The allantois is the middle layer of the placenta derived from the embryonic hindgut ; blood vessels originating from the umbilicus traverse this membrane. The outermost layer of the placenta, the chorion, comes into contact with the endometrium; it is composed of two layers of cells - inner cytotrophoblast and outer syncytiotrophoblast. The chorion and allantois fuse to form the chorioallantoic membrane. An allantoic cavity is significant in quadrupeds Figure In primates the amnion and chorioallantois eventually fuse.
The structure of the placenta varies among species. Differences in placental anatomy are due to gross arrangement in sites of attachment Table and endometrial tissue layers interposed between the circulatory systems of the mother and fetus Table Endometrial tissues, especially those of invasive implanters, are broken down; the fetal side chorion remains intact. Transport processes. Oxygen, carbon dioxide, lipid-soluble vitamins A, D, E, K , some electrolytes eg.
Simple carbohydrates and certain trace minerals eg. Amino acids, water soluble vitamins B complex, C , and other ions eg. These villi invade and destroy the uterine decidua while at the same time they absorb nutritive materials from it to support the growth of the embryo.
Chorionic artery : An image showing the chorionic villi and the maternal vessels. During the primary stage the end of fourth week , the chorionic villi are small, nonvascular, and contain only the trophoblast. During the secondary stage the fifth week , the villi increase in size and ramify, while the mesoderm grows into them; at this point the villi contain trophoblast and mesoderm. During the tertiary stage fifth to sixth week , the branches of the umbilical vessels grow into the mesoderm; in this way, the chorionic villi are vascularized.
At this point, the villi contain trophoblast, mesoderm, and blood vessels. Embryonic blood is carried to the villi by the branches of the umbilical arteries. After circulating through the capillaries of the villi, it is returned to the embryo by the umbilical veins. Chorionic villi are vital in pregnancy from a histomorphologic perspective and are, by definition, products of conception. The placenta begins to develop upon implantation of the blastocyst into the maternal endometrium.
The placenta functions as a fetomaternal organ with two components: the fetal placenta chorion frondosum , which develops from the same blastocyst that forms the fetus; and the maternal placenta decidua basalis , which develops from the maternal uterine tissue. The outer layer of the blastocyst becomes the trophoblast, which forms the outer layer of the placenta. This layer is divided into two further layers: the underlying cytotrophoblast layer and the overlying syncytiotrophoblast layer.
Chorion and chorionic villi are differentiated from the blastocyst during the implantation. During the fetal period, chorionic villi further develop and become a part of the placenta. The remaining part of the chorion, together with amnion forms the transparent fetal membranes. Yashoda Ramyajith Somarathna holds a B.
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