Endometrial pinopodes: some more understanding on human implantation?

Endometrial receptivity is a prerequisite for blastocyst implantation. During receptivity, the hairy-like epithelial cell microvilli transiently fuse to a single flower-like membrane projection called the 'pinopode'. Scanning electron microscopy in sequential endometrial biopsies shows that pinopodes appear about 1 week after ovulation, and they develop and regress within just 2 days. Interestingly, the cycle days when pinopodes appear can vary by up to 5 days between different individuals. On average, they occur on days 20-21 in natural cycles and earlier (days 19-20) in stimulated cycles. The abundance of pinopodes relates to implantation success and many patients with multiple implantation failures fail to produce pinopodes. Based on these findings, biopsies from candidate embryo recipients have been examined in mock cycles and pinopode numbers and timing of their appearance assessed. A similar cycle follows where embryos are replaced earlier or later, according to the reported timing of pinopode formation. If pinopodes are absent, the cycle can be modified. Accumulating evidence supports their clinical use as a marker to assess endometrial receptivity. Pinopode appearance, loss of steroid receptors and maximal expression of a(v)b(3) integrin, osteopontin and leukaemia inhibitory factor and receptor have been demonstrated in the same biopsy, showing a consistent association of pinopode appearance and other receptivity changes.