Enbryology / All About Embryology

Late in the sixth day after ovulation the naked, sticky blastocyst comes into contact with the uterine lining and adheres to it (Figure 1F). The site of attachment is variable and not predetermined. The uterine lining has already been preparing, under the influence of ovarian hormones, for the reception of the blastocyst. Among these preparations has been the elaboration and expulsion, by the uterine glands, of a secretion that serves as nourishment for the blastocyst, both when it is free and during its implantation. Directly after blastocyst attachment come its establishment within the thickened uterine lining and the participation of its trophoblastic capsule in the differentiation of a placenta, a structure that enables the developing embryo to enter into a prompt physiological dependence on the mother.

The trophoblast of the blastocyst exerts an enzymic, destructive influence on the swollen uterine lining, leading to erosion of both the superficial epithelium of the uterine lining and also its deeper, cellular connective tissue (Figure 1G). This early stage of invasion ends in a few days; the blastocyst is then completely buried within a more superficial and compact layer of the total uterine lining (Figure 1G, H). While the blastocyst is completing this phase of implantation, its original shell of cellular trophoblast steadily proliferates a multitude of cells that lose their outermost membranes and merge. The result is a thick peripheral layer consisting of a common mass of cytoplasm in which many nuclei are embedded. This external investment is called syncytial trophoblast.


The implanted blastocyst next proceeds to establish itself as dependent upon the uterus. The syncytial trophoblast becomes a spongy shell containing irregular cavities (Figure 1H above). This expanding mass destroys cellular connective tissue and glands encountered in its path. Both the cellular and derivative syncytial trophoblast have the capacity of ingesting such tissue. The erosive process also taps capillaries and small blood vessels, and blood liberated in this way is likewise taken up into the trophoblast. Such engulfed blood cells and fragments of tissue are presumably utilized for nourishment in the early period of establishment of the expanding blastocystic sac. Yet these erosive activities decline in intensity by the end of the third week of development, and at this time the sac is completing the first phase of its specialization (Figure 1L).

Occasionally a fertilized egg fails to reach the uterus, implanting and beginning to develop elsewhere. This outcome is called an ectopic or extra-uterine pregnancy. The most common ectopic site is the uterine tube—this type of pregnancy, if not treated, can be fatal for the mother—but the peritoneum lining the abdominal cavity and even the interior of the ovary are also involved, though rarely. The unsuitability of all these sites for continued development usually leads to early death and resorption of the embryo.

Placentation

The irregular strands of invasive syncytial trophoblast constitute a first stage in the formation of true villi (which form part of the placenta and are briefly described below). Primitive connective tissue soon lines the interior of the blastocyst wall, and this complex (trophoblast and connective tissue) is then named the chorion . Connective tissue promptly grows into the trophoblastic strands, and blood vessels develop in the tissue. The result is the production of many chorionic villi, each resembling a tiny, branching bush (Figure 1I, Ja).


In the fourth week of development, the essential arrangements have been established that make possible those physiological exchanges between mother and fetus that characterize the remainder of pregnancy. The deepest embedded portion of the chorionic wall becomes the so-called chorionic plate of the developing placenta. From the plate extend the main stems of chorionic villi, which give off progressively subdividing branches (Figure 1N, P). In general, the side branches are free, whereas apical ones tend to attach to the maternal tissue and serve as anchors. The villous trees occupy a labyrinthine space between the villi that was created by erosion of the uterine lining. Trophoblast not only covers the chorionic plate and its villi but also spreads like a carpet over the eroded surface of the maternal tissue.

Tapped uterine arteries open into the trophoblast-lined intervillous space, their blood bathing the branches and twigs of the villous trees. This blood drains from the intervillous space through similarly tapped veins. Arterial blood of the embryo, and later fetus, passes through vessels of the umbilical cord to the chorionic plate. Thence it is distributed to the villous stems, branches, and twigs through vessels in their connective-tissue cores. Return of this blood to the fetus is by a reverse route.

The circulation of maternal blood through the intervillous space is wholly separate from fetal blood coursing through the chorion and its villi. Communication between the two is solely by diffusive interchange. The barrier between the two circulations consists of the trophoblastic covering of villi, the connective tissue of the villous cores, and the thin lining of the capillaries that are contained in the villous cores. The placenta serves the fetus in several ways, most of which involve interchanges of materials carried in the bloodstreams of the mother and fetus. These functions are of the following kinds: (1) nutrition, (2) respiration, (3) excretion, (4) barrier action (e.g., prevention of intrusions by bacteria), and (5) synthesis of hormones and enzymes.

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