Sexual differentiation of somatic and germ cell lineages of the mouse gonad

Abstract

The process of gonadal sex differentiation in mammals remains poorly understood despite the identification of a few key genes such as Sry and Sox9. To further our understanding of gonad organogenesis, a PCR based subtractive hybridization was performed to identify mouse genes that demonstrate sexually dimorphic expression patterns during embryonic development of the testis and ovary. I successfully identified nineteen genes that are upregulated during testis differentiation and two that are upregulated during ovary development. These genes are expressed predominantly in testicular and ovarian somatic cell types and encode several different classes of proteins. I analyzed the expression of these genes during the initial stages of gonadal sex differentiation and found that testis genes are upregulated in a center-to-pole pattern in which expression occurs in the central region of XY gonads and subsequently spreads to the anterior and posterior poles. In XX gonads I found that one ovarian gene, follistatin, is also expressed in a center-to-pole pattern, whereas a second ovarian gene, Adamtsl9, is expressed in an anterior-to-posterior wave. In addition, these ovarian genes are upregulated at the poles of XY gonads, suggesting that ovarian gene expression patterns are initiated at the poles of XY gonads before testis specific expression patterns are established. I have also found that the sex differentiation and meiotic entry of female germ cells occurs in an anterior-to-posterior wave. The presence of this wave indicates that local environmental cues in the ovary may regulate meiotic entry.

(cont.) I identified this anterior-to-posterior wave of differentiation using the Stra8 gene, which I show is a molecular marker of female sex differentiation. Through the generation of Stra8 knockout mice I have further demonstrated that this gene is required for male and female meiosis. The absence of Stra8 homologs in non-vertebrate species suggests that the study of this gene may provide unique insights into vertebrate meiosis. Finally, I describe the characterization of Tetl4, a testis specific X-linked gene. Tet14 encodes a novel open reading frame that lacks homology to other known genes. I have generated a conditional knockout to determine the functional requirement for TET14 in embryonic and adult mouse testes.