This chapter discusses some of the physiological and pathophysiological actions of hormones in controlling the development of the sexual phenotypes of eutherian mammals, the American opossum, and the chicken. The importance of steroid metabolites formed in extraglandular tissues to overall sexual development is also emphasized. Chromosomal sex determines gonadal sex, and gonadal sex determines phenotypic sex. A minimum of 19 genes are implicated in sexual differentiation in man. Some of these are located on sex chromosomes and some on autosomes. Thus, the relatively simple mechanism that imposes male development on the indifferent embryo requires the participation of many genes common to both the male and female embryo. Determinants on the Y chromosome cause the indifferent gonad to develop into a testis. Two hormonal secretions from the fetal testis, mullerian-inhibiting substance and testosterone, then transform the indifferent urogenital tract into one characteristic of the male. Mullerian-inhibiting substance secreted by the Sertoli cells causes regression of the female duct system.
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