ABSTRACT
Ovulation and fertilization trigger embryonic development and endometrial differentiation
by corpus luteum progesterone production. These two synchronous processes couple about
1 week later, when the blastocyst begins to implant in the now receptive endometrium
(implantation window). Receptivity is a state of endometrial differentiation marked
by a change in epithelial morphology: the hairy-like cell microvilli fuse to a single
flower-like membrane projection called the ``pinopode.'' Scanning electron microscopy
of sequential endometrial biopsies shows that pinopodes form briefly (1-2 days), and
their numbers correlate with implantation. On average, the formation of pinopodes
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 wide (up to 5 days) variation
between women in the cycle days on which pinopodes form. These results suggest the
existence of a narrow and discrete implantation window in humans. Detection of pinopodes
is a potential clinical marker to assess endometrial receptivity.
KEYWORD
pinopodes - human implantation - uterine receptivity
