Changes in the fine structure of the apical plasma membrane of endometrial epithelial cells during implantation in the rat.

In previous work we have shown that ovarian hormones, when injected into ovariectomized rats, alter the fine structure of the plasma membrane of endometrial epithelial cells. In this paper freeze-fractures have been used to study the apical plasma membrane of endometrial epithelial cells of rats during the period of blastocyst implantation of normal pregnancy. On day 1 of pregnancy there were 2354 +/- 114 intramembranous particles (IMPs) per micrometer2 of membrane. The particles were spherical and randomly distributed. On day 5 of pregnancy IMP density rose to 2899 +/- 289 per micrometer2 and some rod-shaped particles were also visible. By day 6 of pregnancy IMP density had risen to 4014 +/- 206 per micrometer2 and there were more rod-shaped IMPs than before. In addition, on day 6 IMPs were also present as rows of particles and some gap-junction-like arrays of particles were also seen. Our findings indicate that there are fine-structural alterations in the apical plasma membrane of endometrial epithelial cells, the site of first contact between maternal and embryonic cells, during the period of early pregnancy. The findings are discussed in the light of suggested mechanisms of blastocyst attachment to the uterine epithelium at implantation.