When the gene for fibroblast growth factor 9 (Fgf9) is knocked out in mice, the homozygous mutants are almost always female. Fgf9 protein, whose expression is dependent on Sox9 (Capel et al. 1999; Colvin et al. 2001), plays several roles in testis formation:
- Fgf9 causes proliferation of the Sertoli cell precursors and stimulates their differentiation (Schmahl et al. 2004; Willerton et al. 2004).
- It activates the migration of blood vessel cells from the adjacent mesonephros into the XY gonad. While this is normally a male-specific process, incubating XX gonads in Fgf9 leads to the migration of endothelial cells into XX gonads. These blood vessel cells form the major artery of the testis and play an instructive role in inducing the Sertoli cell precursors to form the testis cords; in their absence, testis cords do not form (Brennan et al. 2002; Combes et al. 2009).
- It is required for maintaining Sox9 expression in the presumptive Sertoli cells and directs their formation into tubules. Moreover, since it can act as both an autocrine and a paracrine factor, Fgf9 may coordinate Sertoli cell development by reinforcing Sox9 expression in all the cells of the tissue (Hiramatsu et al. 2009). Such a “community effect” may be important in achieving the integrated assembly of testis tubules (Palmer and Burgoyne 1991; Cool and Capel 2009).
- It represses Wnt4 signaling, which would otherwise direct ovarian development (Maatouk et al. 2008; Jameson et al. 2012). If the Fgf receptor recognizing Fgf9 is knocked out of the genome of XY mice, the Wnt pathway is not inhibited, and the mice develop ovaries rather than testes (Bagheri-Fam et al. 2015, 2017).
- Finally, Fgf9 appears to help coordinate the sex determination of the gonad with that of the germ cells. As we will see later in this chapter, those mammalian germ cells destined to become eggs enter meiosis quickly upon entering the gonad, whereas germ cells destined to become sperm delay their entry into meiosis until puberty. Fgf9 is one of the factors that block the immediate entry of germ cells into meiosis, thereby placing them in the sperm-forming pathway (Barrios et al. 2010; Bowles et al. 2010).
The transcription factor steroidogenic factor 1 (Sf1, also called Nr5a1) is necessary to make the bipotential gonad. But whereas Sf1 levels decline in the genital ridge of XX mouse embryos, they remain high in the developing testis. It is thought that Sry either directly or indirectly maintains Sf1 gene expression. Sf1 protein appears to be active in masculinizing both the Leydig and the Sertoli cells. In the Sertoli cells, Sf1 works in collaboration with Sry to activate Sox9 (Sekido and Lovell-Badge 2008) and then, working with Sox9, elevates levels of anti-Müllerian hormone transcription (Shen et al. 1994; Arango et al. 1999). In the Leydig cells, Sf1 activates genes encoding the enzymes that make testosterone.