Endometrial pinopodes indicate a shift in the window of receptivity in IVF cycles

The formation of endometrial pinopodes detected by scanning electron microscopy may be a specific marker for uterine receptivity. Aiming to assess the effects of ovarian stimulation on pinopode formation, we examined sequential endometrial biopsies from 17 oocyte donors. Seven normally menstruating women served as controls. Up to four samples were taken from each woman at 24-72 h intervals between days 14 and 24, giving a total of 69 samples. The day of oocyte retrieval was designated day 14 in ovarian stimulation cycles and the day of luteinizing hormone surge was designated day 13 in natural cycles. Endometrial morphology and pinopode numbers were similar in both groups. Fully developed pinopodes appeared in only one sample per cycle, indicating their short life span. However, the cycle day these structures appeared varied up to 5 days between women and the distribution was as follows: day 18 (n = 2), day 19 (n = 7), day 20 (n = 4), day 21 (n = 3), day 22 (n = 1) in ovarian stimulation cycles, and day 20 (n = 2), day 21 (n = 2), day 22 (n = 3) in natural cycles. Furthermore, accelerated pinopode formation in ovarian stimulation cycles was positively correlated with day 13 progesterone. Our findings show that ovarian stimulation does not affect endometrial pinopode formation in terms of quantity and life span. The cycle days when pinopodes form are specific to the individual, being on average 1-2 days earlier in ovarian stimulation than in natural cycles. These changes in pinopode expression may reflect shifts in the window of receptivity, resulting in ovo-endometrial asynchrony and limiting implantation success in in-vitro fertilization.