During gonadal development in mice, FGFR2 signalling in response to activation by FGF9 is required for male sex determination at embryonic day E11.5. However, whether FGFR2 is required for either testis or ovary development after sex determination remains unresolved. To investigate the role of FGFR2 in the fetal testis, Fgfr2 was deleted conditionally in Sertoli cells after sex determination at E13.5 by generating XY Fgfr2fl/fl; Amh-Cre mice. XY mutant gonads exhibited no testicular phenotype when analysed at 15.5 dpc and 5 months of age. Re-examination of FGFR2 protein localisation in XY wildtype gonads using immunofluorescence demonstrated that Sertoli cells lacked FGFR2 expression after E11.5. Intriguingly, in XX wildtype gonads, immunofluorescence analysis showed strong FGFR2 protein expression in somatic cells at E12.5-E15.5. Therefore, we investigated the role of FGFR2 in the developing ovary.
Gene expression analysis of sorted gonadal cells from E12.5 XX Wt1-RG reporter mice indicated that ovarian somatic cells mostly expressed the FGFR2 splice variant; Fgfr2c. To investigate the function of FGFR2c in the ovary, we analysed XX gonads from Fgfr2c-/-fetal mice. At E13.5-15.5, XX Fgfr2c-/-gonads exhibited normal morphology and granulosa cell differentiation, however a severe loss of germ cells was observed without evidence of increased cell apoptosis or reduced cell proliferation. To determine whether the FGFR2c-dependent germ cell survival involves the FOXL2 pathway, we analysed XX Fgfr2c-/-; Foxl2-/- gonads. Loss of Foxl2 from XX Fgfr2c-/- gonads lead to the rescue of germ cell numbers. Together, our results indicate that after the sex determination stage, FGFR2 signalling is not required in Sertoli cells of the testis but plays an important role in the ovary. In particular, the FGFR2C isoform specifically in the pregranulosa cells supports proper germ cell development and prevents germ cell apoptosis.