Pinopodes

ABSTRACT

Pinopodes are balloon-like projections that arise from the endometrial surface in mice, rats and humans during the window of receptivity and are best viewed using scanning electron microscopy. Differences are seen in the shape, size and content of pinopodes between rodents and humans. The diameter of pinopodes in rats is 3.0-4.0 microm and in mice and humans it is 6 microm. The percentage of luminal epithelium covered by pinopodes during the window of receptivity varies in rats from 5.5 to 20%, in mice from 0.6 to 20% and in humans from 0 to 12%. The formation of pinopodes in both the rodent and human endometrium appears to be progesterone dependent, whereas administration of estradiol results in their rapid loss. Although the biological function of pinopodes is less well understood, a correlation between the number of pinopodes and implantation rate has been revealed. During implantation, pinopodes interdigitate with microvilli on the syncytiotrophoblast surface of blastocysts. Very few markers have revealed the presence of pinopodes; these include integrins, leukaemia inhibitory factor, heparin-binding epidermal-like growth factor, glutaredoxin, Hoxa-10 and Mucin 1. Clinically, pinopodes are the best studied endometrial factors involved in implantation. In conclusion, pinopodes are a good marker for endometrial receptivity. Further studies other than endometrial biopsy are needed to demonstrate the presence of pinopodes for timing of endometrial receptivity to increase the pregnancy rate in programmes on in-vitro fertilization