Inner Cell Mass Differentiation
While the syncytiotrophoblast starts to penetrate into the wall of the uterus, the inner cell mass (embryoblast) also develops.
The embryoblast forms a bilaminar (two layered) embryo, composed of the epiblast and the hypoblast. The epiblast is adjacent to the trophoblast and made of columnar cells; the hypoblast is closest to the blastocyst cavity, and made of cuboidal cells. The epiblast, now called primitive ectoderm will perform gastrulation, approximately at day 16 (week 3) after fertilization. In this process, it gives rise to all three germ layers of the embryo: ectoderm, mesoderm, and endoderm. The hypoblast, or primitive endoderm, will give rise to extraembryonic structures only, such as the lining of the primary (primitive) yolk sac (exocoelomic cavity).
Read more about this topic: Human Embryogenesis
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