BIOLOGY Volume 3 - A Guide to General Biology - 2004

21. REPRODUCTION

21.8. Human Reproduction

21.8.6. Early Embryonic Development and Extraembryonic Membranes

As previously mentioned, the outer Cells of the blastocyst, which make up the trophoblast, grow and develop to form an outer membrane called the chorion. The chorion plays a vital role in nourishing the developing embryo and excreting Metabolic waste products. Meanwhile, two cavities appear within the inner Cell mass, and the cells lining these cavities give rise to two additional membranes: the amnion and the yolk sac (Fig. 21.48).

The amnion is a thin membrane that envelops the embryo like an umbrella and serves a protective function. The space between the amnion and the embryo, known as the amniotic cavity, is filled with Amniotic Fluid secreted by the amniotic cells. As the embryo grows in size, the amnion expands so that it remains constantly pressed against the uterine wall opposite the embryo. The amniotic fluid Supports the embryo and protects it from mechanical injury.

In humans, the yolk sac does not perform any significant Functions; however, in reptiles and birds, it plays a crucial role by absorbing nutrients from the isolated yolk and transporting them to the gut of the developing embryo.

The cells of the inner cell mass, located between the early amnion and the yolk sac, form a Structure called the embryonic disc, which gives rise to the embryo proper. The cells of this disc differentiate at an early stage (when the disc is less than 2 mm in diameter) to form an outer and an inner cell layer—the ectoderm and the endoderm. At a later stage, the mesoderm is formed, and these three germ layers give rise to all the Tissues of the developing fetus. The Development of these three germ layers is called Gastrulation and occurs 10–11 days after Fertilization. The Development of the Brain AND SPINAL cord begins during the third week from the neural tube, which forms from the ectoderm.

Initially, the tube appears as a groove, but gradually the edges of this groove elevate and fold inward until they fuse completely, forming a hollow tube with a Swelling at one end. This swelling develops into the first organ—the brain.

In the Cytology/cytology/16.html">Early stages of embryonic development, Metabolic exchange between the embryo and the maternal Organism occurs via the trophoblast villi. However, soon afterward, a fourth membrane—the allantois—develops from the hindgut of the embryo. The chorion, amnion, yolk sac, and allantois are referred to as extraembryonic membranes (Fig. 21.49). The allantois grows outward until it comes into contact with the chorion, forming a highly vascularized structure known as the chorioallantois, which participates in The formation of the Placenta—an organ that carries out exchange between the embryo and the maternal organism much more efficiently.

Class="center">

Fig. 21.49. Simplified diagram showing the relationships between the human embryo and the four extraembryonic membranes (amnion, chorion, allantois, and yolk sac) five weeks after fertilization. The region where the allantois contacts the chorion is called the chorioallantois, which subsequently forms the placenta.