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Chapter 241.Development of the Reproductive System (workbook question 15-19)Initially, in embryonic development the reproductive structures of male and female are homologous (the same). They consist of one pair of primary sex organs, or gonads, and two pairs of ducts: the mesonephric ducts (Wolffian) and the paramesonephric ducts (Mullerian). The Mullerian ducts are the precursor of the internal female sex organs (ovaries, uterus, cervix, and upper vagina). Mullerian ducts are initially formed regardless of genotypic sex and require no sex-determining region on the Y chromosome (SRY) signaling for development. SRY signaling is required in males to cause regression of the Mullerian ducts, which in turn prevents the development of the female reproductive tract. The Wolffian ducts are the precursors of male internal sex organs (secrete testosterone and promote the development of the male sex organs).Both pairs of ducts empty into an opening called the urogenital sinus. 2.Male sex organ development during gestation (workbook question 15)The first sign of development of the reproductive organs occurs during the 5thweek of gestation. Between 6-7 weeks, the male embryo will differentiate under the influence of testes-determining factor (TDF), a protein expressed by a gene in the SRY. When the SRY gene is expressed, male gonads develop into the two testes, and by 8 weeks of gestation, testosterone secretion begins. Mullerian inhibitory hormone (MIH), secreted by Sertoli cells in the testes, promotes degeneration of the Mullerian ducts. Without MIH, the Mullerian ducts would develop, and the Wolffian would degenerate with loss of male sex organ development. By 9 months, the male gonads have descended into the scrotum. The testes produce sperm after puberty. 3.Female sex organ development (workbook question 16, 17)Female gonadal development occurs in the absence of SRY expression and with the expression of other genes. The presence of estrogen and the absence of testosterone and MIH cause a loss of the Wolffian system and, at 6-8 weeks gestation, the two female gonads develop into ovaries, which will produce ova. By the 10thweek, the mesonephric ducts deteriorate, and the upper ends of the paramesonephric ducts become the fallopian tubules whereas the lower ends join tobecome the uterus, cervix, and upper two-thirds of the vagina. The fallopian tubes carry ova from the ovaries to the uterus during a woman's reproductive years. Lack of testosterone and the presence of estrogen promote the development of external genitalia (lower end of the vagina, labia, and clitoris.