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Chapter 27: Reproductive System Development by John F. NeasSex Determination The genetic sex of the zygote (fertilized egg) becomes determined at conception when the ovum is fertilized by a sperm that contains either an X or a Y chromosome. A sperm that contains an X chromosome will pair with the X chromosome of the ovum and a female child will develop. A sperm that carries a Y chromosome results in an XY combination, and a male child will develop. Genetic sex determines whether the gonads will develop into testes or ovaries. If testes develop, they will produce and secrete male sex hormones during late embryonic and early fetal development that cause the secondary sex organs of the male to develop. Development of Gonads Indifferent Stage Endodermal cells, called primordial germ cells, migrate from the wall of the yolk sac near the allantois to the dorsal wall of the abdominal cavity during the third week. The primordial germ cells enter the gonadal ridges that parallel the mesonephros. Each ridge has a thick epithelium continuous with columns of cells called primary sex cords that extend into the center (medulla) of the ridge. At this gender indifferent stage, male embryos are indistinguishable from female embryos. Development of the Testes The primary sex cords in the male proliferate and the germ cells migrate into the sex cords. The resulting testis cords produce the seminiferous tubules. Connections develop between the arching testis cords and the adjacent mesonephric tubules. The mesonephric tubules later degenerate, but the seminiferous tubules remain connected to the mesonephric duct. Development of the Ovaries The primary sex cords in the female embryo degenerate and the primordial germ cells migrate into the outer region (cortex) of the genital ridge that will become the ovary. Development of Ducts and Accessory Organs Indifferent Stage A paramesonephric (Müllerian) duct develops anterior to each mesonephric duct in both sexes. The paramesonephric duct has no connection to the kidneys; instead it extends along the genital ridge and continues toward the cloaca. Thus, both genders have mesonephric and paramesonephric ducts during the indifferent stage of sexual development. The embryo—regardless of its genetic sex—will develop into a female unless exposed to androgens. In a normal male embryo, cells in the medulla of the gonadal ridge begin to produce testosterone sometime after the sixth week. Testosterone triggers changes in the duct system and external genitalia shown in these figures. Development of Genital Ducts in the Male In the male, the paramesonephric ducts degenerate. After 4 months of development, the testis cords are connected to the remnants of the mesonephric tubules by the rete testis. The testes acquire their definitive organization after they descend into the scrotum. Development of Genital Ducts in the Female The mesonephric tubules and duct degenerate in the female embryo. The paramesonephric ducts develop a broad proximal opening into the peritoneal cavity; they fuse distally to form the uterus. Development of External Genitalia Indifferent Stage After four weeks of development, mesenchymal swellings called cloacal folds develop around the cloacal membrane. A genital tubercle develops that will form the glans of the penis in males and the clitoris in females. At six weeks, the cloaca has been subdivided, separating the cloacal membrane into a posterior anal membrane bounded by anal folds, and an anterior urogenital membrane bounded by the urethral folds. A prominent genital swelling develops lateral to each urethral fold. Development of Male External Genitalia In the male embryo at 10 weeks, the genital tubercle has enlarged, the tips of the urethral folds have moved together to form the penile urethra, and paired scrotal swellings have developed from the genital swellings. Development of Female External Genitalia In the female, the urethral folds do not fuse but develop into the labia minora. The genital swellings will become the labia majora. The genital tubercle develops into the clitoris, immediately posterior to which the urethra opens to the exterior. The hymen persists as an elaboration of the urogenital membrane.
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