Sexual differentiation can be defined as the series of events whereby the sexually indifferent fetus progressively acquires male or female characteristics in the gonads, genital tract and external genitalia.
Normal sex development consists of several sequential stages. Genetic sex, as determined by the chromosome constitution, drives the primitive gonad to differentiate into a testis or an ovary.
Subsequently, internal and external genitalia will follow the male pathway in the presence of specific testicular hormones, or the female pathway in their absence.
Since the presence of the fetal testis plays a determining role in the differentiation of the reproductive tract, the term " sex determination " has been coined to designate the differentiation of the gonad during early fetal development.
Testicular development is guided by testis determining factor (TDF), which is encoded by the SRY (sex-determining region on the Y chromosome) gene located on the short arm of the Y chromosome.
Under the influence of testis determining substance TDF, germ cells in the genital ridge differentiate (sexual cords) into Sertoli cells, which secrete müllerian inhibiting substance (MIS) (or müllerian inhibiting factor or MIF), and Leydig cells, which produce testosterone. MIF causes complete regression of the müllerian ducts, whereas testosterone promotes maturation of spermatogonia and regulates development of the male phenotype by means of paracrine and endocrine actions. By means of paracrine action, the wolffian duct develops into the epididymis, vas deferens, ejaculatory duct, and seminal vesicles.
In the absence of the Y chromosome, gonads differentiate into ovaries at around 11–13 weeks gestation. Ovarian hormones are thought to play no role in female phenotype differentiation. Absence of MIF leads to persistence of müllerian structures, which develop into the fallopian tubes, uterus, cervix, and upper vagina. Because of the absence of testosterone, the wolffian ducts involute.
Pathology (Anomalies of sex differentiation)
disorders of sexual differentiation (DSDs) or intersex disorders (intersexuality or intersex)
See also
primitive gonad
- gonadal ridge
- primitive germ cells
male sex determination
- testicular differentiation
female sex determination
- ovarian differentiation
sexual dimorphism
Online textbooks
References
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