Abembryonic Pole

Figure 6.8 Structure of villi at various stages of development. A. During the fourth week. The extraembryonic mesoderm penetrates the stem villi in the direction of the decidual plate. B. During the fourth month. In many small villi the wall of the capillaries is in direct contact with the syncytium. C and D. Enlargement of the villus as shown in A and B, respectively.

on the opposite side of the uterus and the two fuse (Figs. 6.10 to 6.12), obliterating the uterine lumen. Hence the only portion of the chorion participating in the exchange process is the chorion frondosum, which, together with the decidua basalis, makes up the placenta. Similarly, fusion of the amnion and chorion to form the amniochorionic membrane obliterates the chorionic cavity (Fig. 6.10, A and B). It is this membrane that ruptures during labor (breaking of the water).

Structure of the Placenta

By the beginning of the fourth month, the placenta has two components: (a) a fetal portion, formed by the chorion frondosum; and (b) a maternal portion, formed by the decidua basalis (Fig. 6.10B). On the fetal side, the placenta is bordered by the chorionic plate (Fig. 6.13); on its maternal side, it is bordered by the decidua basalis, of which the decidual plate is most intimately incorporated

Chorionic Villi And Maternal Decidua
Figure 6.9 A 6-week embryo. The amniotic sac and chorionic cavity have been opened to expose the embryo showing the bushy appearance of the trophoblast at the embryonic pole in contrast to small villi at the abembryonic pole and the connecting stalk and yolk sac with its extremely long duct.
Chorion Laeve

Figure 6.10 Relation of fetal membranes to wall of the uterus. A. End of the second month. Note the yolk sac in the chorionic cavity between the amnion and chorion. At the abembryonic pole, villi have disappeared (chorion laeve). B. End of the third month. The amnion and chorion have fused, and the uterine cavity is obliterated by fusion of the chorion laeve and the decidua parietalis.

Figure 6.10 Relation of fetal membranes to wall of the uterus. A. End of the second month. Note the yolk sac in the chorionic cavity between the amnion and chorion. At the abembryonic pole, villi have disappeared (chorion laeve). B. End of the third month. The amnion and chorion have fused, and the uterine cavity is obliterated by fusion of the chorion laeve and the decidua parietalis.

into the placenta. In the junctional zone, trophoblast and decidua cells intermingle. This zone, characterized by decidual and syncytial giant cells, is rich in amorphous extracellular material. By this time most cytotrophoblast cells have degenerated. Between the chorionic and decidual plates are the intervillous spaces, which are filled with maternal blood. They are derived from lacunae in the syncytiotrophoblast and are lined with syncytium of fetal origin. The villous trees grow into the intervillous blood lakes (Figs. 6.17 and 6.13).

During the fourth and fifth months the decidua forms a number of decidual septa, which project into intervillous spaces but do not reach the chorionic plate (Fig. 6.13). These septa have a core of maternal tissue, but their surface is covered by a layer of syncytial cells, so that at all times a syncytial layer separates maternal blood in intervillous lakes from fetal tissue of the villi. As a result of this septum formation, the placenta is divided into a number of compartments, or cotyledons (Fig. 6.14). Since the decidual septa do not reach the chorionic plate, contact between intervillous spaces in the various cotyledons is maintained.

As a result of the continuous growth of the fetus and expansion of the uterus, the placenta also enlarges. Its increase in surface area roughly parallels that of the expanding uterus and throughout pregnancy it covers approximately 15 to 30% of the internal surface of the uterus. The increase in thickness of the placenta results from arborization of existing villi and is not caused by further penetration into maternal tissues.

Decidual Lining Uterus Pregnancy
Figure 6.11 A 19-week-old fetus in its natural position in the uterus showing the umbilical cord and placenta. The lumen of the uterus is obliterated. In the wall of the uterus is a large growth, a myofibroma.
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Responses

  • gustava trentini
    When does amnion and chorion fuse?
    7 years ago
  • Edda
    What is a chorion in the uterus?
    7 years ago
  • katja
    What is lining in pregnancy?
    7 years ago
  • lauren
    What is an embryonic pole?
    7 years ago
  • fnan
    What does the amniochorionic membrane look like?
    7 years ago
  • Tiina
    What is chorion frondosum?
    2 years ago
  • kisanet
    What is chorionic frondosum?
    2 years ago

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