Sophronia L. Gallop
Sexual differentiation is a sequential process beginning with the establishment of chromosomal sex at fertilization, followed by the development of gonadal sex, and culmination in the development of secondary Sexual characteristics, collectively termed the male and female phenotypes.
Chromosomal Sex
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Gonadal Sex
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Phenotypic Sex
The paradigm for sexual differentiation.
Development of Gonadal Sex
The gonadal ridges are formed during the third and fourth weeks of embryogenesis by a proliferation of the coelomic epithelium and condensation of the underlying mesenchyme on each side of the midline between the primitive kidney (mesomesenchyme) and the dorsal mesentery. Initially these gonadal primordia do not contain germ cells; the germ cells are located in the endodum of the yolk sac near the allantoic evagination. During the fifth week of gestation the germ cells begin to leave the primitive gut and migrate through the mesentery to the gonadal ridges. During their migration, they increase in number by means of mitotic division.
By the sixth week of gestation, germ cell migration is completed and the gonads of the male and female embryo are indistinguishable. At this time they are composed of three distinct cell types: (1) germ cells, (2) supporting cells that are derived from the coelomic epithelium of the genital ridge and that will differentiate either into Sertoli cells of the testes or the granulosa cells of the ovary, and (3) stromal (interstitial) cells derived from the mesenchyme of the gonadal ridge. During the seventh week of gestation, the fetal testis begins to differentiate: The primitive seminiferous tubules form first, followed by differentiation of the interstitial cells and the onset of testosterone formation on the interstitial cell compartment at approximately eight weeks.
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The limiting of prenatal sexual differentiation in the human.
Male Development
The initial event in the virilization of the male urogenital tract the onset of miillerian duct regression, which coincides with the development of the spermatogenic cords in the fetal testis at approximately eight weeks of gestation.
The transformation of the wolffian ducts into the male genital tract begins after the onset of miillerian duct regression. The mesonephric tubules adjacent to the testis (the epigenital tubules) lose their primitive glomeruli and establish contact with the developing rete and spermatogenic tubules of the testis to form the efferent ductules of the testis. The portion of the wolffrain duct immediately caudal to the efferent ductules becomes elongated and convoluted to form the epididymis, and the middle portion of the duct develops thick muscular walls to become the vas deferens.
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At about the thirteenth week of gestation, the seminal vesicles begin to develop from the lower portions of the wolffian ducts near the urogenital sinus. The terminal portions of the ducts between the developing seminal vesicles and the urethra become the ejaculatory ducts and the ampullae of the vas deferens.
The prostatic and membranous portions of the male urethra develop from the pelvic portion of the urogenital sinus. At about 10 weeks of gestation prostatic buds begin to form in the mesenchyme surrounding the pelvic urethra. The budding is most extensive in the area surrounding the entry of the wolffian ducts (the ejaculatory ducts) into the male urethra. Although differentiation of the prostate occurs early in embryogenesis, growth and development of the gland continue into postnatal life.
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Female Development
The internal reproductive tract of the female is formed from the miillerian ducts, and the wolffian ducts for the most part degenerate. The cephalic ends of the miillerian ducts (the portions derived from coelomic epithelium) are the anlagen of the Fallopian tubes, and the caudal portions fuse to form the uterus. The cervix of the uterus can be recognized by nine weeks of development, and the formation of the muscular walls of the uterus (myometrium) from the mesenchyme that surrounds the miillerian ducts is completed by approximately 17 weeks of age.
Development of the vagina begins at approximately nine weeks of gestation wit the formation of a solid mass of cells (the uterovaginal plate) between the caudal buds of the miillerian ducts and the dorsal wall of the urogenital sinus. The cells of the uterovaginal plate subsequently proliferate, thus increasing the distance between the uterus and urogenital sinus. At 11 weeks of development a lumen begins to from in the caudal end of the vaginal plate, and by 20 weeks gestation the vagina is completely canalized. It is currently believed that the upper one-third of the vagina is derived from the miillerian ducts and the remainder is derived from the urogenital sinus.
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