Relationship of apical domes in the rabbit uterine epithelium during the peri-implantation period to endocytosis, apocrine secretion and fixation.

The luminal surfaces of non-ciliated uterine epithelial cells of 17 rabbits on Days 4--6 of pregnancy were studied histologically with light microscopy, transmission and scanning electron microscopy. The luminal surfaces of most epithelial cells exhibited short microvilli and did not project into the uterine lumen. However, the surfaces of some cells showed fewer microvilli and bulged out into the uterine lumen, giving the cells a dome-like appearance. The domes appeared most frequently on Day 6 of pregnancy and contained a few ribosomes, mitochondria, and membranous elements; large vacuoles and granules were absent. The domes were not involved in endocytosis. Ferritin introduced into the uterine lumen was incorporated into the cells by coated micropinocytotic invaginations at the base of the microvilli, rather than by any activity of the domes. There was also no indication that the domes pinched off to produce an apocrine secretion. The size and frequency of domes were, however, clearly related to the fixation procedure. Epithelial cells from uteri fixed by vascular perfusion showed fewer and less prominent domes, while cells from uteri fixed by immersion displayed a larger number of domes that projected further into the uterine lumen. The domes in the rabbit epithelium therefore differ structurally and functionally from the apical projections (pinopods) that occur in the uterine epithelium of the rat and mouse during the peri-implantation period.