The plasma membrane transformation does not last: microvilli return to the apical plasma membrane of uterine epithelial cells after the period of uterine receptivity.

Abstract

Transmission electron microscopy was used to investigate morphological changes in surface ultrastructure of rat uterine luminal epithelial cells and to determine how quickly and to what extent microvilli return to the apical surface after the period of uterine receptivity for blastocyst attachment. Major, observable, differences in the surface morphology of uterine epithelial cells from six groups of pregnant rats were found. On the afternoon of day 6 of pregnancy, the apical surface is typically flattened and consists mostly of irregular projections, but by the morning of day 7, short, irregular microvilli are already evident among the remaining irregular projections giving the cell surface a 'taller' profile. By the afternoon of day 7, the microvilli have increased in height and frequency. This trend continues until by day 9 of pregnancy, the apical surface bears microvilli that are long, thin, and comparatively regularly distributed. These ultrastructural alterations demonstrate that the plasma membrane transformation of early pregnancy which is essential for uterine receptivity for blastocyst attachment, is transient and is followed by the return of regular microvilli.