PLACENTA, FETAL MEMBRANES & TWINNING
Langman’s Medical Embryology, 9th ed ,
Chapter 6 - pp 125-148
Objectives:
1. The components of fetal membrane and their functions
2. Structure and function of placenta
3. Composition of placenta barrier
A. Introduction
As the embryo grows and develops, its nutrient needs can no longer be supplied by
simple diffusion. The placenta (Latin - flat cake) allows the fetus to "plug in" to the
mother, accessing nutrients and disposing of waste products. The placenta is unique in
its function from several points of view. Its lifetime is short (relative to the fetus) and its
function changes during the course of gestation. For the fetus, the placenta serves as
lung, GI tract, kidneys, liver and even endocrine organs.
The "fetal membranes" are a collective group which include the following: 1)
Chorion --gives rise to the placenta, 2) Amnion--surrounds the fetus, 3) Yolk Sac--a
portion of which is incorporated into the GI tubes (the remainder of the yolk sac is
rudementary in humans), and 4) Allantois -- caudal outlet of bladder remains
rudementary in humans. 5) Umbilical cord,
B. Chorion Development
1. Chorionic Components
- Early in development, the villi cover the entire surface of the chorion.
- During the second month, the villi at the embryonic pole expand and proliferate,
forming a bushy array of villi called the chorion frondosum.
- In contrast, the villi over the aembryonic pole degenerate, forming a smooth surface
known as the chorion laeve.
2. Endometrial Contribution
-The endometrial cells have accumulated lipids and glycogen, and are now called
decidua.
-The decidua underlying the chorion frondosum is known as the decidua
basalis. A layer of decidual cells tightly adherent to the underlying chorion is
known as the decidual plate.
- The decidua atop the aembryonic pole is known as the decidua capsularis.
-The decidua not in intimate contact with the developing embryo is called the decidua
parietalis.
- As the embryo develops and the chorion expands, the decidua capsularis degenerates
and the chorionic leave fuses with the decidua parietalis obliterating the uterus
lumen.
3. Development of Villi (week 2 to week 3)
Finger-like projections of proliferating cytotrophoblasts surrounded by
syncytiotrophoblasts, form – primary villi.
- Secondary villi form as extraembryonic
cytotrophoblastic core of the primary villi.
mesoderm
penetrates
the
- With the development of blood vessels, the villi are now called tertiary villi.
- During this time, cytotrophoblasts proliferate and extend through the syncytium to
form a thin, outer cytotrophoblastic that adheres to the endometrial tissues
4.Function of chorion
1) Exchange of metabolite: a portion of placenta, involved in the fetal circulatory
system
2) Hormone production: human chorionic gonadotropin (HCG)
C. Amnion/Amniotic Fluid
- Amniotic fluid is produced by 1) amniotic cells, and 2) infusion of fluid from
maternal blood. Additionally, in late fetal life 3) urine output from the fetus and 4)
Pulmonary secretions also contribute to the production of amniotic fluid.
At the end of pregnancy, urine is daily added to the amniotic fluid. This urine is
mostly water, since the placenta is functioning as an exchange for metabolic wastes.
- During the latter parts of gestation (fifth month), the fetus swallows and breathes
about half the total volume of amniotic fluid per day.
Note: The water in the amniotic fluid changes every 3 hours, indicating the enormous
exchange between the amniotic cavity and the maternal circulation.
- Amniotic fluid functions to: (1) mechanically cushion the fetus, (2) allow fetal
movements, (3) prevent fetal adhesions with the amnion, and (4) acts as a
temperature sink to maintain constant temperature
- An overabundance of amniotic fluid (polyhydramnios) is frequently associated
with congenital anomalies, particularly those which prevent the infant from normal
swallowing. (either because of esophageal atresia or through lack of nervous
control of the swallowing mechanism ( as in anencephaly)
- Oligohydramnios, or a decreased amount in amniotic fluid, is associated with
poor production of amniotic fluid (i.e. low fetal urine output, such as poor
development of kidney, urethra atresia, or inadequate amniotic fluid production).
Note: At the end of the 3rd month, the amnion has expanded to such an extent that it
comes in contact with the chorion, thereby obliterating the chorion cavity. The yolk sac
then usually shrinks and is gradually obliterated.
D. Yolk Sac
- Early in gestation the yolk sac may provide some limited nutrition.
- During the 3rd through 6th weeks of gestation, blood islands form and
contribute to the embryo's early circulation.
- Also during the 3rd week of gestation, germ cells from the yolk sac migrate into
the embryo.
- Finally, as the embryo folds and takes shape, a portion of the yolk sac is
incorporated into the embryo as the primitive gut.
- The remainder of the yolk sac is rudimentary.
E. Allantois
- Is rudimentary in humans
- During fetal life, it fuses to become the ligamentous urachus.
- Genes rise to the umbilical vessles
F. Umbilical Cord
- Cranial and caudal folding of the embryonic disc, accompanied by the lateral
folding, produces a purse string closure around the primitive umbilical ring.
- The primitive umbilical ring contains: the connecting stalk (with umbilical vessels
and allantois), the yolk sac and an intercoelomic canal.
- Vessels from the chorionic plate (two arteries and one umbilical vein) intertwine
as they enter the connecting stalk. A gelatinous mesoderm called Wharton's Jelly,
surrounds umbilical vessels.
- The umbilical vein caries O2 and nutrient-rich blood to the fetus, while the
umbilical arteries return O2/nutrient depleted blood from the fetus to the placenta
- The intercoelomic canal connects the intraembryonic coelom with the
extraembryonic coelom (chorionic cavity) and, during early fetal development
(~1st trimester), contains the herniated portion of the developing midgut.
- Amnion covers the umbilical cord.
- The tortuosity of the umbilical cord often gives the impression of knot formation,
however no vascular compromise is seen with vessel tortuosity. True knots can
form in abnormally long umbilical cords.
- In contrast, short umbilical cords can detach the placenta from the uterus before
delivery, eliminating adequate oxygenation for the fetus during the delivery
process.
- Occasionally, a single umbilical artery is found and is often associated with other
congenital anomalies.
- Abnormal insertions of the umbilical cord into the placenta can cause rupture of
the vessels during delivery and exsanguination of the fetus.
G. Placental Structure
- The placenta proper is composed of the chorion frondosum from the fetus and
the decidua basalis from the mother.
- The fetal border of the placenta is the chorionic plate (extraembryonic somatic
mesoderm and cytotrophoblasts).
- The decidual plate of the decidua basalis is the placental border on the maternal
side.
- The placenta becomes compartmentalized into cotyledons by evaginating
decidual septa extending from the decidual plate. The decidual septa do not reach
the chorionic plate, thus the compartmentalization is incomplete.
- The placenta is usually composed of fifteen to twenty cotyledons.
- At term, the placenta is discoid shape and weighs five to six hundred grams. The
placenta is expelled from the uterine cavity approximately thirty minutes following
the birth of the infant.
H. Placental circulation and barrier
- Villi extending from the chorionic plate and anchoring to the cytotrophoblastic
shell are termed anchoring villi.
- The intervillous spaces are lined by syncytiotrophoblasts and bordered by
adjacent anchoring villi, the chorionic plate, and the cytotrophoblastic shell. The
incorporated maternal blood vessels extend through the cytotrophoblastic shell and
bleed into the intervillous space forming lakes of maternal blood that bathe the
branching or free villi
- Branching or free villi originating from the anchoring villi, extend into the
intervillous spaces, and are the chief villi involved in circulatory exchange.
- During the fourth month, there is a loss of cytotrophoblasts and connective tissue
in the branching villi, approximating the endothelial wall of the capillary and the
syncytium. Thus, only two layers separate the fetal and maternal circulations.
- The syncytial cells that overlie tightly adherent blood vessels form microvilli to aid
in the exchange between the fetal and maternal circulations.
- In an effort to enhance functional maternal-fetal exchange, nuclei of
syncytiotrophoblasts shift away from underlying blood vessels and clump together
to form syncytial knots.
- Maternal spiral arteries that enter the intervillous space inject blood deep within
the space toward the chorionic plate. The blood then percolates through the
branching villi back toward the decidual plate and endometrial veins.
I. Placental Function
1. Exchange of Metabolites
- The placenta allows exchange of oxygen, carbon dioxide, nutrients, and
electrolytes, which maintains the active embryonic and fetal cell metabolism.
- Factors affecting exchanges include: Concentration differences, area of exchange,
permeability, molecular size, lipid solubility, degree of ionization and molecular
configuration,
- Additionally, the placenta allows exchange of maternal antibodies, providing
passive immunologic protection for the developing fetus. The placenta also
functions to remove metabolic wastes from the developing fetus as the kidneys and
GI tract are unable to support waste elimination.
- Unfortunately, these same features that allow nutrient and supportive transfer
between the fetal and maternal circulations also allows drugs and infectious agents
to traverse into the fetal circulation.
2.Denfense barrier
- The placental barrier, is initially composed of four layers: (a)the endothelial
lining of the fetal vessels,(b) the connective tissue in the core of the villus, (c) the
cytotrophoblastic layer; and (d) the syncytium. From the fourth month on, the
placental barrier becomes much thinner, since the endothelial lining of the vessels
comes in intimate contact with the syncytial membrane, thus greatly increasing the
rate of exchange.
3. Hormone Production
- The placenta (chiefly the syncytiotrophoblasts) produces chorionic gonadotrophin
(Beta HCG--Placental LH) which support the corpus luteum in the production of
progesterone.( HCG is excreted by the mother in the urine, their presence is used
as an indicator of early stages of gestation)
- The placenta will eventually (around the 4th month of gestation) produce adequate
amounts of estrogen and progesterone that the corpus luteum is no longer needed.
- Estrogen and progesterone, enhance and stabilize uterine growth and the
development of the mammary gland.
- Placental Lactogen (somatomanotrophin) is also produced by the placenta and
facilitates glucose extraction from maternal blood.
J. Multiple Gestations
- Dizygotic or fraternal twins are the most common (70 percent) type of multiple
gestations and results from fertilizations of two or more simultaneously shed
oocytes.
- Because each zygote is different, the sex may differ and the offspring share no
more resemblance to each other than other brothers and/or sisters.
- Each zygote will implant individually, develop its own placenta, amnion, and
chorion.
- Monozygotic or identical twins. Occasionally, a zygote will split in the early
stages of development.
- If the zygote is split at the two-cell stage, two completely separate zygotes will
develop with their own placenta, amnion, and chorion.
- If separation occurs at the blastocyst stage, the inner cell mass will split into two
embryos, with a common placenta and a chorionic cavity, but separate amniotic
cavities. * Most common for monozygotic twins.
- Rarely does separation occur at the bilaminar germ disc stage, resulting in two
embryos with a single placenta, a common chorion, and a common amnion.
- Even more rarely, splitting of the zygote may be incomplete and thus, the
developing embryos remain partially fused at some point, giving rise to conjoined
twins.