Ultrastructure of human cumulus-oocyte complexes from healthy and atretic follicles

Follicular fluids with cumulus-oocyte complexes and granulosa cells were aspirated from antral follicles in healthy women at different stages of the normal menstrual cycle. The cumulus-oocyte complexes were processed for light and electron microscopy and classified as pre-ovulatory, healthy non-pre-ovulatory and atretic by certain light microscopical criteria, i.e. degree of expansion, number of layers and pycnotic nuclei in the cumulus investment, size of the peri-vitelline space and oocyte nuclear morphology. Their follicles were similarly classified by follicular steroid measurements and/or flow cytometric DNA analyses of the aspirated granulosa cells. A high degree of correlation between these classifications was found. Moreover, pre-ovulatory and atretic cumulus-oocyte complexes showed particular fine morphological characteristics, some of which were different while others appeared to be identical. Similarities were increased number of lipid droplets in the cumulus cells, widened peri-vitelline space, peripheral displacement or breakdown of the oocyte nucleus and disconnection of the junctions between cumulus cell projections and the oolemma. In addition to the criteria used for the light microscopical classification, pre-ovulatory complexes were characterized by release of the content of cortical granules while atretic ones displayed cumulus cells with pseudopodia-like projections, cumulus cells in the perivitelline space and lipid droplets in the ooplasm.