membrane. Tis innermost avascular etal membrane is contiguous
with amnionic uid and provides almost all o the tensile strength
o the etal membranes. Its resilience to rupture is vital to success-
ul pregnancy outcomes. Indeed, preterm rupture o etal membranes is a major cause o preterm delivery (Chap. 45, p. 787).
Bourne (1962) described ve separate amnion layers. Here,
progression o discussed layers moves rom amnionic uid to
the chorion. Te innermost layer, which is bathed by amnionic
uid, is a single-layer cuboidal epithelium (Fig. 5-14). Tis epithelium attaches rmly to a distinct basement membrane. Next,
an acellular compact layer composed primarily o interstitial collagens is ollowed by the fbroblast-like mesenchymal cell layer.
Te outermost layer is the relatively acellular zona spongiosa,
which is contiguous with the chorion laeve. Te amnion also
contains a ew etal macrophages, which predominate in the
outer two layers. Te amnion lacks smooth muscle cells, nerves,
lymphatics, and importantly, blood vessels.
■ Amnion Development
Early during implantation, a space develops between the
embryonic cell mass and adjacent trophoblastic cells (see
Fig. 5-7). Small cells that line this inner surace o trophoblasts
are precursors o amnionic epithelium, and the amnion is rst
identiable on the 7th or 8th day o embryo development. It is
initially a minute vesicle, which then develops into a small sac
that covers the dorsal embryo surace. As the amnion enlarges,
it gradually enguls the growing embryo, which prolapses into
its cavity (Benirschke, 2012).
Distention o the amnionic sac eventually brings it into contact with the interior surace o the chorion laeve. Apposition
o the chorion laeve and amnion near the end o the rst trimester obliterates the extraembryonic coelom. Te amnion and
chorion laeve, although slightly adhered, are never intimately
connected and can be separated easily. Placental amnion covers
the placental surace and thereby is in contact with the chorionic vessels. Umbilical amnion covers the umbilical cord. As
discussed in Chapter 48 (p. 842), with monochorionic-diamnionic placentas, no tissue intervenes between the used amnions. With dichorionic-diamnionic twin placentas, amnions are
separated by used chorion laeves.
Amnionic uid lls the amnionic sac. As pregnancy progresses, this normally clear uid increases in volume until
approximately 34 weeks’ gestation. Ater this, the volume
declines. At term, it averages 1000 mL, although this may vary
widely in normal and especially abnormal conditions. Amnionic uid origin, composition, circulation, and unction o are
discussed urther in Chapter 14 (p. 256).
■ Amnion Cell Histogenesis
Amnionic epithelium derives rom etal ectoderm o the embryonic disc and not rom trophoblasts. Tis is an important consideration both embryologically and unctionally. For example,
HLA class I gene expression in amnion is more akin to that in
embryonic cells than to that in trophoblasts.
Te broblast-like mesenchymal cell layer likely originates
rom embryonic mesoderm. Early in human embryogenesis,
the amnionic mesenchymal cells lie immediately adjacent to
the basal surace o the amnion epithelium. At this time, the
amnion has two-cell layers and approximately equal numbers o
epithelial and mesenchymal cells. Simultaneously with growth
and development, interstitial collagens are deposited between
these two cell layers. Tis marks ormation o the amnion compact layer, which separates the two early layers.
Amnionic epithelium early in pregnancy replicates at a rate
appreciably aster than mesenchymal cells. Tus, as the amnionic sac expands, its epithelial cells orm a continuous, uninterrupted layer. Instead, mesenchymal cells become more sparsely
distributed. Connected by a lattice network o extracellular
matrix, they appear as long, slender brils.
Amnion Epithelial Cells
Te apical surace o the amnionic epithelium is replete with
highly developed microvilli. Tis structure reects its unction
as a major site o transer between amnionic uid and amnion
layers. Tis epithelium is metabolically active, and its cells synthesize tissue inhibitor o MMP-1, prostaglandin E2 (PGE2),
and etal bronectin (FN) (Rowe, 1997). Although epithelia
produce FN, studies suggest that FN acts in the underlying
mesenchymal cells. Here, FN promotes synthesis o MMPs that
break down strength-bearing collagens. It also enhances prostaglandin synthesis to prompt uterine contractions (Mogami,
2013). Tis pathway is upregulated with premature rupture o
membranes induced by thrombin or inection-induced release
o FN (Chigusa, 2016; Mogami, 2014).
Epithelial cells may respond to signals derived rom the etus
or the mother, and they are responsive to various endocrine or
paracrine modulators. Examples include oxytocin and vasopressin, both o which increase PGE2 production in vitro (Moore,
1988). Tese cells may also produce cytokines such as IL-8 during labor initiation (Elliott, 2001).
FIGURE 5-14 Photomicrograph of fetal membranes. From left to
right: AE = amnion epithelium; AM = amnion mesenchyme; S =
zona spongiosa; CM = chorionic mesenchyme; TR = trophoblast;
D = decidua. (Reproduced with permission from Dr. Judith R. Head.)Implantation and Placental Development 95
CHAPTER 5
Amnionic epithelium also synthesizes vasoactive peptides,
which unction in both maternal and etal tissues in diverse
physiological processes. Tese peptides include endothelin
and parathyroid hormone-related protein (Economos, 1992;
Germain, 1992). Others are brain natriuretic peptide (BNP)
and corticotropin-releasing hormone, which are peptides that
invoke smooth-muscle relaxation (Riley, 1991; Warren, 1995).
BNP production is positively regulated by mechanical stretch
in etal membranes and is proposed to unction in uterine quiescence. Epidermal growth actor, a negative regulator o BNP,
is upregulated in the membranes at term and leads to a decline
in BNP-regulated uterine quiescence (Carvajal, 2013).
Amnion Mesenchymal Cells
Tese cells are responsible or other major unctions. Mesenchymal cells synthesize the interstitial collagens that compose
the amnionic compact layer—the major source o its tensile
strength (Casey, 1996). At term, the generation o cortisol by
11β-hydroxysteroid dehydrogenase may contribute to membrane rupture by reducing collagen abundance (Mi, 2017).
Mesenchymal cells also synthesize cytokines that include IL-6,
IL-8, and MCP-1. Cytokine synthesis rises in response to bacterial toxins and IL-1. Tis ability o amnion mesenchymal
cells to synthesize chemokines is an important consideration in
interpreting studies o labor-associated accumulation o inammatory mediators in amnionic uid (Garcia-Velasco, 1999).
Last, mesenchymal cells may be a greater source o PGE2 than
epithelial cells, especially in the case o premature membrane
rupture (Mogami, 2013; Whittle, 2000).
■ Tensile Strength
During tests o tensile strength, the decidua and then the chorion laeve give way long beore the amnion ruptures. Indeed,
the membranes are elastic and can expand to twice normal
size during pregnancy (Benirschke, 2012). Te amnion tensile
strength resides almost exclusively in the compact layer, which
is composed o cross-linked interstitial collagens I and III, and
lesser amounts o collagens V and VI.
Amnion tensile strength is regulated in part by brillar collagen assembly. Tis process is inuenced by the interaction
between brils and proteoglycans such as decorin and biglycan (Chap. 21, p. 405). Reduction o these proteoglycans is
reported to perturb etal membrane unction (Horgan, 2014;
Wu, 2014). Fetal membranes overlying the cervix have a
regional shit in gene expression and lymphocyte activation that
set in motion an inammatory cascade (Marcellin, 2017). Tis
change may contribute to tissue remodeling and loss o tensile
strength in the amnion (Moore, 2009).
■ Metabolic Functions
Te amnion is metabolically active, is involved in solute and
water transport or amnionic uid homeostasis, and produces
an impressive array o bioactive compounds. Te amnion is
responsive both acutely and chronically to mechanical stretch,
which alters amnionic gene expression (Carvajal, 2013;
Nemeth, 2000). Tis in turn may trigger both autocrine and
paracrine responses that include production o MMPs, IL-8,
and collagenase (Bryant-Greenwood, 1998; Mogami, 2013).
Such actors may modulate changes in membrane properties
during labor.
UMBILICAL CORD
Te yolk sac and the umbilical vesicle into which it develops
are prominent early in pregnancy. Initially, the embryo is a
attened disc interposed between amnion and yolk sac (see
Fig. 5-7). Te embryonic dorsal surace, in association with
the elongation o its neural tube, grows aster than the ventral
surace. As a result, the embryo bulges into the amnionic sac,
and the embryo body incorporates the adjacent yolk sac to
orm the gut. Te body stalk connects the caudal embryo to
the chorion. Te etal allantois orms as a diverticulum rom the
caudal wall o the yolk sac and projects into the base o
the body stalk.
As pregnancy advances, the yolk sac becomes smaller and
its pedicle relatively longer. By the middle o the third month,
the expanding amnion uses with the chorion laeve to obliterate the extraembryonic coelom. In its expansion, the amnion
covers the bulging placental disc and the lateral surace o the
body stalk. Te latter is then called the umbilical cord or unis. A
more detailed description o this cord and potential abnormalities is ound in Chapter 6 (p. 113).
Te cord at term normally has two arteries and one vein.
Te right umbilical vein usually disappears early during etal
development, leaving only the original let vein. Te umbilical
cord extends rom the etal umbilicus to the etal surace o
the placenta, that is, the chorionic plate. As discussed in detail
in Chapter 7 (p. 126), blood ows rom the umbilical vein
toward the etus. Blood then takes a path o least resistance via
two routes within the etus. One is the ductus venosus, which
empties directly into the inerior vena cava. Te other route
consists o numerous smaller openings into the hepatic circulation. Blood rom the liver ows into the hepatic vein and
then the inerior vena cava. Resistance in the ductus venosus is
controlled by a sphincter situated at the origin o the ductus at
the umbilical recess and is innervated by a vagus nerve branch.
Blood exits the etus via the two umbilical arteries. Tese
are anterior branches o the internal iliac artery and become
obliterated ater birth to orm the medial umbilical ligaments.
PLACENTAL HORMONES
Te production o steroid and protein hormones by human
trophoblasts is greater in amount and diversity than that o
any single endocrine tissue in all o mammalian physiology.
Table 5-1 is a compendium o average production rates or various steroid hormones in nonpregnant and in near-term pregnant women. It demonstrates the remarkable increase in steroid
hormone production during pregnancy. Te human placenta
also synthesizes an enormous amount o protein and peptide
hormones, summarized in Table 5-2. Te successul physiological adaptations o pregnant women to this unique endocrine
milieu is discussed throughout Chapter 4.96 Placentation, Embryogenesis, and Fetal Development
Section 3
■ Human Chorionic Gonadotropin
Biosynthesis
Chorionic gonadotropin is a glycoprotein with biological activity similar to that o LH, and both act via the LH-hCG receptor.
hCG varies in molecular weight rom 36,000 to 40,000 Da and
is highly glycosylated with the most carbohydrate content o
any human hormone—30 percent. Tis glycosylation protects
the molecule rom catabolism and results in a 36-hour plasma
hal-lie or intact hCG compared with 2 hours or LH. Te
hCG molecule is composed o two dissimilar subunits, α and
β. Tese are noncovalently linked but held together by electrostatic and hydrophobic orces. Isolated subunits are unable to
bind the LH-hCG receptor and thus lack biological activity.
Te hCG hormone is structurally related to three other
glycoprotein hormones—LH, FSH, and thyroid-stimulating
hormone (SH). All our glycoproteins share a common
α-subunit. However, each o their β-subunits is characterized
by a distinct (although related) amino acid sequence.
Synthesis o the α- and β-chains o hCG is regulated separately rom gene loci on dierent chromosomes. A single gene
TABLE 5-1. Steroid Production Rates in Nonpregnant
and Near-Term Pregnant Women
Production Rates (mg/24 hr)
Steroida Nonpregnant Pregnant
Estradiol 0.1–0.6 15–20
Estriol 0.02–0.1 50–150
Progesterone 0.1–40 250–600
Aldosterone 0.05–0.1 0.250–0.600
Deoxycorticosterone 0.05–0.5 1–12
Cortisol 10–30 10–20
aEstrogens and progesterone are produced by placenta.
Aldosterone is produced by the maternal adrenal in
response to the stimulus of angiotensin II. Deoxycorticosterone
is produced in extraglandular tissue sites by way of the
21-hydroxylation of plasma progesterone. Cortisol production during pregnancy is not increased, even though the
blood levels are elevated because of decreased clearance
caused by increased cortisol-binding globulin.
TABLE 5-2. Protein Hormones Produced by the Human Placenta
Hormone
Primary Non-placental
Site of Expression
Shares Structural or
Function Similarity Functions
Human chorionic
gonadotropin (hCG)
— LH, FSH, TSH Maintains corpus luteum function
Regulates fetal testis testosterone
secretion
Stimulates maternal thyroid
Placental lactogen (PL) — GH, prolactin Aids maternal adaptation to fetal energy
requirements
Adrenocorticotropin (ACTH) Hypothalamus —
Corticotropin-releasing
hormone (CRH)
Hypothalamus — Relaxes smooth-muscle; initiates
parturition?
Promotes fetal and maternal glucocorticoid
production
Gonadotropin-releasing
hormone (GnRH)
Hypothalamus — Regulates trophoblast hCG production
Thyrotropin (TRH) Hypothalamus Unknown
Growth hormone-releasing
hormone (GHRH)
Hypothalamus — Unknown
Growth hormone variant
(hGH-V)
— GH variant not found
in pituitary
Potentially mediates pregnancy insulin
resistance
Neuropeptide Y Brain Potential regulates CRH release by
trophoblasts
Parathyroid-releasing
protein (PTH-rp)
— Regulates transfer of calcium and
other solutes; regulates fetal mineral
homeostasis
Inhibin Ovary/testis Potentially inhibits FSH-mediated ovulation;
regulates hCG synthesis
Activin Ovary/testis Regulates placental GnRH synthesis
GH = growth hormone; FSH = follicle-stimulating hormone; LH = luteinizing hormone; TSH = thyroid-stimulating
hormone.Implantation and Placental Development 97
CHAPTER 5
on chromosome 6 encodes the α-subunit. Chromosome 19
encodes the β-hCG–β-LH amily o subunits with six genes or
β-hCG and one or β-LH (Miller-Lindholm, 1997). Both subunits are synthesized as larger precursors and then cleaved by
endopeptidases to their mature orm. Intact hCG is assembled
and released by secretory granule exocytosis (Morrish, 1987).
Modications during synthesis and subsequent enzymatic
degradation give rise to multiple orms o hCG in maternal
plasma and urine that vary enormously in bioactivity and
immunoreactivity.
Beore 5 weeks, hCG is expressed both in the syncytiotrophoblast and cytotrophoblasts (Maruo, 1992). Later in the rst
trimester, hCG is produced almost solely in the syncytiotrophoblast, peaks around 9 weeks’ gestation, and then declines
to a plateau or the remainder o gestation (Beck, 1986;
Kurman, 1984). Dynamic changes in hCG concentration in
the rst trimester highlight the importance o accurate gestational age estimation when interpreting aneuploidy screening
strategies that include hCG.
Circulating levels o ree β-subunit are low to undetectable
throughout pregnancy, and their concentration is the limiting
actor or secretion o complete hCG molecules. Levels o the
α-subunit rise gradually and roughly correspond to placental
mass, until they plateau at approximately 36 weeks’ gestation
(Cole, 1997).
Concentrations in Serum and Urine
Te combined hCG molecule is detectable in plasma o pregnant women 7 to 9 days ater the midcycle LH surge preceding ovulation. Tus, hCG likely enters maternal blood at the
time o blastocyst implantation. Plasma levels rise rapidly,
doubling approximately every 2 days in the rst trimester
(Fig. 5-15). Appreciable uctuations in levels or a given patient
are observed on the same day.
Intact hCG circulates as multiple, highly related isoorms
that have variable cross-reactivity between commercial assays.
Tis emphasizes the need to use the same assay type when measuring serial hCG levels or clinical indications such as evaluating pregnancy o unknown location or medical management o
ectopic pregnancy. Peak maternal plasma levels reach approximately 50,000 to 100,000 mIU/mL between the 60th and
80th days ater menses. Plasma levels then decline, and a nadir
is reached by approximately 16 weeks’ gestation. Plasma levels
remain at this lower level or the rest o pregnancy.
Although maternal urine, like plasma, contains various
hCG degradation products, the principal urinary orm is the
terminal product o hCG degradation, namely, the β-core ragment. Concentrations o this ragment ollow the same general
pattern as that in maternal plasma, peaking at approximately
10 weeks’ gestation. Importantly, the β-subunit antibody used
in most pregnancy tests reacts with both intact hCG (the major
orm in the plasma) and with ragments o hCG (the major
orm ound in urine).
hCG Regulation
Placental GnRH is likely involved in the regulation o hCG
ormation. Both GnRH and its receptor are expressed by
cytotrophoblasts and syncytiotrophoblast (Wolahrt, 1998).
GnRH administration elevates circulating hCG levels, and cultured trophoblasts respond to GnRH treatment with increased
hCG secretion (Iwashita, 1993; Siler-Khodr, 1981). Pituitary
GnRH production is regulated by inhibin and activin. Likewise, in cultured placental cells, activin stimulates and inhibin
inhibits GnRH and hCG production (Petraglia, 1989; Steele,
1993).
hCG is cleared by the kidney (about 30 percent), and the
remainder is likely cleared by liver metabolism (Wehmann,
1980). Tus, levels can be markedly altered in gravidas with
chronic renal disease.
Biological Functions
Noted earlier, hCG maintains corpus luteum unction—that
is, continued progesterone production. Both hCG subunits are
required or binding to the LH-hCG receptor in the corpus
luteum. However, maximum plasma hCG concentrations are
attained well ater hCG-stimulated corpus luteum secretion o
progesterone has ceased. Specically, luteal progesterone synthesis begins to decline at approximately 6 weeks’ gestation
despite continued and increasing hCG production. Tereore,
this incompletely explains the physiological unction o hCG
in pregnancy. LH-hCG receptors are present in various other
tissues, and roles are discussed subsequently.
In pregnancies with male etuses, hCG stimulates etal testicular testosterone secretion. Tis reaches a maximum when
hCG levels peak. Tus, at a critical time in male sexual dierentiation, hCG enters etal plasma rom the syncytiotrophoblast.
In the etus, it acts as an LH surrogate to stimulate Leydig
cell replication and testosterone synthesis to promote male
sexual dierentiation (Chap. 3, p. 35). Beore approximately
110 days, the etal anterior pituitary lacks vascularization rom
the hypothalamus and produces minimal LH secretion. Tereater, as hCG levels all, pituitary LH maintains modest testicular stimulation.
Te maternal thyroid gland also is stimulated by large
quantities o hCG. In women with gestational trophoblastic
disease, biochemical and clinical evidence o hyperthyroidism sometimes develops (Chap. 13, p. 238). Some orms o
140 500
400
300
200
100
120
100
80
60
40
hCG
hPL
CRH
20
0
0 10 20
Weeks’ gestation
hCG (IU/mL)
hPL(µg/mL)
CRH(pmol/mL)
30 40
8 7 6 5 4 3 2 1 0
FIGURE 5-15 Distinct profiles for the concentrations of human
chorionic gonadotropin (hCG), human placental lactogen (hPL),
and corticotropin-releasing hormone (CRH) in serum of women
throughout normal pregnancy.98 Placentation, Embryogenesis, and Fetal Development
Section 3
hCG bind to SH receptors on thyrocytes (Hershman, 1999).
Te thyroid-stimulatory activity in plasma o rst-trimester
pregnant women varies appreciably rom sample to sample.
Modications o hCG oligosaccharides likely are important in
the capacity o hCG to stimulate thyroid unction. Acidic iso-
orms stimulate thyroid activity, and some more basic isoorms
stimulate iodine uptake (Kraiem, 1994; suruta, 1995). Last,
the LH-hCG receptor is also expressed by thyrocytes, which
suggests that hCG stimulates thyroid activity via the LH-hCG
receptor as well (omer, 1992).
LH-hCG receptors are ound in myometrium and in uterine vascular tissue. It has been hypothesized that hCG may
promote uterine vascular vasodilation and myometrial smooth
muscle relaxation (Kurtzman, 2001). hCG also modulates
maternal immune cell unctions in the decidua during early
stages o placentation (Schumacher, 2019; Silasi, 2020).
Abnormally High or Low Levels
Several clinical circumstances display substantively higher
maternal plasma hCG levels. Some examples include multietal
pregnancy, erythroblastosis etalis associated with etal hemolytic anemia, and gestational trophoblastic disease. Relatively
higher hCG levels may be ound in women carrying a etus with
Down syndrome, and this has been incorporated into screening
strategies (able 17-4, p. 336). Various malignant tumors also
produce hCG, sometimes in large amounts— especially trophoblastic neoplasms (Chap. 13, p. 241).
Relatively lower hCG plasma levels are ound in women
with ailing early pregnancies and ectopic pregnancy (Chap.
12, p. 222). hCG is produced in very small amounts in normal
tissues o men and nonpregnant women, perhaps primarily in
the anterior pituitary gland. Nonetheless, the detection o hCG
in blood or urine almost always indicates pregnancy (Chap. 10,
p. 176).
■ Human Placental Lactogen
Biosynthesis
Tis single, nonglycosylated polypeptide chain shares a 96-
percent amino-acid-sequence homology with human growth
hormone (hGH) and a 67-percent homology with human prolactin (hPRL). Because o these similarities, it was called chorionic growth hormone or human placental lactogen. Currently,
the latter term is used by most.
Five genes in the growth hormone–placental lactogen gene
cluster are linked and located on chromosome 17: GH1, GH2,
CSHL1, CSH1, and CSH2. Human placental lactogen (hPL) is
encoded by the last two genes. hPL is concentrated in syncytiotrophoblast, but similar to hCG, hPL is demonstrated in cytotrophoblasts beore 6 weeks (Grumbach, 1964; Maruo, 1992).
Within 5 to 10 days ater conception, hPL is demonstrable in
the placenta and can be detected in maternal serum as early as
3 weeks. Levels o mRNA or hPL in syncytiotrophoblast
remain relatively constant throughout pregnancy. Tis nding
supports the idea that the hPL secretion rate is proportional
to placental mass. Levels rise steadily until 34 to 36 weeks’
gestation. Te hPL production rate near term—approximately
1 g/d—is by ar the greatest o any known hormone in humans.
It is rapidly cleared and has a hal-lie between 10 and 30 minutes (Walker, 1991). In late pregnancy, maternal serum concentrations reach levels o 5 to 15 μg/mL (see Fig. 5-15).
Very little hPL is detected in etal blood, and amnionic uid
levels are somewhat lower than that in maternal plasma. Tus,
although hPL may have a direct eect on etal tissues, such as
modulating etal vasculature ormation, its primary role is to
act on maternal physiology to ensure adequate nutrient delivery
to the placenta (Corbacho, 2002).
Metabolic Actions
hPL has putative actions in several important maternal metabolic processes. First, hPL promotes lipolysis to raise circulating
ree atty acid levels. Tis provides an energy source or maternal metabolism and etal nutrition. In vitro studies suggest that
hPL inhibits secretion by term syncytiotrophoblast o leptin
(Coya, 2005). Prolonged maternal starvation in the rst hal o
pregnancy leads to higher hPL plasma concentrations.
Second, hPL aids maternal adaptation to etal energy requirements (Hill, 2018). For example, increased maternal insulin
resistance ensures nutrient ow to the etus. It also avors protein synthesis and provides a readily available amino acid source
to the etus. o counterbalance the greater insulin resistance
and prevent maternal hyperglycemia, maternal insulin levels
rise. Both hPL and prolactin signal through the prolactin receptor to increase maternal beta cell prolieration, which augments
insulin secretion (Georgia, 2010). In animals, prolactin and
hPL upregulate serotonin synthesis, which increases beta cell
prolieration (Kim, 2010). Short-term changes in plasma glucose or insulin, however, have relatively little eect on plasma
hPL levels. In vitro studies o syncytiotrophoblast suggest that
hPL synthesis is stimulated by insulin and insulin-like growth
actor-1 and inhibited by PGE2 and PGF2α (Bhaumick, 1987;
Genbacev, 1977).
■ Other Placental Protein Hormones
Te placenta has a remarkable capacity to synthesize numerous peptide hormones, including some that are analogous or
related to hypothalamic and pituitary hormones. In contrast
to their counterparts, some o these placental peptide/protein
hormones are not subject to eedback inhibition.
Hypothalamic-Like Releasing Hormones
Te known hypothalamic-releasing or -inhibiting hormones
include GnRH, corticotropin-releasing hormone (CRH),
thyrotropin-releasing hormone (RH), growth hormone–
releasing hormone (GHRH), and somatostatin. For each o
these, the human placenta produces an analogous hormone
(Petraglia, 1992; Siler-Khodr, 1988).
GnRH in the placenta shows its highest expression in the rst
trimester (Siler-Khodr, 1978, 1988). Interestingly, it is ound
in cytotrophoblasts but not syncytiotrophoblast. Placentaderived GnRH unctions to regulate trophoblast hCG production and extravillous trophoblast invasion via regulation o
MMP-2 and MMP-9 (Peng, 2016). Placenta-derived GnRH
is also the likely source o elevated maternal GnRH levels in
pregnancy (Siler-Khodr, 1984).Implantation and Placental Development 99
CHAPTER 5
CRH is a member o a larger amily o CRH-related peptides that includes CRH and urocortins (Dautzenberg, 2002).
Maternal serum CRH levels rise rom 5 to 10 pmol/L in the
nonpregnant woman to approximately 100 pmol/L in the early
third trimester and then to almost 500 pmol/L abruptly during the last 5 to 6 weeks (see Fig. 5-15). Ater labor begins,
maternal plasma CRH levels rise even urther (Petraglia, 1989,
1990). Urocortin, involved in the stress response, is also produced by the placenta and secreted into the maternal circulation, but at much lower levels than that seen or CRH (Florio,
2002).
Te unction o CRH synthesized in the placenta, membranes, and decidua has been somewhat dened. For example,
trophoblast, amniochorion, and decidua express both CRH-R1
and CRH-R2 receptors and several variant receptors (Florio,
2000). Both CRH and urocortin enhance trophoblast secretion
o adrenocorticotropic hormone (ACH), and this suggests an
autocrine–paracrine role (Petraglia, 1999). Large amounts o
trophoblast CRH enter maternal blood.
CRH receptors are also present in many tissues outside
the placenta. Proposed biological roles include induction o
smooth-muscle relaxation in vascular and myometrial tissue
and immunosuppression. Te physiological reverse, however,
induction o myometrial contractions, has been proposed or
the rising CRH levels seen near term. Some hypothesize that
CRH may be involved with parturition initiation (Wadhwa,
1998).
Glucocorticoids act in the hypothalamus to inhibit CRH
release. But, in the trophoblast, glucocorticoids stimulate CRH
gene expression (Jones, 1989a; Robinson, 1988). Tus, a novel
positive eedback loop in the placenta may allow placental
CRH to stimulate placental ACH and thereby prompt etal
and maternal adrenal glucocorticoid production, with subsequent stimulation o placental CRH expression (Nicholson,
2001; Riley, 1991).
GHRH is expressed in placenta, but its unction is unclear
(Berry, 1992). GHRH may play an autocrine role in trophoblast survival via the GHRH receptor (Liu, 2016). Ghrelin is
another regulator o hGH secretion and is produced by placental tissue (Horvath, 2001). rophoblast ghrelin expression
peaks at midpregnancy and is a paracrine regulator o dierentiation or is a potential regulator o human growth hormone
variant production, described next (Fuglsang, 2005; Gualillo,
2001).
Pituitary-Like Hormones
A human growth hormone variant (hGH-V) that is not expressed
in the pituitary is expressed in the placenta. Te gene encoding
hGH-V is located in the hGH–hPL gene cluster on chromosome 17. Sometimes reerred to as placental growth hormone,
hGH-V is a 191-amino-acid protein that diers in 15 amino
acid positions rom the sequence or hGH. Although hGH-V
retains growth-promoting and antilipogenic unctions that are
similar to those o hGH, it has reduced diabetogenic and lactogenic unctions relative to hGH (Vickers, 2009). Placental
hGH-V presumably is synthesized in the syncytiotrophoblast.
It is believed that hGH-V is present in maternal plasma by 21 to
26 weeks’ gestation, rises in concentration until approximately
36 weeks, and remains relatively constant thereater. hGH-V
levels in maternal plasma and those o insulin-like growth actor 1 positively correlate. Moreover, hGH-V secretion by trophoblast in vitro is inhibited by glucose in a dose-dependent
manner (Patel, 1995). Overexpression o hGH-V in mice
causes severe insulin resistance, making it a likely candidate to
mediate insulin resistance o pregnancy (Liao, 2016).
Pro-opiomelanocortin (POMC) is a polypeptide produced
in the pituitary and other tissues including the placenta. It is
proteolytically cleaved into numerous active hormones including ACH, β-lipotropic hormone, melanocyte-stimulating
hormone (α-, β-, and γ-MSH), and β–endorphin (Harno,
2018). Tese hormones play a role in maintaining energy balance. As discussed, placental CRH stimulates synthesis and
release o placental ACH, demonstrating the autocrine and
paracrine unctions o the placenta in addition to its systemic
endocrine activity.
Relaxin
Te peptide is expressed in human corpus luteum, decidua,
and placenta (Bogic, 1995). wo o the three relaxin genes—
H2 and H3—are transcribed in the corpus luteum (Bathgate,
2002; Hudson, 1983, 1984). Decidua, placenta, and membranes express H1 and H2 (Hansell, 1991). Relaxin is synthesized as a single, 105-amino-acid preprorelaxin molecule that is
cleaved to A and B molecules. Relaxin is structurally similar to
insulin and insulin-like growth actor.
Te rise in maternal circulating relaxin levels in early pregnancy is attributed to corpus luteum secretion, and levels parallel those o hCG. Relaxin, along with rising progesterone
levels, may act on myometrium to promote relaxation and the
quiescence o early pregnancy (Chap. 21, p. 404). In addition,
the production o relaxin and relaxin-like actors within the
placenta and etal membranes may play an autocrine-paracrine
role in postpartum regulation o extracellular matrix remodeling (Qin, 1997a,b). One important relaxin unction is enhancement o the maternal glomerular ltration rate that is apparent
early in gestation (Chap. 4, p. 68).
Parathyroid Hormone-Related Protein
Levels o this peptide are elevated within maternal but not
etal circulation (Bertelloni, 1994; Saxe, 1997). Parathyroid
hormone-related protein (PH-rP) synthesis is ound in several normal adult tissues, especially in reproductive organs
that include myometrium, endometrium, corpus luteum, and
lactating mammary tissue. PH-rP is not produced in the
parathyroid glands o normal adults. Although yet undened,
placenta-derived PH-rP may regulate genes involved in trans-
er o calcium and other solutes. It also contributes to mineral
homeostasis in etal bone, amnionic uid, and the etal circulation (Simmonds, 2010).
Leptin
Tis hormone is normally secreted by adipocytes, but cytotrophoblasts and syncytiotrophoblast also synthesize leptin
(Henson, 2002). Relative contributions o leptin rom maternal adipose tissue versus placenta are currently undened.
Leptin unctions as an antiobesity hormone that decreases ood100 Placentation, Embryogenesis, and Fetal Development
Section 3
intake through its hypothalamic receptor. It also regulates bone
growth and immune unction (Cock, 2003; La Cava, 2004).
Placental leptin promotes placental cell prolieration, protein
synthesis, and activation o immune tolerance and antiapoptotic responses (Rosario, 2016a; Schanton, 2018). Maternal
serum levels are signicantly higher than those in nonpregnant
women. Fetal leptin levels correlate positively with birthweight
and likely unction in etal development and growth. Studies suggest that reductions in leptin availability contribute to
adverse etal metabolic programing in intrauterine growthrestricted ospring (Nusken, 2016, Perez-Perez, 2018).
Neuropeptide Y
Tis 36-amino-acid peptide is widely distributed in brain. It
also is ound in sympathetic neurons innervating the cardiovascular, respiratory, gastrointestinal, and genitourinary systems.
Neuropeptide Y has been isolated rom the placenta and localized in cytotrophoblasts (Petraglia, 1989). rophoblasts possess
neuropeptide Y receptors, and treatment o these with neuropeptide Y causes CRH release (Robidoux, 2000).
Transforming Growth Factor Beta Superfamily
Tis amily o cytokines regulates various cellular unctions that
include placental development, trophoblast dierentiation, and
invasion into the decidua (Adu-Gyama, 2020). Tis process
is nely tuned, and extreme invasion results in placenta accreta
spectrum disorders, whereas shallow implantation can lead to
early pregnancy loss or preeclampsia. Members o the amily
include transorming growth actor beta (GF-β), activin,
inhibin, nodal, bone morphogenic proteins (BMPs), antimüllerian hormone, and growth dierentiation actors (GDFs).
Tese cytokines bind to a suite o receptors composed o
a type 1 subunit and a type 2 receptor subunit to ultimately
direct gene expression. Tis combinatorial diversity leads to
divergent unctions in the placenta and decidua. Activin A
promotes syncytialization, extravillous trophoblast ormation,
and invasion. BMP2 enhances invasion, and BMP4 may direct
embryonic stem cells toward a trophoblast lineage (Zhang
2018, Xu 2002). Inhibin A promotes syncytialization but
inhibits invasion (Debiève, 2000; Jones, 2006). Nodal inhibits
prolieration, extravillous trophoblast ormation, and invasion.
Te complete role this superamily in normal and abnormal
placental development is yet to be ully dened.
■ Placental Progesterone Production
Ater 6 to 7 weeks’ gestation, little progesterone is produced in
the ovary (Diczalusy, 1961). Surgical removal o the corpus
luteum or even bilateral oophorectomy during the 7th to 10th
week does not decrease excretion rates o urinary pregnanediol,
the principal urinary metabolite o progesterone. Beore this
time, however, corpus luteum removal will lead to spontaneous
abortion unless an exogenous progestin is given, and Chapter
66 (p. 1170) lists suitable dosing. Ater approximately 8 weeks,
the placenta assumes progesterone secretion, and maternal
serum levels throughout pregnancy gradually rise (Fig. 5-16).
By term, these levels are 10 to 5000 times o those in nonpregnant women, depending on the ovarian cycle stage.
Te daily production rate o progesterone in late, normal,
singleton pregnancies approximates 250 mg. In multietal pregnancies, the daily production rate may exceed 600 mg. Progesterone is synthesized rom cholesterol in a two-step enzymatic
reaction. First, cholesterol is converted to pregnenolone within
the mitochondria in a reaction catalyzed by cytochrome P450
cholesterol side-chain cleavage enzyme. Pregnenolone leaves
the mitochondria and is converted to progesterone in the endoplasmic reticulum by 3β-hydroxysteroid dehydrogenase. Progesterone is released immediately by diusion.
Although the placenta produces a prodigious amount o
progesterone, the syncytiotrophoblast has a limited capacity or
cholesterol biosynthesis. Te rate-limiting enzyme in its biosynthesis is 3-hydroxy-3-methylglutaryl coenzyme A (HMGCoA) reductase. Because o this, the placenta must rely on an
exogenous source, that is, maternal cholesterol, or progesterone ormation. Te trophoblast preerentially uses LDL cholesterol or progesterone biosynthesis (Simpson, 1979, 1980).
Tis mechanism diers rom placental production o estrogens,
which relies principally on etal adrenal precursors.
Although etal well-being and placental estrogen production
show a relationship, this is not the case or placental progesterone. Tus, placental endocrine unction, including progesterone biosynthesis and ormation o protein hormones such as
hCG, may persist or weeks ater etal demise.
Estetrol
Gestational age (weeks)
Plasma unconjugated steroid (ng/mL)
4
0.05
0.1
0.5
1.0
5.0
10.0
50.0
100.0
8 12 16 20 24 28 32 36 40
Estrone
Estriol
Estradiol
Estradiol
(nonpregnant)
Progesterone
(nonpregnant)
Progesterone
FIGURE 5-16 Plasma levels of progesterone, estradiol, estrone,
estetrol, and estriol in women during the course of gestation.
(Modified and redrawn with permission from Mesiano S: The endocrinology of human pregnancy and fetoplacental neuroendocrine
development. In Strauss JF, Barbieri RL (eds) Yen and Jaffe’s Reproductive Endocrinology: Physiology, Pathophysiology, and Clinical
Management, 6th ed. Philadephia, PA: Saunders; 2009.)Implantation and Placental Development 101
CHAPTER 5
Te metabolic clearance rate o progesterone in pregnant
women is similar to that ound in men and nonpregnant women.
One metabolite is 5α-dihydroprogesterone (5α-DHP), and levels disproportionately rise due to synthesis in syncytiotrophoblast
rom both placenta-produced progesterone and etus-derived
precursor (Dombroski, 1997). Tus, the concentration ratio o
5α-DHP to progesterone is elevated in pregnancy, although the
mechanisms or this are incompletely dened. Progesterone also
is converted to the potent mineralocorticoid deoxycorticosterone in pregnant women and in the etus. Te concentration
o deoxycorticosterone is strikingly higher in both maternal and
etal compartments (see able 5-1). Te extraadrenal ormation
o deoxycorticosterone rom circulating progesterone accounts
or most o its production in pregnancy (Casey, 1982a,b).
■ Placental Estrogen Production
During the rst 2 to 4 weeks or pregnancy, rising hCG levels
maintain production o estradiol in the corpus luteum. Ovarian production o both progesterone and estrogens drops signicantly by the 7th week o pregnancy. At this time, there is
a luteal–placental transition. Subsequently, more than hal o
estrogen entering maternal circulation is produced in the placenta, and it produces a continually increasing magnitude o
estrogen (MacDonald, 1965a; Siiteri, 1963, 1966). Near term,
normal human pregnancy is a hyperestrogenic state, and syncytiotrophoblast is producing estrogen in amounts equivalent
to that produced in one day by the ovaries o no ewer than
1000 ovulatory women. Tis hyperestrogenic state terminates
abruptly ater delivery o the placenta.
Biosynthesis
In human trophoblast, neither
cholesterol nor in turn progesterone can serve as precursor or
estrogen biosynthesis. Tis is because
steroid 17α-hydroxylase/17,20-lyase
(CYP17A1) is not expressed in the
human placenta. Tis essential
enzyme converts 17-OH progesterone (a C21 steroid) to androstenedione, which is a C19 steroid and an
estrogen precursor. Consequently,
conversion o C21 steroids to C19
steroids is not possible.
However, dehydroepiandrosterone (DHEA) and its sulate
(DHEA-S) are also C19 steroids
and are produced by maternal
and etal adrenal glands. Tese
two steroids can serve as estrogen precursors in the placenta
(Fig. 5-17). Ryan (1959a) ound
that the placenta had an exceptionally high capacity to convert
appropriate C19 steroids to estrone
and estradiol. Te conversion o
DHEA-S to estradiol requires
placental expression o our key enzymes that are located
principally in syncytiotrophoblast (Bonenant, 2000;
Salido, 1990). First, the placenta expresses high levels o
steroid sulatase (SS), which converts the conjugated DHEA-S
to DHEA. DHEA is then acted upon by 3β-hydroxysteroid
dehydrogenase type 1 (3βHSD) to produce androstenedione.
Cytochrome P450 aromatase (CYP19) then converts androstenedione to estrone, which is then converted to estradiol
by 17β-hydroxysteroid dehydrogenase type 1 (17βHSD1).
DHEA-S is the major precursor o estrogens in pregnancy
(Baulieu, 1963; Siiteri, 1963). However, maternal adrenal
glands do not produce sufcient amounts o DHEA-S to
account or more than a raction o total placental estrogen biosynthesis. Te etal adrenal glands are quantitatively the most
important source o placental estrogen precursors in human
pregnancy. Tus, estrogen production during pregnancy reects
the unique interactions among etal adrenal glands, etal liver,
placenta, and maternal adrenal glands.
Directional Secretion
O estradiol and estriol ormed in syncytiotrophoblast, >90
percent enters maternal plasma (Gurpide, 1966). O placental
progesterone production, ≥85 percent enters maternal plasma,
and little maternal progesterone crosses the placenta to the etus
(Gurpide, 1972).
Tis directional secretion stems rom the architecture o
hemochorioendothelial placentation. Steroids produced in the
syncytiotrophoblast are secreted directly into maternal blood.
o reach the etus, they must rst traverse the cytotrophoblast
layer and then enter the stroma o the villous core and then etal
capillaries. From either o these spaces, steroids can reenter the
FIGURE 5-17 Schematic presentation of estrogen biosynthesis in the human placenta. Dehydroepiandrosterone sulfate (DHEA-S) is secreted in prodigious amounts by the fetal adrenal glands,
and a portion is converted to 16α-hydroxydehydroepiandrosterone sulfate (16OH-DHEA-S) in the
fetal liver. DHEA-S and 16OH-DHEA-S are converted in the placenta to the estrogens 17β-estradiol
(E2) and estriol (E3). These estrogens then enter the maternal circulation. Near term, half of E2 is
derived from fetal adrenal DHEA-S and half from maternal DHEA-S. On the other hand, 90 percent
of E3 in the placenta arises from fetal 16OH-DHEA-S and only 10 percent from all other sources.
3βHSD1 = 3β-hydroxysteroid dehydrogenase type 1; 17βHSD1 = 17β-hydroxysteroid dehydrogenase type 1; CYP17 = steroid 17α-hydroxylase/17,20-lyase; LDL = low-density lipoprotein;
SCC = cholesterol side-chain cleavage enzyme; StAR = steroidogenic acute regulatory protein.102 Placentation, Embryogenesis, and Fetal Development
Section 3
syncytium. Te net result o this hemochorial arrangement is
that entry o steroids into the maternal circulation is substantially greater than that into etal blood.
FETAL ADRENAL GLAND–PLACENTAL
INTERACTIONS
As noted, the etal adrenal gland is a vital source o steroid
precursors or placental estrogen synthesis. Tis etal gland is
remarkable both morphologically and unctionally. At term, its
mass exceeds that in adults (Chap. 7, p. 134). More than 85
percent o the etal gland is composed o a unique etal zone,
which has a great capacity or steroid biosynthesis. Its daily steroid production near term is 100 to 200 mg/d, which exceeds
the adult steroid secretion o 30 to 40 mg/d. Te unique etal
zone subsequently regresses in the rst year o lie.
In addition to responding to ACH rom the etal brain,
etal adrenal gland growth is inuenced by actors secreted by
the placenta. Tis is exemplied by the continued adrenal gland
growth throughout gestation and by its rapid involution immediately ater birth and placenta delivery.
■ Placental Estriol Synthesis
Estradiol is the primary placental estrogen product at term.
In addition, signicant levels o estriol and estetrol are ound
in the maternal circulation, particularly late in gestation (see
Fig. 5-16). Tese hydroxylated orms o estrogen derive rom
the placenta using substrates ormed by the combined eorts o
the etal adrenal gland and etal liver enzymes (see Fig. 5-17).
For this, high levels o hepatic 16α-hydroxylase act on adrenal-derived steroids (MacDonald, 1965b; Ryan, 1959b). Tus,
the disproportionate rise in estriol ormation during pregnancy
is accounted or by placental synthesis rom plasma-borne
16-OH-DHEA-S. Near term, the etus produces 90 percent o
placental estriol and estetrol precursors in normal human pregnancy. Tus, in the past, levels o these steroids were used as an
indicator o etal well-being.
■ Fetal Adrenal Steroid Precursor
Cholesterol is the precursor or etal adrenal steroidogenesis.
Here, the steroid biosynthesis rate is so great that its steroidogenesis alone is equivalent to a ourth o the total daily LDL
cholesterol turnover in adults. Fetal adrenal glands synthesize
cholesterol rom acetate, and all enzymes involved in this biosynthesis are elevated compared with those o the adult adrenal
gland (Rainey, 2001). Tus, the de novo cholesterol synthesis
rate by etal adrenal tissue is extremely high. Even so, it is insu-
cient to account or the steroids produced by etal adrenal
glands. Tereore, cholesterol must be assimilated rom the etal
circulation and mainly rom LDL produced in the etal liver
(Carr, 1980, 1984; Simpson, 1979).
■ Fetal Conditions Affecting Estrogen
Production
Several etal disorders alter the availability o substrate or placental steroid synthesis and thus highlight the interdependence
o etal development and placental unction. Fetal demise is
ollowed by a striking reduction in maternal urinary estrogen
levels. Similarly, ater ligation o the umbilical cord with the
etus and placenta let in situ, placental estrogen production
declines markedly (Cassmer, 1959). However, as previously
discussed, placental progesterone production is maintained.
Tese observations indicate that an important source o precursors or placental estrogen—but not or progesterone—biosynthesis derive rom the etus.
Anencephalic etuses have markedly atrophic adrenal glands
due to absent hypothalamic and pituitary structures that
would otherwise release ACH or adrenal stimulation. In the
absence o the adrenal cortical etal zone development, placental ormation o estrogen is severely limited because o diminished availability o C19 steroid precursors. Indeed, urinary
estrogen levels in women pregnant with an anencephalic etus
approximate only 10 percent o those ound in normal pregnancy (Frandsen, 1961). With an anencephalic etus, almost
all estrogens produced arise rom placental use o maternal
plasma DHEA-S.
Fetal adrenal cortical hypoplasia occurs in perhaps 1 in
12,500 births (McCabe, 2001). Estrogen production in these
pregnancies is also limited, which suggests the absence o C19
precursors.
Fetal–placental sulatase defciency is associated with very
low estrogen levels in otherwise normal pregnancies (France,
1969). Namely, sulatase deciency precludes the hydrolysis o
C19 steroid sulates, the rst enzymatic step in the placental
use o these circulating prehormones or estrogen biosynthesis.
Tis deciency is an X-linked disorder, and thus all aected
etuses are male. Its estimated requency is 1 case in 2000 to
5000 births and is associated with delayed labor onset. It also
is associated with development o ichthyosis in aected males
later in lie (Bradshaw, 1986).
Fetal–placental aromatase defciency is a rare autosomal recessive disorder in which individuals cannot synthesize endogenous
estrogens (Grumbach, 2011; Simpson, 2000). Fetal adrenal
DHEA-S is converted in the placenta to androstenedione, but
in cases o placental aromatase deciency, androstenedione cannot be converted to estradiol. Rather, androgen metabolites o
DHEA produced in the placenta, including androstenedione
and some testosterone, are secreted into the maternal or etal
circulation, or both. Tis can virilize the mother and the emale
etus (Belgorosky, 2009; Harada, 1992).
Trisomy 21—Down syndrome—screening searches or
abnormal levels o hCG, alpha-etoprotein, and other analytes
(Chap. 17, p. 338). O these, serum unconjugated estriol levels can be low in women with Down syndrome etuses (Benn,
2002). Tis likely stems rom inadequate ormation o C19 steroids in the adrenal glands o these trisomic etuses.
Fetal erythroblastosis in some cases o severe etal D-antigen
alloimmunization can lead to elevated maternal plasma estrogen levels. A suspected cause is the greater placental mass rom
hypertrophy, which can be seen with such etal hemolytic anemia (Chap. 18, p. 360).
Complete hydatidiorm mole and gestational trophoblastic neoplasias lack a etus and also a etal adrenal source o C19 steroid precursors or trophoblast estrogen biosynthesis. Placental
estrogen ormation is consequently limited to the use o C19Implantation and Placental Development 103
CHAPTER 5
steroids rom the maternal plasma, and thus estradiol is principally produced (MacDonald, 1964, 1966).
■ Maternal Conditions Affecting
Estrogen Production
Maternal conditions can likewise aect placental estrogen production. Glucocorticoid treatment can inhibit ACH secretion
rom the maternal and etal pituitary glands. Tis diminishes
maternal and etal adrenal secretion o the placental estrogen
precursor DHEA-S and leads to a striking reduction in placental estrogen ormation.
With Addison disease, pregnant women show lower estrogen
levels, principally estrone and estradiol levels (Baulieu, 1956).
However, the etal adrenal gland contribution to estriol synthesis, particularly in later pregnancy, is quantitatively much more
important than that o the maternal adrenal gland.
Maternal androgen-producing tumors can present the placenta with elevated androgen levels. Fortunately, placenta is
extraordinarily efcient in the aromatization o C19 steroids.
For example, virtually all androstenedione entering the intervillous space is taken up by syncytiotrophoblast and converted
to estradiol (Edman, 1981). None o the C19 steroid enters
the etus, and a emale etus is rarely virilized by a maternal
androgen-secreting tumor. Indeed, virilized emale etuses o
women with an androgen-producing tumor may be cases in
which a nonaromatizable C19 steroid androgen is produced
by the tumor—or example, 5α-dihydrotestosterone. Alternatively, tumor-derived testosterone could be produced very early
in pregnancy in amounts that exceed the placental aromatase
capacity at that time
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