Some Characteristics of Small White Ovarian Follicles: Implications for Recruitment and Atresia in the Domestic Hen

Granulosa cells from resting or slow-growing follicles that have yet to enter the preovulatory hierarchy (1to 8-mm in diameter) of the hen ovary are steroidogenically incompetent. The inability of granulosa tissue to synthesize the steroid precursor, pregnenolone, is due to the absence of cytochrome P450 side-chain cleavage (P450scc) enzyme activity. Cytochrome P450scc mRNA levels and enzyme activity can be induced in granulosa cells collected from 6to 8-mm follicles, in vitro, by pretreatment with agents that induce cyclic adenosine monophosphate (cAMP) formation (e.g. forskolin, FSH). Prior and up to the 6to 8-mm stage of development, the theca layer is solely responsible for ovarian progesterone, androgen and estrogen production; such steroidogenesis is under the regulatory control of pituitary gonadotropins (LH and, to a lesser extent, FSH). Spontaneous atresia is common in follicles 3to 5-mm in diameter, and is functionally characterized by the activation of a nuclear endonuclease that cleaves genomic DNA into a series of 185-b.p. fragments and ultimately results in resorption of the follicle. We propose that this process (termed apoptosis) also mediates ovarian regression at the termination of the breeding period in seasonally-breeding birds.