MoBio Gastrulation Chapter 8

Gastrulation is the stage of development during which three germ layers (ectoderm, mesoderm, and endoderm) are formed from the blastula or blastocyst (in mammals).


Figure 5.1: The formation of tissues in human embryos. The blastocyst consists of trophoblast and inner cell mass. The latter first splits into two layers: hypoblast and epiblast. The hypoblast develops into yolk sac which, as indicated by the dashed line, may also contribute to the formation of the extraembryonic mesoderm . The epiblast develops into the three germ layers which constitute the embryonic tissues. The rest (amnionic ectoderm, yolk sac, extraembryonic mesoderm and syncytiotrophoblast) forms the extraembryonic tissues.


During gastrulation, cells continue to divide and involve dramatic cell movements (illustration). The blastocyst implants in the uterine wall about 8 to 9 days after fertilization and the three germ layers are formed by the end of the third week.