Ultrastructure of oocyte migration through the mouse ovarian surface epithelium during neonatal development.

Previous studies using light microscopy have described the presence of oocytes within the ovarian surface epithelium (OSE) of the neonate ovary and their subsequent release into the periovarian space. The ultrastructural examination and quantitative estimate of oocyte migration through the OSE is described in this study. The surface of the mouse ovary is covered by a simple squamous to simple cuboidal epithelium resting on a distinct basal lamina. Healthy, non-atretic primordial follicles located in the periphery of the ovarian cortex interact with the OSE. Oocytes within the primordial follicles are large (50-70 microns), spherical cells surrounded by a single layer of squamous granulosa cells. Migratory oocytes initially display a ruffled border and extend pseudopodia-like cellular extensions towards the OSE. These processes subsequently compromise the basal lamina of the OSE and extend between the epithelial cells. Granulosa cells retract as junctional complexes between them and the oocyte are no longer observed. The oocyte migrates between the OSE cells and passes into the periovarian space without the loss of either OSE or granulosa cells. The rate of oocyte migration reaches a peak during the first week of neonatal development and then gradually diminishes until by day 28 of development no oocyte migration was observed.