Relevance of endometrial pinopodes for human blastocyst implantation

Endometrial pinopodes, hormone-dependent protrusions of the endometrial apical plasma membrane, have been suggested as indicators of endometrial receptivity. The lifespan of endometrial pinopodes appears to be tightly regulated. Interestingly, considerable interpatient variations according to the onset of pinopode formation have been observed. In normal fertile women, pinopodes are present on days 6-8 post-ovulation, whereas pinopode formation is observed 1-2 days earlier in patients undergoing controlled ovarian hyperstimulation. In oocyte recipients treated with oestradiol and progesterone in hormone-controlled cycles, pinopode formation seems to be delayed. In-vitro experimental studies on human blastocyst attachment to an artificial endometrium indicate a preference for human blastocysts to attach to pinopode presenting areas on the endometrial surface. Transmission electron microscopy shows that the blastocysts do not attach to the apical plasma membrane of pinopode presenting cells. Trophoblast cells display contact with endometrial epithelial cells at the lateral plasma membranes by sharing apical junctional complexes. The function of endometrial pinopodes during human blastocyst implantation still remains to be elucidated. Ultrastructural studies indicate that the pinopode presenting apical plasma membrane does not participate directly in embryo-endometrial interactions.