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What causes some ovarian follicles to degenerate instead of maturing?

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Detailed cross-sectional illustration of a human ovary showing the stages of follicle development, ovulation, and corpus luteum formation, with labeled structures including primordial, primary, secondary, and mature follicles, a ruptured follicle releasing an ovum, and corpus luteum and albicans. An inset at the bottom shows the ovary's location in the female reproductive system. — ovary The steps of ovulation, beginning with a dormant primordial follicle that grows and matures and is eventually released from the ovary into the fallopian tube.

In all vertebrates, the degeneration of ovarian follicles—a process known as atresia—is a normal biological mechanism that serves to reduce the number of eggs that are ultimately ovulated.1

While follicles begin to grow and mature under the influence of gonadotropins (such as follicle-stimulating hormone and luteinizing hormone), they may cease development and undergo atresia at any time prior to ovulation. This process effectively limits the number of ovulatable eggs; for example, in small laboratory rodents, approximately 50 percent of Graafian follicles undergo atresia shortly before ovulation. Once a follicle becomes atretic, it is eventually lost within the stroma of the ovarian cortex.21