Development of preovulatory follicles and oocytes during the oestrous cycle of mature and aged rats.

Abstract

Abstract. The percentage of viable granulosa cells and the viability of the oocyte of each antral follicle on each day of the oestrous cycle of mature and aged rats was determined by trypan blue exclusion and fluorescein diacetate assays, respectively. In mature rats, the number of mid-sized follicles (350–550 μm in diameter) increased from oestrus through di-oestrus. The percentage of viable granulosa cells increased in only mid-sized follicles which contained a viable oocyte. On pro-oestrus, two types of preovulatory follicles (> 550 μm in diameter) were observed in the mature pro-oestrous rat: those with a high granulosa cell viability (> 60%) and a viable oocyte and those with a lower percentage of viable granulosa cells (< 50%) and a non-viable oocyte. This suggests that in mature rats only viable oocytes would be ovulated since presumably, only those preovulatory follicles with a high percentage of viable granulosa cells are able to ovulate. In aged rats, preovulatory-sized follicles were present on each day of the cycle. The percentage of viable granulosa cells within those preovulatory-sized follicles with nonviable oocytes increased on met-oestrus, decreased on di-oestrus and increased by pro-oestrus. This pattern resulted in the development of 3 to 4 preovulatory follicles per ovary with all preovulatory-sized follicles possessing a high percentage of viable granulosa cells regardless of the functional state of the oocyte. It is proposed that in aged rats, all the preovulatory follicles would ovulate but many of the ovulated oocytes would be defective. This mechanism would account for the lower fertility of these older animals.