Second Week of Development:
Bilaminar Germ Disc
At the eighth day of development, the blastocyst is partially embedded in the endometrial stroma. In the area over the embryoblast, the trophoblast has differentiated into two layers:
(a) an inner layer of mononucleated cells, the cytotrophoblast,and
(b) an outer multinucleated zone without distinct cell boundaries, the syncytiotrophoblast.
Mitotic figures are found in the cytotrophoblast but not in the syncytiotrophoblast. Thus,cells in the cytotrophoblast divide and migrate into the syncytiotro-phoblast, where they fuse and lose their individual cell membranes.Cells of the inner cell mass or embryoblast also differentiate into two layers:
(a) a layer of small cuboidal cells adjacent to the blastocyst cavity,known as thehypoblast layer,and
(b) a layer of high columnar cells adjacent to the amniotic cavity, theepiblast layer.
Together, the layers form a flat disc. At the same time, a small cavity appears within the epiblast. This cavity enlarges to become the amniotic cavity.Epiblast cells adjacent to the cytotrophoblast are called amnioblasts together with the rest of the epiblast, they line the amniotic cavity. The endometrial stroma adjacent to the implantation site is edematous and highly vascular. The large, tortuous glands secrete abundant glycogen and mucus.
The blastocyst ismore deeply embedded in the endometrium, and the penetra-tion defect in the surface epithelium is closed by a fibrin coagulum.The trophoblast shows considerable progress in development, particularly at the embryonic pole, where vacuoles appear in the syncytium. When these vac-uoles fuse, they form large lacunae, and this phase of trophoblast development is thus known as the lacunar stage.At the embryonic pole, meanwhile, flattened cells probably originating from the hypoblast form a thin membrane, the exocoelomic (Heuser’s) mem-brane, that lines the inner surface of the cytotrophoblast. This mem-brane, together with the hypoblast, forms the lining of theexocoelomic cavity,orprimitive yolk sac.
Days 11 and 12
By the 11th to 12th day of development, the blastocyst is completely embedded in the endometrial stroma, and the surface epithelium almost entirely covers .The original defect in the uterine wall. 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 intercommuni-cating network. This network is particularly evident at the embryonic pole; at the abembryonic pole, the trophoblast still consists mainly of cytotrophoblastic cells.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. As the trophoblast continues to erodemore 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 in-ner surface of the cytotrophoblast and the outer surface of the exocoelomic avity. These cells, derived from yolk sac cells, form a fine, loose connec-tive tissue, the extraembryonic mesoderm,which eventually fills all of the space between the trophoblast externally and the amnion and exocoelomic membrane internally. Soon, large cavities develop in the extraembryonic mesoderm, and when these become confluent, they form a new space known as the extraembryonic coelom,orchorionic cavity.This space surrounds the primitive yolk sac and amniotic cavity ex-cept where the germdisc is connected to the trophoblast by the connecting stalk. 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.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 area immediately surrounding the implantation site but soon occur throughout the endometrium.
By the 13th day of development, the surface defect in the endometrium has usually healed. Occasionally, however, bleeding occurs at the implantation site as a result of increased blood flow into the lacunar spaces. Because this bleeding occurs near the 28th day of the menstrual cycle, it may be confused with normal menstrual bleeding and, therefore, cause inaccuracy in determining the expected delivery date.
The trophoblast is characterized by villous structures. Cells of the cy-totrophoblast proliferate locally and penetrate into the syncytiotrophoblast,forming cellular columns surrounded by syncytium. Cellular columns with the syncytial covering are known as primary villi. In the meantime, the hypoblast produces additional cells that migrate along the inside of the exocoelomic membrane. These cells proliferate and gradually form a new cavity within the exocoelomic cavity. This new cavity is known asthe secondary yolk sacordefinitive yolk sac. This yolk sac is much smaller than the original exocoelomic cavity, or primitive yolk sac. During its formation, large portions of the exocoelomic cavity are pinched off. These portions are represented byexocoelomic cysts,which are often found in the extraembryonic coelom or chorionic cavity.Meanwhile, the extraembryonic coelom expands and forms a large cavity,the chorionic cavity.The extraembryonic mesoderm lining the inside of the cytotrophoblast is then known as thechorionic plate.The only place where extraembryonic mesoderm traverses the chorionic cavity is in the connecting stalk. With development of blood vessels, the stalk becomes the umbilical cord.