Chapter 6 - Sex Determination and Differentiation

Abstract

This chapter focuses on sex determination and sex differentiation. In mammals, the primary sex determinant is genetic, and therefore the first step in sexual differentiation is the establishment of chromosomal sex at fertilization. For the most part, the initial stages of fetal development do not show any morphological differences between the sexes until the gonad develops. The second step in sexual differentiation is the choice of the bipotential gonad to become an ovary or a testis. The third step is the development of the sexual phenotype of the fetus, some aspects of which are controlled by diffusible substances produced by the gonads. Disorders of sexual differentiation can affect all three steps, and their analysis is essential in the identification of genes regulating this process. The development of most male-specific structures is dependent on testosterone produced by the Leydig cells of the embryonic testis. Testosterone regulates three main aspects of male phenotypic development: the conversion of the wolffian ducts into the epididymis, vas deferens, and seminal vesicles; the formation of the male urethra and prostate from the urogenital sinus; and the formation of the phallus and scrotum from the genital tubercle and urethral folds.