BIOLOGY Lecture Notes - Golden Pages 2003

5. FUNDAMENTALS OF EMBRYOLOGY

Formation of Extraembryonic Membranes

Extraembryonic membranes act as intermediaries between the embryo and its environment. Although mammals have developed a Placenta instead of an eggshell, they retain the exact same extraembryonic membranes.

Extraembryonic membranes are formed through the proliferation of ectodermal and endodermal epithelia underpinned by mesoderm.

The combination of ectoderm and mesoderm (somatopleure) forms the amnion and chorion, whereas the combination of endoderm and mesoderm (splanchnopleure) gives rise to the yolk sac and allantois.

The Cells of the ectodermal and endodermal Tissues function as active epithelial cells, while the mesoderm provides an extensive network of Blood Vessels that transport blood from the embryo to the epithelium and back.

The aqueous environment for the embryo is provided by the amnion. Amniotic cells secrete Amniotic Fluid, ensuring that Embryogenesis still takes place, in effect, within an aquatic medium.

Gas exchange is mediated by the chorion, the outermost extraembryonic membrane. In mammals, the chorion becomes incorporated into the placenta.

Functions of the chorion:

1) transport of soluble substances between the fetus and the mother;

2) regulatory function (it produces human chorionic gonadotropin, a peptide hormone that stimulates placental and ovarian cells to synthesize progesterone, as well as chorionic somatomammotropin, which stimulates The Development of the mother's Mammary Glands and promotes milk production);

3) protective function (it shields the fetus from the maternal Immune Response by producing substances that block maternal Immunity, which are contained within The Cell membranes of the syncytiotrophoblast).

The allantois accumulates End products of Nitrogen METABOLISM and participates in gas exchange. In mammals, the size of the allantois depends on how efficiently the chorionic placenta removes nitrogenous wastes from the embryo. In humans, the allantois is a rudimentary sac.