Ultrastructural evaluation of oocytes during atresia in rat ovarian follicles

Mammalian females are born with a finite number of ovarian oocytes, the vas t majority of which ultimately undergo degeneration by atresia. The overall process of ovarian follicular atresia has been morphologically well descri bed only in large antral follicles. Additionally, little attention has been focused on ultrastructural changes in the oocyte. furthermore, most such m orphological studies were performed prior to identification of apoptosis as a mechanism of physiological cell death. Therefore, the purpose of this st udy was to use electron microscopy to compare the process of atretic oocyte degradation in ovarian follicles of female Fischer 344 rats (38 days old) with ultrastructural characteristics of apoptosis. Examination of ovarian f ollicles revealed that nucleolar segregation, cytoplasmic or nuclear conden sation, apoptotic body formation, and chromatin margination along the nucle ar membrane are never observed in atretic oocytes during the degenerative p rocess. Instead, early morphological changes in atretic oocytes include ret raction of granulosa cell- and oocyte-derived microvilli and condensation o f mitochondria and loss of cristae. These occurrences coincide with initiat ion of granulosa cell apoptosis. After most granulosa cells are lost, more severe changes occur, including segmentation of the oocyte and cytoplasmic vacuolization as atresia progresses. Thus, these results suggest that, duri ng atresia, oocytes are removed by physiological oocyte cell death, a metho d that does not involve classically described apoptosis.