Activation of Primordial Follicles
The development of ovarian follicles is a protracted process. Even in rodents, it is estimated to take about 8 weeks for development from the primordial to the preovulatory stage (Hirshfield 1991), and estimates for larger mammals like humans and cattle are around 6 months (Gougeon 1996). Although follicular growth and differentiation are gradual and continuous processes, there are critical transitional points in follicular development. They include initiation of follicular growth, selection of a dominant follicle capable of ovulating, and the transformation of a preovulatory follicle into a periovulatory follicle in response to the luteinizing hormone/follicle-stimulating hormone (LH/FSH) surge. This review will focus on the first of these critical transitions, the initiation of follicular growth. Understanding the signals that regulate the activation of primordial follicles is of practical importance since it could stimulate new ideas for contraceptive approaches or lead to methods to enhance fertility in women, domestic animals, and endangered species.
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