Days 11 and

By the 11th to 12th day of development, the blastocyst is completely embedded in the endometrial stroma, and the surface epithelium almost entirely covers

Blastocyst 12th Day

Figure 3.4 Human blastocyst of approximately 12 days. The trophoblastic lacunae at the embryonic pole are in open connection with maternal sinusoids in the endometrial stroma. Extraembryonic mesoderm proliferates and fills the space between the exo-coelomic membrane and the inner aspect of the trophoblast.

Figure 3.4 Human blastocyst of approximately 12 days. The trophoblastic lacunae at the embryonic pole are in open connection with maternal sinusoids in the endometrial stroma. Extraembryonic mesoderm proliferates and fills the space between the exo-coelomic membrane and the inner aspect of the trophoblast.

the original defect in the uterine wall (Figs. 3.4 and 3.5). The blastocyst now produces a slight protrusion into the lumen of the uterus. The trophoblast is characterized by lacunar spaces in the syncytium that form an intercommunicating network. This network is particularly evident at the embryonic pole; at the abembryonic pole, the trophoblast still consists mainly of cytotrophoblastic cells (Figs. 3.4 and 3.5).

Concurrently, cells of the syncytiotrophoblast penetrate deeper into the stroma and erode the endothelial lining of the maternal capillaries. These capillaries, which are congested and dilated, are known as sinusoids. The syncytial lacunae become continuous with the sinusoids and maternal blood enters the lacunar system (Fig. 3.4). As the trophoblast continues to erode more and more sinusoids, maternal blood begins to flow through the trophoblastic system, establishing the uteroplacental circulation.

In the meantime, a new population of cells appears between the inner surface of the cytotrophoblast and the outer surface of the exocoelomic

Figure 3.5 Fully implanted 12-day human blastocyst (x 100). Note maternal blood cells in the lacunae, the exocoelomic membrane lining the primitive yolk sac, and the hypoblast and epiblast.

cavity. These cells, derived from yolk sac cells, form a fine, loose connective tissue, the extraembryonic mesoderm, which eventually fills all of the space between the trophoblast externally and the amnion and exocoelomic membrane internally (Figs. 3.4 and 3.5). Soon, large cavities develop in the extraembryonic mesoderm, and when these become confluent, they form a new space known as the extraembryonic coelom, or chorionic cavity (Fig. 3.4). This space surrounds the primitive yolk sac and amniotic cavity except where the germ disc is connected to the trophoblast by the connecting stalk (Fig. 3.6). The extraembryonic mesoderm lining the cytotrophoblast and amnion is called the extraembryonic somatopleuric mesoderm; the lining covering the yolk sac is known as the extraembryonic splanchnopleuric mesoderm (Fig. 3.4).

Growth of the bilaminar disc is relatively slow compared with that of the trophoblast; consequently, the disc remains very small (0.1-0.2 mm). Cells of the endometrium, meanwhile, become polyhedral and loaded with glycogen and lipids; intercellular spaces are filled with extravasate, and the tissue is edema-tous. These changes, known as the decidua reaction, at first are confined to the

Extraembryonic Pathway
Figure 3.6 A 13-day human blastocyst. Trophoblastic lacunae are present at the embryonic as well as the abembryonic pole, and the uteroplacental circulation has begun. Note the primary villi and the extraembryonic coelom or chorionic cavity. The secondary yolk sac is entirely lined with endoderm.

area immediately surrounding the implantation site but soon occur throughout the endometrium.

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Responses

  • amalia
    Where is abembryonic pole?
    3 years ago
  • ivone
    What is meant by extraembryonic mesoderm?
    3 years ago

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