D, TEM showed that the microvilli, markers of uterine receptivity, were observed at the surface of intact luminal epithelium on day 4 of pregnancy and epithelial cells with cytoplasmic lipid droplets were observed at 2000h on day 5 of pregnancy in the control mice (Rb1f/f mice). Scale bar = 2µm; red dotted line, stroma; green dotted line, luminal epithelium; blue dotted line, trophoblast. 