SPONTANEOUS CORTICAL GRANULE RELEASE AND ALTERATION OF ZONA-PELLUCIDAPROPERTIES DURING AND AFTER MEIOTIC MATURATION OF MOUSE OOCYTES

Exocytosis of cortical granules (CGs) and the concomitant electron den sity changes of the zona pellucida (ZP) in the absence of sperm penetr ation were investigated in mouse oocytes processed with tannic acid co ntaining fixation at various stages during and after maturation. After fusion of the CG membrane with the plasma membrane, the CG contents b ecame very electron-dense, due to tannic acid. CG material is seen to be made up of coarse granular structures which gradually change to fin e amorphous structures, which accumulate within the developing perivit elline space (PVS). When the coarse CG material attaches to the ZP, sm all domains exhibiting higher electron density appeared, and the numbe r of these domains gradually increased. Release of CG was observed fro m metaphase I through metaphase II. In metaphase I to immediately afte r ovulation, the higher electron density of ZP and CG release was rest ricted to the cortical area overlying the meiotic spindle. Finally, th e CG-free domain formed itself overlying the meiotic spindle as a resu lt of CG release. However, in oviductal ova, CG release additionally o ccurred in the hemisphere opposite the spindle. At this stage the enti re PVS was well developed and contained numerous fine electron-dense m aterials. Moreover, the inner half of the ZP increased in electron den sity as well. This change in electron density of the ZP might be assoc iated with released CG material. These results suggest that the ''part ial cortical reaction'' may play an important role in conditioning the ZP prior to ZP reaction. (C) 1993 Wiley-Liss, Inc.