ENDOCRINE CONTROL OF SEXUAL DIFFERENTIATION IN THE HUMAN ,

Abstract

Publisher Summary This chapter discusses the endocrine control of sexual differentiation in the human. It describes sexual differentiation as an ordered and sequential process. Chromosomal sex, established during fertilization, directs the development of gonadal sex. Hormonal secretions from the fetal testes then transform the phenotypically indifferent embryonic urogenital tract into that of the male. Despite the difference in chromosomal constitution, the gonads and the urogenital tracts of male and female embryos develop initially in an identical manner. The first evidence of sexual dimorphism in the human embryo is the appearance of the spermatogenic cords in the fetal testes between week 6 and week 7 of gestation. The factors involved in the differentiation of the ovaries and testes are not fully understood, but the characterization of the male-specific histocompatibility antigen (HY antigen) has provided a working model to explain the mechanisms by which the Y chromosome directs testicular development. During the indifferent stage of sexual development, the urogenital tract of the embyro consists of two components: (1) two duct systems (wolffian and mullerian), derived from the mesonephros, and (2) the urogenital sinus and genital tubercle. The development of the internal urogenital tract of the male involves regression and ultimate disappearance of the mullerian ducts as well as the growth and differentiation of the wolffian ducts into the epididymides, vasa deferentia, and seminal vesicles. In females, the mullerian ducts persist to form the fallopian tubes, the uterus, and the upper portion of the vagina, and the wolffian ducts degenerate. Thus, the internal genital tracts develop from different anlagen in the two sexes.