Bilaminar Germ Disc

Second Week of Development: Bilaminar Germ Disc Copyright © 2010 Wolters Kluwer Health | Lippincott Williams & Wilkins DAY 8 At the eighth day of development, the blastocyst is partially embedded in the endometrial stroma. The trophoblast has differentiated into two layers: (1) an inner layer of mononucleated cells, the cytotrophoblast , and (2) 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 syncytiotrophoblast, where they fuse and lose their individual cell membranes. Cells of the inner cell mass or embryoblast also differentiate into two layers: (1) a layer of small cuboidal cells adjacent to the blastocyst cavity, known as the hypoblast layer (2)a layer of high columnar cells adjacent to the amniotic cavity,the epiblast layer  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. DAY 9 The blastocyst is more deeply embedded in the endometrium, and the penetration defect in the surface epithelium is closed by a fibrin coagulum At the embryonic pole, vacuoles appear in the syncytium. When these vacuoles fuse, they form large lacunae, and this phase of trophoblast development is thus known as the lacunar stage At the abembryonic pole, flattened cells probably originating from the hypoblast form a thin membrane, the exocoelomic (Heuser) membrane that lines the inner surface of the cytotrophoblast This membrane, together with the hypoblast, forms the lining of the exocoelomic cavity , or primitive yolk sac . DAYS 11 AND 12 The blastocyst is completely embedded in the endometrial stroma, and the surface epithelium almost entirely covers the original defect in the uterine wall. 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 continous with the sinosoids and maternal blood enters the lacunar system Establishing the uteroplacental circulation. The extra embryonic mesoderm :A new population of cells appears between the inner surface of the cytotrophoblast and the outer surface of the exocoelomic cavity. These cells, derived from yolk sac cells, form a fine, loose connective tissue, which eventually fills all of the space between the trophoblast externally and the amnion and exocoelomic membrane internally. The extra embryonic cavity , or chorionic cavity: large cavities develop in the extra embryonic mesoderm, and when these become confluent, they form a new space, named Extraembryonic cavity or chorionic cavity. This space surrounds the primitive yolk sac and amniotic cavity, except where the germ disc is connected to the trophoblast by the connecting stalk. The extraembryonic mesoderm lining the cytotrophoblast and amnion is called the extraembryonic somatic mesoderm ; the lining covering the yolk sac is known as the extraembryonic splanchnic Mesoderm. Growth of the bilaminar disc is relatively slow compared with that of the trophoblast; consequently, the disc remains very small (0.1to 0.2mm). Decidua reaction : Cells of the endometrium, meanwhile, become polyhedral and loaded with glycogen and lipids; intercellular spaces are filled With extra vasate, and the tissue is edematous. At first are confined to the area immediately surrounding the implantation site but soon occur throughout the endometrium. DAY 13 By the 13th day of development, the surface defect in the endometrium has usually healed. Primary villi : Cells of the cytotrophoblast proliferate locally and penetrate into the syncytio trophoblast, forming cellular columns surrounded by syncytium. Secondary yolk sac or definative yolk sac : 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. - Exocoelomic cysts Chorionic cavity: the extraembryonic coelom expands and forms a large cavity. The chorionic plate :The extra embryonic mesoderm lining the inside of the cytotrophoblast. With development of blood vessels, the connecting stalk becomes the umbilical cord . Sites for ectopic pregnancy: Tubal pregnancy: