Embryonic Membranes

The placenta and umbilical cord are not the only accessory organs of the conceptus. There are also four membranes—the amnion, yolk sac, allantois, and chorion (fig. 29.10). To understand these, it helps to realize that all mammals evolved from egg-laying reptiles. Within the shelled, self-contained egg of a reptile, the embryo rests atop a yolk, which is enclosed in the yolk sac; it is suspended in a little sea of liquid contained in the amnion; it stores its toxic wastes in the allantois; and to breathe, it has a chorion permeable to gases (the "skin" that you may have noticed just beneath the shell of a boiled egg). All of these membranes persist in mammals, including humans, but are modified in their functions.

The amnion is a transparent sac that develops from cells of the embryonic disc. It grows to completely enclose the embryo and is penetrated only by the umbilical cord. The amnion becomes filled with amniotic fluid, which protects the embryo from trauma and temperature fluctuations, allows the freedom of movement important to muscle development, enables the embryo to develop symmetrically, and prevents adhesion of, for example, an arm to the trunk. At first, the amniotic fluid forms by filtration of the mother's blood plasma, but beginning at 8 to 9 weeks,

Saladin: Anatomy & I 29. Human Development I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

1098 Part Five Reproduction and Development

1098 Part Five Reproduction and Development

Embryo Diagram
Figure 29.10 The Embryonic Membranes. (a) Diagram of a frontal section of the uterus. (b) Photograph of a human fetus at 8 weeks of gestation. The scale bar is 3 cm.

the fetus urinates into the amniotic cavity about once an hour and contributes substantially to the fluid volume. The volume increases only slowly, however, because the fetus swallows amniotic fluid at a nearly equal rate. At term, the amnion contains 700 to 1,000 mL of fluid.

The yolk sac arises partly from cells of the embryonic disc opposite the amnion. It is a small sac suspended from the ventral side of the embryo. It contributes to the formation of the digestive tract and produces the first blood cells and future egg or sperm cells.

The allantois (ah-LON-toe-iss) is an outpocketing of the posterior end of the yolk sac. It forms the foundation for the umbilical cord and becomes part of the urinary bladder. It can be seen in proximal cross sections of the cord.

The chorion is the outermost membrane, enclosing all the rest of the membranes and the embryo. Initially it has villi around its entire surface, but as the pregnancy advances, the villi of the placenta grow and branch while the rest of them degenerate. The chorion forms the fetal portion of the placenta; its functions are essentially those listed in table 29.1.

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  • neftalem
    Which accessory organs protects the embryo from trauma?
    6 years ago

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