Stem cell factor and c-kit gene expression and protein localization in thesheep ovary during fetal development

The aim of this study was to investigate stem cell factor and c-kit gene ex pression and protein localization in the mesonephros and ovary of sheep fet uses at different days of gestation, using RNA in situ hybridization and im munohistochemical procedures. At days 24 and 26 of gestation, stem cell fac tor mRNA and protein were present in cells throughout the developing gonad and mesonephros. From day 28 to day 40 of gestation, stem cell. factor mRNA and protein became increasingly localized to the cortical region of the ov ary, where most germ cells were present as actively proliferating oogonia. From day 40 to day 90 of gestation, stem cell factor mRNA and protein local ization were confined mainly to the ovarian cortex. Moreover, within the co rtical region, stem cell factor mRNA was low or absent where follicles were first forming and highest in the outer ovarian cortex, where germ cells we re undergoing mitosis or the early stages of meiosis. in contrast, stem cel l factor protein was present in newly forming follicles, as well as in mito tic and meiotic germ cells, which is consistent with the presence of both m embrane-bound and soluble forms of this ligand. However, by day 90 of gesta tion, both stem cell factor mRNA and protein were observed in the granulosa cells of most (> 90%) primordial follicles. C-kit mRNA and protein were ob served from day 24 of gestation in both germ cells and somatic cells but, w ith increasing gestational age, preferentially in germ cells (for example, pre-meiotic germ cells and both isolated oocytes and follicle-enclosed oocy tes). C-kit protein, but not mRNA, was also observed in germ cells that wer e undergoing meiosis. The results indicate that the cells containing stem c ell factor mRNA within the ovary up to day 90 of gestation originated from the gonadal blastema and from cells that migrated from the mesonephros befo re day 28 of gestation. Since stem cell factor mRNA was absent in both meso nephric cells migrating after day 28 of gestation and in regions where foll icles were first forming, it is suggested that these later migrating mesone phric cells are the progenitors of the granulosa cells in the first forming follicles. in conclusion, during follicle formation, c-kit mRNA is localiz ed to germ cells whereas c-kit, together with stem cell factor protein, is localized to both germ cells and somatic cells, consistent with the hypothe sis that the presence of this receptor-ligand pair is essential to prevent apoptosis.