Decoding the transcriptome of pre-granulosa cells during the formation of primordial follicles in the mouse

Mammalian eggs are produced by the maturation of primordial follicles a finite reservoir of eggs in ovaries. Primordial follicles are composed of single oocyte and its supporting somatic cells termed granulosa cells. Although their formation must require reciprocal interplays between oocytes and pre-granulosa cells, the knowledge is limited because there are many cell populations in the developing ovaries. In this study, we focused on Lgr5-EGFP-positive pre-granulosa cells since this is a most reliable molecular marker to trace pre-granulosa cells. Using FACS sorted cells from wild-type and oocyte defective mutants Nanos3-KO and Figla-KO, we conducted transcriptome analyses. By sorting with developmental timing and each mutant group, we made several findings.1) The initial development of pre-granulosa cells is germ cell-independent.2) A drastic gene expression changes occurs between embryonic day17.5 and postnatal Day1 when oocytes become competent to undergo cyst-breakdown. The Notch signaling pathway genes play a central role via an oocyte-dependent manner. 3) The proper differentiation of granulosa cells require oocyte and this process is accompanied by the down-regulation of WNT signaling pathway.