BIOLOGY Lecture Notes - Golden Pages 2003

5. FUNDAMENTALS OF EMBRYOLOGY

Gastrulation: Cell Redistribution (in Mammals)

The initial Cell segregation within the inner cell mass involves The formation of the hypoblast layer, or primary endoderm. Detaching from the inner cell mass, these Cells migrate into the blastocyst cavity, where they give rise to the endoderm of the yolk sac (these cells do not contribute to any part of the newborn Organism). The remaining surface portion of the inner cell mass (above the hypoblast) is referred to as the epiblast. It is subsequently cleft by minor fissures that coalesce, separating the embryonic epiblast from other epiblast cells that form the amniotic lining. The amnion fills with a secretion known as Amniotic Fluid (acting as a 'Shock absorber' that prevents desiccation of the embryo).

The embryonic epiblast (EE) contains all the cells required to form the embryo proper.

At the posterior margin of the EE, a localized thickening develops, giving rise to the primitive streak, through which the progenitor Cells of the endoderm and mesoderm migrate. These cells are coated with hyaluronic acid.

Extraembryonic tissue cells give rise to Organs unique to mammals that ensure the survival of the fetus within the Uterus.

Some trophoblast cells form the cytotrophoblast layer—actively dividing cells—while others form the syncytiotrophoblast. This tissue invades the uterine mucosa. In response, the uterus directs Blood Vessels to this area.

Subsequently, the mesodermal tissue extends outward from the gastrulating embryo (migrating through the primitive streak and entering the trophoblast villi). Extraembryonic mesoderm gives rise to blood vessels that transport nutrients from the mother to the embryo. A narrow connecting stalk of extraembryonic mesoderm links the embryo to the trophoblast and later forms the umbilical cord vessels. The fully developed organ, composed of trophoblast tissue and extraembryonic blood vessels, is called the chorion; fusing with the uterine wall, the chorion forms the Placenta.