Development of a cortical granule-free area of cortex and the perivitelline space in the hamster oocyte during maturation and following ovulation.

The distribution and behavior of cortical granules (CGs) in hamster oocytes at various stages of maturation were examined using both light (phase-contrast) and electron microscopy. At the germinal vesicle stage, CGs were distributed almost evenly in the cortex of the oocyte. A 'small' CG-free area of the cortex, with prominent cytoplasmic protrusions, appeared twice during the progression of meiosis. The first time, immediately above the metaphase spindle of the first meiosis and secondly, above the metaphase spindle of the second meiosis. Both peripheral migration and exocytosis of CGs appeared to be responsible for the formation of CG-free cortex above the metaphase spindle of first meiosis. The development of CG-free cortex above the metaphase spindle of the second meiosis was definitely due to exocytosis of CGs. The CG-free cortex above the metaphase spindle of the second meiosis increased its size steadily even after the oocytes had been transported into oviducts. Concomitantly, the size of the perivitelline space increased. The enlargement of the perivitelline space in unfertilized oviductal oocytes seems to be attributed not only to the exocytosis of CGs, but also to an extrusion of non-CG materials by the oocytes and perhaps an accumulation of some materials secreted by the oviduct epithelium. The limited (premature) CG-exocytosis during oocyte maturation and prior to fertilization may alter the physico-chemical properties of the zona pellucida slightly in such a way that the zona can be penetrated only by a very 'strong' spermatozoon. The presence of the perivitelline space in the mature oocyte prior to fertilization seems to be essential or at least beneficial to normal fertilization. Distinct cytoplasmic protrusions appear on the cortex above the metaphase spindle. The plasma membrane covering these protrusions seems to represent the membrane synthetized by the oocyte in preparation for the extrusion of polar body.