Morphological changes in mouse eggs due to aging in the fallopian tube

Morphological changes in mouse eggs due to aging become evident after the eggs have spent approximately 12 hours in the oviduct. The spindle rotates at first and is radially disposed. Later the entire spindle migrates to the center of the egg. In most eggs the spindle is maintained throughout the first 24 hours after ovulation. In some eggs a nucleus is reconstituted without formation of polar body II. Other eggs break into several cytoplasmic fragments but a nucleus is seen only in the largest fragment. The cortical granules lose their proximity to the cell membrane, migrate centrad and frequently swell. The cumulus cells and the cells of the corona radiata degenerate and/or disperse.