This study examined structural changes in oocyte and follicular wall during oocyte maturation in Japanese quails. The structures of the germinal disc and the surrounding follicular wall were observed by light and electron microscopy 25, 6 and 1 h before the expected time of ovulation. The germinal disc of the oocyte was located near the oocyte plasma membrane at 25 h before ovulation, and the germinal vesicle was located in the centre of the germinal disc. Numerous cytoplasmic elements, such as elongated membrane-bound vesicles, mitochondria and glycogen granules were also observed in the germinal disc. The surface of the oocyte made close contact with the cytoplasmic processes of the granulosa cells. Six hours before ovulation, fluid filled spaces formed between the oocyte and follicular wall. At 6 h before ovulation, the germinal disc was similar to that at 25 h before ovulation, whereas the oocyte and the granulosa cells were disconnected. Myelin bodies and dense bodies developed in the cytoplasmic processes of the granulosa cells, suggesting that lysosomal enzymes were activated. In the follicle at 1 h before ovulation, the second maturation spindle was located just beneath the surface of the oocyte, and the first polar body was in the perivitelline space. In the germinal disc, the membrane-bound vesicles were swollen and well developed. We suggest that, during the process of early oocyte maturation, the junctions between the oocyte and granulosa cells are disconnected, and factors that promote oocyte maturation may be activated in the germinal disc since the membrane-bound vesicles are developed.
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