Ultrastructure of mammalian oocytes and ova.

The complex ultrastructural changes of mammalian oocytes are presented. The follicular oocyte can be divided into 3 descriptive stages. First is the quiescent stage in which the majority of cytoplasmic organelles are concentrated in a crescentric basophilic juxtanuclear area (the Balbiani Vitelline body),. Second is the maturative stage, in which the follicle cells undergo mitoses and secrete the liquor folliculi. In this stage, the most salient changes in the oocyte are the development of microvilli, completion of the first meiotic division and changes in the structural organization of the cytoplasm. Third is the preovulatory stage in which the most prominent changes are the relationship between the oocyte and the cumulus cells. The changes that occur in the fallopian tubes before and after sperm penetration are described from mouse reproductive tracts fixed in situ by vascular perfusion. Shortly before sperm penetration the spaces between the cumulus and corona cells and the surface of the ovum widens and the cytoplasmic "bridges" are broken.