Expression of bcl-2 and nr-13 in hen ovarian follicles during development.

Follicle atresia is initiated within the granulosa cell layer of ovarian follicles and is mediated via the process of apoptosis. In the hen, at least two populations of granulosa cells can be distinguished during follicle development on the basis of their inherent susceptibility or resistance to apoptosis, in vitro. Given the previously established correlation between expression of bcl-xLong and hen granulosa cell resistance to apoptosis, the present studies were conducted to characterize expression of bcl-2 and an avian bcl-2 homologue, nr-13, in follicles at various stages of development. Levels of nr-13 mRNA were significantly higher only in granulosa cells from the largest (F1) preovulatory follicle compared to 3- to 5-mm prehierarchal follicles. By comparison, bcl-2 mRNA levels were 5- to 9-fold higher in granulosa cells from the three largest preovulatory follicles compared to those from follicles 9 to 12 mm in diameter and prehierarchal follicles. The increase in neither nr-13 nor bcl-2 was correlated with the stage of follicle development associated with the acquisition of resistance to apoptosis in granulosa cells (e.g., at the 9- to 12-mm stage). Results from the present studies do not support a close correlation between constitutive expression of nr-13 or bcl-2 mRNA and the transition from a state of apoptosis susceptibility to apoptosis resistance in hen granulosa cells. Thus, it is proposed that nr-13 and bcl-2 play more of a supportive role in regulating additional aspects of ovarian cell function such as cell proliferation and/or differentiation.