Endometrial receptivity: Changes in cell-surface morphology

Ovulation and fertilization trigger embryonic development and endometrial d ifferentiation by corpus luteum progesterone production. These two synchron ous processes couple about I week later, when the blastocyst begins to impl ant in the Mow receptive endometrium (implantation window). Receptivity is a state of endometrial differentiation marked by a change in epithelial mor phology: the hairy-like cell microvilli fuse to a single flower-like membra ne projection called the "pinopode." Scanning electron microscopy of sequen tial endometrial biopsies shows that pinopodes form briefly (1-2 days), and their numbers correlate with implantation. On average, the formation of pi nopodes is earlier in stimulated (days 19-20) and later in artificial (days 21-22) compared with natural cycles (days 20-21). There is, however, a wid e (up to 5 days) variation between women in the cycle days on which pinopod es form. These results suggest the existence of a narrow and discrete impla ntation window in humans. Detection of pinopodes is a potential clinical ma rker to assess endometrial receptivity.