RELATIONSHIP BETWEEN THE SUPPLY OF PRIMORDIAL FOLLICLES AND THE ONSET OF FOLLICULAR-GROWTH IN RATS

Primordial follicles enter a state df suspended animation when they are formed and constitute a ''stockpile'' from which all growing follicles are derived. The factors that release individual follicles from their quiescent state are unknown. Many investigators believe that the number of follicles in the primordial stockpile is a major factor in determining the rate at which follicles begin to grow. However, the relationship between the size of the stockpile of primordial follicles and the rate at which follicles move into the growing pool is unclear. The purpose of the present study was to attempt to clarify this relationship. The initial size of the stockpile of primordial follicles was experimentally reduced by exposing rats to various doses of busulphan (BUS) in utero. Ovaries were collected at various ages postpartum and prepared for histological analysis. A computer-controlled image analyzer was used to perform size/frequency analysis of oocytes in control and treated ovaries; onset of follicular growth was recognized by enlargement of the oocyte. There was an inverse correlation between the number of primordial follicles in the ovary at birth and the rate at which they moved into the growing pool. In rats most severely affected by the BUS, all of the remaining follicles began to grow very early in life. By the time these severely affected rats reached adulthood, the stockpile of primordial follicles had been nearly exhausted. Nevertheless, the number of large antral follicles remained at normal levels until the follicular reserve was completely depleted. These observations may provide insight into the mechanisms that maintain normal ovarian function during the waning reproductive years but result in accelerated follicular loss at the approach of menopause.