Changes in the localization of MHC class II positive cells in hen ovarian follicles during the processes of follicular growth, postovulatory regression and atresia

The aim of this study was to determine the changes in the population of major histocompatibility complex class II positive (MHC-II+) cells in ovarian follicles during the processes of follicular growth, postovulatory regression and follicular atresia in hens. Cryostat sections of ovarian stroma containing cortical follicles, small white follicles, the largest (F-1) and third largest (F-3) preovulatory follicles, postovulatory and atretic follicles of laying hens were prepared. The sections were immunostained for MHC-II molecules using mouse anti-chicken MHC-II monoclonal antibody and positive cells were counted using a computer-assisted image analyser under a light microscope. MHC-II+ cells were localized in the theca layer of normally growing follicles including cortical follicles, small white follicles and F-3 and F-1 preovulatory follicles, whereas they were found in both the theca and granulosa layers in postovulatory and atretic follicles, The frequency of MHC-II+ cells in the theca layer was significantly increased during follicular growth from cortical follicles to F-3 preovulatory follicles. Although the population of MHC-II+ cells did not differ between F-3 and F-1 preovulatory follicles, it increased significantly in postovulatory follicles (P < 0.01). The population of MHC-II+ cells was significantly greater in the theca layer of atretic follicles than in non-atretic follicles (P < 0.01). These results indicate that the antigen-presenting function via MHC-II increases in association with follicular growth. A marked increase in MHC-II+ cells indicates that these cells may be involved in regression of postovulatory and atretic follicular tissues.