Expression of rabbit zone pellucida-1 messenger ribonucleic acid during early follicular development

Progress in research on initiation of folliculogenesis has progressed slowl y because of a lack of markers for early folliculogenesis. The rabbit zona pellucida protein (ZP1) is synthesized in follicles during early stages of folliculogenesis. In order to establish ZP1 as a marker for initiation of f olliculogenesis, in situ hybridization was used to localize ZP1 mRNA in imm ature follicles. ZP1 mRNA was first detected in oocytes of some but not all primordial follicles. The primordial follicles expressing ZP1 mRNA were lo cated at the cortico-medullary junction, indicating that they were newly ac tivated follicles. ZP1 mRNA accumulated in oocytes of intermediate, primary , and secondary follicles. In contrast, ZP1 mRNA was first detectable in gr anulosa cells of intermediate follicles and is present in cuboidal granulos a cells of primary and early secondary follicles, but was undetectable in g ranulosa cells of more mature follicles. These data demonstrate that 1) ZP1 mRNA is expressed in both oocytes and granulosa cells, 2) ZP1 mRNA is init ially expressed in oocytes of activated follicles, and 3) ZP1 mRNA is trans iently expressed in granulosa cells during early stages of folliculogenesis . Therefore, rabbit ZP1 is a molecular marker that can be used in future st udies to measure initiation of folliculogenesis.