Trophoblast is the outer sphere of cells of the blastocyst, a hollow cavity, and an inner mass of cells which enables implantation to occur.
During pre-embryonic development stage about day six or seven, this layer (the trophoblast cells) will contact with the endometrial lining and secrete enzymes that digest endometrial cells. The trophoblast releases the hormone human chorionic gonadotropin (hCG), keeping the corpus luteum alive and is useful for the detection of pregnancy. As the trophoblast enzymes digest the endometrium, the blastocyst trophoblast develops into the chorion (Outermost fetal membrane that contributes to placenta formation.)
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Image: http://www.nature.com/nm/journal/v12/n9/images/nm0906-991-F1.gif
