Irx3 is differentially up-regulated in female gonads during sex determination

Irx3 is a member of the Iroquois homeobox gene family that encodes a protein known for its essential role in spinal cord development. Transcript screening of male and female gonads during the critical period of sex determination (E12-13.5) revealed a sexually dimorphic expression pattern for Irx3 with female gonads exhibiting a sixfold increase in expression over time. Whole mount in situ hybridization confirmed the sexually dimorphic nature of Irx3 expression and immunohistochemical analysis of gonads at E13.5 determined that IRX3 and GATA4 proteins co-localized to somatic cells of XX gonads. The Irx3 signal persisted in germ cell-depleted XX gonads resulting from Busulfan treatment suggesting that its expression was independent of germ cell regulation. Quantitative real-time PCR analysis over an extended time course determined that Irx3 message was low initially and then increased in XX gonads until E13.5, remained elevated until birth, diminished shortly after birth, and remained low in the adult ovary. In contrast, Irx3 message was 50% lower in male compared to female gonads at the initial time point, and continued to decrease over time. Further analysis of adult ovaries suggested that IRX3 expression is not present in any subpopulations of cells of the differentiated ovary. Together, these results suggest that the Irx3 signal is restricted to the somatic cell component of XX gonads and is present at a discreet period of ovarian development that ends abruptly at birth. This timing coincides with the transition of female primordial germ cells from mitotic proliferation to meiotic division, and the organization of germ cell cysts prior to primordial follicle development at birth.