Structural changes in the oviductal wall during passage of unfertilized cumulus-oocyte complexes in mice

Little information is available on the structural relationship of cumulus-oocyte complexes and the oviductal wall during the transport of cumulus-oocyte complexes.

To this end, morphological changes of the oviductal wall during the passage of unfertilized cumulus-oocyte complexes were examined chronologically in ICR mice 25-27 days of age injected with PMSG and hCG. Mice were sacrificed at 12, 14, 16, 18, and 24 hr after the injection of hCG to remove oviducts, and the height of mucosal folds, muscle layers, and epithelial cells was measured in the serial sections stained with hematoxylineosin or colloidal iron.

The height of the mucosal fold and muscle layer where cumulus-oocyte complexes were located was less than that of the adjacent portions. At 12-18 hr of hCG injection (approximately 2-8 hr after ovulation), the ova with surrounding cumulus cells lie free in a wide lumen, and the muscular tissue consists of only 2 or 3 layers of cells, arranged mostly longitudinally. However, a neighboring portion without cumulus-oocyte complexes, where the folds meet in the middle, appreciably restricts the free space in the lumen. After 24 hr of hCG administration, structural changes in the oviductal wall, where cumulus-oocyte complexes were located, were no longer apparent. The number of cumulus cells surrounding the oocyte decreased during the passage through the oviduct. At 12-18 hr after hCG injection, approximately 140 cells were identified in the largest cross section of a cumulus-oocyte complex, but after 24 hr of hCG administration (approximately 14 hr after ovulation), an oocyte was surrounded with only approximately 25 cells.

These results indicate that oocyte-cumulus cell complexes influence the structure of the oviductal wall during the passage in the oviduct.