Download 3.Lecturenotes(Placenta and fetal membrane)

PLACENTA, FETAL MEMBRANES
Langman’s Medical Embryology, 13th ed ,
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 laeve 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.
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Secondary villi form as extraembryonic mesoderm penetrates the cytotrophoblastic core of the primary villi.
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With the development of blood vessels, the villi are now called tertiary villi.
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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.
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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.
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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.
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During the 3rd through 6th weeks of gestation, blood islands form and contribute to the embryo's early
circulation.
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Also during the 3rd week of gestation, germ cells from the yolk sac migrate into the embryo.
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Finally, as the embryo folds and takes shape, a portion of the yolk sac is incorporated into the embryo as the
primitive gut.
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The remainder of the yolk sac is rudimentary.
E. Allantois
- Is rudimentary in humans
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During fetal life, it fuses to become the ligamentous urachus.
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Gives 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.
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The primitive umbilical ring contains: the connecting stalk (with umbilical vessels and allantois), the yolk sac and
an intercoelomic canal.
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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.
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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
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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.
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Amnion covers the umbilical cord.
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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.
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In contrast, short umbilical cords can detach the placenta from the uterus before delivery, eliminating adequate
oxygenation for the fetus during the delivery process.
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Occasionally, a single umbilical artery is found and is often associated with other congenital anomalies.
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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.
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The fetal border of the placenta is the chorionic plate (extraembryonic somatic mesoderm and cytotrophoblasts).
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The decidual plate of the decidua basalis is the placental border on the maternal side.
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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.
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The placenta is usually composed of fifteen to twenty cotyledons.
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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
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Villi extending from the chorionic plate and anchoring to the cytotrophoblastic shell are termed anchoring villi.
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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
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Branching or free villi originating from the anchoring villi, extend into the intervillous spaces, and are the chief
villi involved in circulatory exchange.
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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.
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The syncytial cells that overlie tightly adherent blood vessels form microvilli to aid in the exchange between the
fetal and maternal circulations.
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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.
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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.
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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.
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Factors affecting exchanges include: Concentration differences, area of exchange, permeability, molecular size,
lipid solubility, degree of ionization and molecular configuration,
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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.
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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)
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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.
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Estrogen and progesterone, enhance and stabilize uterine growth and the development of the mammary gland.
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Placental Lactogen (somatomanotrophin) is also produced by the placenta and facilitates glucose extraction from
maternal blood.
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