Chapter 6: Sex Determination & Gametogenesis

Sex Determination & Gametogenesis begins by distinguishing between primary sex determination, which is the chromosomal decision guiding the bipotential gonad to become testes or ovaries, and secondary sex determination, which involves the hormonal structuring of the sexual phenotype. In placental mammals, the Y-chromosome-specific Sry gene is identified as the master switch that activates the autosomal Sox9 gene, initiating a feed-forward loop with Fgf9 to suppress ovarian genes and drive testicular differentiation into Sertoli and Leydig cells. The summary details how the opposing ovarian pathway relies on Wnt4 and R-spondin1 signaling to stabilize beta-catenin and upregulate Foxl2, effectively blocking testis formation. The text further explains the hormonal cascade where testicular anti-Mullerian hormone (AMH) triggers the regression of Mullerian ducts, while testosterone and its potent derivative dihydrotestosterone (DHT) stimulate the development of Wolffian ducts and external genitalia. Comparative analysis is provided through Drosophila melanogaster, where sex is determined by the number of X chromosomes activating the Sex-lethal (Sxl) gene, which orchestrates an RNA splicing cascade involving transformer and doublesex to control somatic and behavioral traits. Additionally, the chapter explores temperature-dependent sex determination in reptiles, highlighting how environmental conditions influence gonadal fate. The final sections detail the distinct processes of gametogenesis, contrasting the continuous, stem-cell-reliant process of spermatogenesis and spermiogenesis in males with the interrupted, finite process of oogenesis in females. This includes an in-depth look at the regulation of meiosis via retinoic acid and Stra8, the formation of the synaptonemal complex, and the clinical correlations between maternal age, cohesin loss, and chromosomal aneuploidy.