Implantation and primitive streak

Implantation

Implantation (Stage 5) requires relatively long time. It begins around embryonic day 7 and finishes by day 12. During this period, the trophoblast differentiates into two structures: the mainly unilayered cytotrophoblast, in which cell boundaries are preserved, and the surrounding syncytiotrophoblast, in which the boundaries between neighbouring cells dissolve. The syncytiotrophoblast penetrates the superficial epithelium of the uterine mucosa and dissolves the endometrium proteolytically. By S5a, it is still compact; by S5b, it becomes lacunar; by S5c, connections appear between the lacunae and the maternal sinusoids. The trophoblast develops most strongly at the embryonic pole, where it has penetrated the uterine mucosa. At the abembryonic pole, i.e. towards the uterus lumen, it remains unilayered for longer.
The embryoblast consists of the epiblast and hypoblast. Small spaces appear in the dorsal aspect of the epiblast, which coalesce to form the amniotic cavity. This cavity is separated from the trophoblast by a thin epithelial layer (amnioblast) of epiblastic origin.
The hypoblast enlarges laterally, and lines the blastocystic cavity as parietal hypoblast (later also termed extra-embryonic endoderm). The part beneath the epiblast is known as the visceral hypoblast.