Pinopode expression during human implantation.

Implantation failure is considered the major impediment to successful in vitro fertilization pregnancy rate. Despite recent advances in reproductive medicine, the role played by uterine receptivity in contributing to human infertility remains unclear. Several clinical studies suggest the importance of blastocyst implantation in a narrow window of uterine receptivity, between 6 and 8 days after ovulation. This brief and precise period, called "implantation or nidation window", lasts less than 48 h and coincides with the formation of large and smooth projections, called "pinopodes", on the apical membranes of the endometrial epithelial cells. Pinopode expression appears to advance or retard depending on the hormonal treatments and an individual's particular response. This displacement can be followed through endometrial biopsy and scanning electron microscopy (SEM). Therefore, pinopode expression could be considered a useful biological marker to assess endometrial receptivity and to locate the implantation window.