Objective:To observe the formation of pinopodes in human endometrium in ovulation normality cycles and to investigate the relation between the development of endometrial pinopodes and the estrogen/progesterone and their receptors.Methods:(1)Twenty-seven women were followed for one cycle by vaginal ultrasonograms,and by testing urine samples for LH surge.(2)An endometrial biopsy sample was obtained randomly between days 3 to 11 after the LH surge and divided into two pieces,one for scanning electron microscopy and the other for immunohistochemistry.The biopsy samples were analyzed for expression of estrogen and progesterone receptors(ER,PR)by using immunohistochemical techniques.(3)Serum samples were obtained on the biopsy day and determined the concentrations of estradiol and progesterone by magnetic solid phase enzyme immunoassay.Receptor expression was compared with hormone levels and the developmental stage of the pinopodes.Results:(1)Biopsy samples were grouped according to developmental stage of the endometrium as determined by scanning electron microscopy,which were defined as before pinopodes,developing pinopodes,fully developed pinopodes and regressing pinopodes.(2)The development of pinopodes was positively correlated with the progesterone level(r=0.515,P<0.01).(3)The expresstion of PR in stromal cells did not change during luteal period.PR in the glandular epithelial was greatly decreased when fully developed pinopodes formed.The expression of the PR in the glandular epithelial cells was negatively correlated with the progesterone level in serum(r=-0.550,P<0.01)and it was negatively correlated with pinopodes developmental stage(r=-0.793,P<0.01).(4)Levels of estrogen receptors and serum estradiol did not correlate with pinopodes formation.Conclusions:Progesterone regulated the expression of pinopodes by PR.The formation of pinopodes was concurrent with serum progesterone increasing and down-regulation of glandular epithelial progesterone receptors.