Apoptosis in the degeneration process of unfertilized mouse ova

In mammals, ova which are not fertilized undergo degeneration. Thus, it seems that the ovum is programmed to die unless fertilization and embryogenesis occur, but little is known about the mechanism of such degeneration. To investigate this process, we observed the morphological changes of cultured unfertilized ova and stained DNA fragmentation by the modified TUNEL method (treating floating samples in liquid reagents) to detect apoptosis. Ova were collected from the oviducts of superovulated mice and cultured for observation. The number of morphologically abnormal ova with shrinkage of the ooplasm and cytoplasmic fragmentation, which are typical features of apoptosis, showed a significant gradual increase from 24 to 32 hr (p < 0.05) and an abrupt increase from 40 hr of incubation (p < 0.001). DNA fragmentation, which is one of the biological changes seen in apoptosis, was observed in the ooplasm of both intact and abnormal ova, cells with cytoplasmic fragmentation, and in the first polar body at the time of ovum collection as well as after incubation. These findings demonstrate that apoptosis is related to the process of degeneration in the mouse ovum and first polar body.