Freeze-fracture observations of ovulated hamster oocytes with their cumulus cells.

Mature unfertilized oocytes in the cumulus oophorus were examined using the freeze-fracture technique. The P face of the oocyte plasma membrane (oolemma) has randomly distributed intramembranous particles (IMPs) with an average diameter of about 10 nm. No marked difference is found between the intermicrovillous and microvillous regions of the oocyte with respect to the size and distribution of IMPs. The E face of the oolemma has fewer IMPs than the P face. The intermicrovillous region is slightly elevated in some places because of close contact with underlying cortical granules. The oolemma of this elevated region and the membrane of the cortical granule have IMPs which are considerably smaller than those in the remaining areas of the oolemma. The IMP density of the P face of the cortical granule membrane is much lower than that of the E face, possibly indicating a membrane specialization required for fusion of the cortical granule membrane with the overlying oolemma at the time of exocytosis. Cytoplasmic lamellae appear as parallel bands of particles in freeze-fractured specimens. No gap junctions are found between the oocyte and cumulus cells, but residual gap junctions are seen among clustered cumulus cells.