The<unk>  proliferates and differentiates into two cell layers at approximately six days after fertilization for humans. Layer Location Description Cytotrophoblast The inner layer A single-celled inner layer of the<unk> . Syncytiotrophoblast The outer layer A thick layer that lacks cell boundaries and grows into the endometrial stroma. It secretes hCG in order to maintain progesterone secretion and sustain a pregnancy. Intermediate<unk>  (IT) The implantation site, chorion, villi (dependent on subtype) An anchor placenta (implantation site IT).
Trophoblast
Hemangioblast
Nurse cell
Clue cell
Elliptocyte
Echinocyte
Lymphoblast
Neuroblast