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The extraembryonic membranes of reptiles and mammals sustain the growing embryo. In reptiles these membranes surround the embryo within the shelled egg. In mammals the extraembryonic membranes form the placenta, an organ that exchanges nutrients, respiratory gases, and metabolic wastes between the mother and the embryo. Human gestation lasts 9 months. At the end of the first trimester, the fetus is very small but most of its organs have begun to form. In the second trimester, limbs elongate and the fetus moves. By the end of the third trimester, most organs have begun to function.
learning outcomes
You should be able to:
Describe the formation of the yolk sac, amnion, chorion, and allantois, using the chick as an example.
Summarize how the placenta forms.
Explain why a first-
Which germ layers participate in the development of the chick’s yolk sac, allantois, chorion, and amnion?
The yolk sac and allantois grow out from the hypoblast and consist of endoderm and mesoderm. The chorion and amnion develop from ectoderm and mesoderm.
What is the origin of the placenta in mammalian development?
The blastocyst divides into a trophoblast group and a hypoblast group of cells. The trophoblast interacts with the uterine lining to embed the embryo into the endometrium and to send out projections to make contact with maternal blood vessels. Cells of the hypoblast grow out to form what in birds would be the yolk sac, but there is no yolk. These hypoblast cells and the trophoblast cells form the placenta.
Why is a first-
The first trimester is a time of great sensitivity of the embryo to environmental risks because this is the time when many signaling cascades are setting up subsequent processes of determination, differentiation, and development. Therefore the earlier any necessary lifestyle or environmental changes are made, the better.