Gastrulation - 7th to 10th Day

Axis formation in mammals is poorly understood and as in eg. xenopus there is an animal-vegetal pole and dorsal-ventral axis (which I won't cover in here) there is no such things in mammalian embryo. Only "axis" that can be said to exist is embryonic-aemryonic axis according to the Inner Cell Mass. But this axis is only geometrical, it has nothing to do with the future germ layers.

Blastocyst implants into the uterine wall and the trophectoderm gives rise to extra-embryonic ectoderm. The gastrulation involves cell movements which yield a three-dimensional embryo, where the germ layers, ectoderm, mesoderm and endoderm are in correct positions. The gastrulation is initiated by the formation of primitive streak and the cells move inside as individual cells, giving rise to endoderm and mesoderm.



The embryo is now 10 days old.

This part of the embryo development wasn't covered in our course, only the gastrulation of sea urchins and xenopus was covered and as we are not interested on them, I will not go through them ;)