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Many oocytes of antral follicles appeared to express reduced levels of KIT protein compared with those of smaller growing follicles (Fig. 4b). Intensely stained granules also were detected at the oolemma and in the cytoplasm of oocytes from primordial through antral follicles (Fig. 4, b-d). KIT protein was weakly detected in the theca cells of some preantral and antral follicles (not shown). All other somatic cells within the ovary were free of KIT immunoreactivity. Specific staining was not detected when antiserum was substituted with preimmune serum (Fig. 4a). Role of KITL During Early Folliculogenesis in the Rabbit In situ hybridization and immunolocalization studies demonstrated that KITL is expressed by the granulosa cells of some primordial follicles, and KITL expression increases during follicle activation and the formation of cuboidal granulosa cells. In contrast, KIT is expressed by all oocytes. The selective distribution of KITL in primordial follicles is consistent with the hypothesis that KITL availability is a limiting factor in follicle activation. Therefore, an in vitro approach was employed to investigate the effect of recombinant mouse KITL on follicle survival, follicular recruitment, and oocyte growth in rabbit ovarian cortical explants.