Reptilian and avian follicular hierarchies: models for the study of ovarian development.

The presence of an ovarian follicular hierarchy is a characteristic feature of reptiles and birds. The hierarchy contains follicles at all stages of maturation and therefore, varying degrees of sensitivity to the ovulation-inducing effects of the gonadotropins. In the hen, ovulability is gained as the ability of the follicle to produce androgens and estrogens declines and the ability to produce progesterone increases. In the mature follicle, the granulosa cells are the site of progesterone production whereas the theca cells produce androgens and estrogens. Small follicles that have not yet been (and may never be) recruited into the yolk-filled hierarchy are the major producers of androgens and estrogens within the ovary. In reptiles the ovarian follicular hierarchy includes non-vitellogenic follicles and in some species includes follicles destined to become atretic. These two features distinguish the reptiles from the birds and provide experimental biologists with a unique model to investigate the physiological events that regulate the most common fate of ovarian follicles, atresia.