THE PLASMA-MEMBRANE TRANSFORMATION DOES NOT LAST - MICROVILLI RETURN TO THE APICAL PLASMA-MEMBRANE OF UTERINE EPITHELIAL-CELLS AFTER THE PERIOD OF UTERINE RECEPTIVITY

Transmission electron microscopy was used to investigate morphological changes in surface ultrastructure of rat uterine luminal epithelial c ells and to determine how quickly and to what extent microvilli return to the apical surface after the period of uterine receptivity for bla stocyst attachment. Major, observable, differences in the surface morp hology of uterine epithelial cells from six groups of pregnant rats we re found. On the afternoon of day 6 of pregnancy, the apical surface i s typically flattened and consists mostly of irregular projections, bu t by the morning of day 7, short, irregular microvilli are already evi dent among the remaining irregular projections giving the cell surface a 'taller' profile. By the afternoon of day 7, the microvilli have in creased in height and frequency. This trend continues until by day 9 o f pregnancy, the apical surface bears microvilli that are long, thin, and comparatively regularly distributed. These ultrastructural alterat ions demonstrate that the plasma membrane transformation of early preg nancy which is essential for uterine receptivity for blastocyst attach ment, is transient and is followed by the return of regular microvilli .