Hormonal dependence of pinopode formation at the uterine luminal surface

The uterine luminal epithelium during the period of receptivity for nidation displays characteristic protrusions of the apical surface named pinopodes. The effects of oestradiol and progesterone, singly or in combination, on the formation and regression of pinopodes were investigated using scanning electron microscopy. The appearance of pinopodes was found to be strictly progesterone dependent. When given together with progesterone, before the development of pinopodes, high doses of oestradiol (plasma level approximately 300 pmol/l) inhibited pinopode formation; on the contrary, low doses of oestradiol (nidatory doses) did not interfere with the process until the 4th day of treatment. When oestradiol was given as a single injection, after pinopode formation, both doses were equivalent in inducing their regression 48-72 h later. It appears that the hormonal conditioning for pinopode formation and for the development of uterine receptivity for egg implantation is the same. These observations support the hypothesis that pinopodes could be an extremely useful tool to estimate uterine receptivity. The experiments we describe here, together with observations made a few years ago, in stimulated cycles in the human, suggest that implantation failure as a result of a hormonal imbalance during the time intervening between ovulation and nidation, may be a general phenomenon.