Human Reproduction | Extraembryonic Membrane | Part 25 | NEET | AIIMS | CSIR | 12th Board

Описание: The extraembryonic membranes consist of the chorion (the combination of trophoblast plus underlying extraembryonic mesoderm), amnion, yolk sac, and allantois.
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The amnion, a thin ectodermal membrane lined with mesoderm, grows to enclose the embryo like a balloon. It is filled with a clear fluid, which is generated from many sources, such as the fetal skin, the amnion itself, the fetal kidneys, and possibly the fetal vessels. At term, the volume of amniotic fluid approaches 1 L. Amniotic fluid is removed by exchange across the amniotic membrane and by fetal swallowing.
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The yolk sac is a ventral, endodermally lined structure that does not serve a nutritive function in mammalian embryos. Mesodermal blood islands in the wall of the yolk sac form the first blood cells and vessels. Primordial germ cells are recognizable in the wall of the yolk sac, but they originate in extraembryonic mesoderm at the base of the allantois.
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The allantois is a small, endodermally lined diverticulum off the ventral side of the hindgut. It does not serve a direct function of respiration or storage of wastes in humans. These functions are carried out through the placenta and the umbilical vessels that arise in conjunction with the allantois.
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Chorionic villi form as outward projections from the trophoblast. Primary villi consist of projections of trophoblast alone. When a mesenchymal core forms within a villus, it is a secondary villus, and when the mesenchyme becomes vascularized, the villus is a tertiary villus. As villi mature, the cytotrophoblast in some villi grows through the syncytiotrophoblast as cytotrophoblastic cell columns and makes contact with the maternal endometrial tissue. Cytotrophoblast continues to grow around the blood-filled space surrounding the chorion to form a cytotrophoblastic shell, which is the direct interface between the fetal and maternal tissues. Villi that make direct contact with maternal tissues are anchoring villi; villi that do not make such contact are floating villi. Because chorionic villi float in a pool of maternal blood, the human placenta is designated a

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