Postovulatory aging of the rat oocyte: Cytologic changes

The purpose of this study was to determine whether the morphologic changes associated with postovulatory aging of the rat oocyte represent a random crumbling of cell structure or an orderly process with a predictable sequence and timing. Unfertilized rat oocytes recovered from the oviducts at various intervals after ovulation were examined by light and electron microscopy to assess the occurrence of time related cytologic changes. Fertilized ova were also studied to compare the normal pattern of development with the process of aging. The majority of the oocytes examined within the first 10 h after ovulation had a pyriform shape, were devoid of polar body and contained a meiotic spindle in the protruding zone. The remaining oocytes during this period were round and in a few of them the first polar body was visible in the perivitelline space. At about 18 h after ovulation, most oocytes had changed from a pyriform to a round shape and the second polar body was present in the perivitelline space. A large proportion of the oocytes recovered at subsequent times had undergone fragmentation giving rise to cytoplasmic masses of various sizes. Nucleus and/or nucleolus were present in the majority of these late round oocytes as well as in many cytoplasmic masses of fragmented oocytes. The changes observed with the light microscope were paralleled by orderly ultrastructural modifications that affected the microvilli, the cortical granules, the chromosomes, and the location of some organelles. Collectively, these changes partially resemble the activation that follows fertilization and suggest that the rat oocyte undergoes spontaneous activation prior to its cytolysis. It is concluded that the rat oocyte fulfills important requirements for studying morphological correlates of the loss of fertilizability and potential for normal development in delayed insemination experiments.