The internal and external genitalia are formed between 6and 13 weeks of gestation. Fetal gonad and external genitalia are bipotential and have the capacity to support development of a normal male or female phenotype (Fig. 177-1). In the presence of a gene called SRY for sex-determining regionon the Y chromosome, the primitive fetal gonad differentiatesinto a testis (Fig. 177-2). The Leydig cells of the testis secretetestosterone, which has direct effects (stimulating development of the wolffian ducts) but also is locally converted todihydrotestosterone (DHT) by the 5α-reductase enzyme.DHT causes enlargement, rugation, and fusion of the labioscrotal folds into a scrotum; fusion of the ventral surface of the penis to enclose a penile urethra; and enlargement of the 16.8 mm 49.0 mm 45.0 mm
Figure 177-1 Differentiation of male and
Labia minora female external genitalia as proceeding from a Scrotum common embryonic anlage. Testosterone acts at 9 to 13 weeks of gestation to virilize the bipotential anlage. In the absence of testosterone action, the female phenotype develops. (From Grumbach MM, Conte FA: Disorders of sexual differentiation. In Wilson JD, Foster DW, editors: Textbook of Endocrinology, ed 8, Philadelphia, 1990, WB Saunders, p 873; adapted from
Anus Spaulding MH, Contrib Embryol Instit 13:69−88, 1921.) phallus with ultimate development of male external genitalia.Testicular production and secretion of müllerian-inhibitorysubstance by Sertoli cells cause the regression and disappearance of the müllerian ducts and their derivatives, such as the fallopian tubes and uterus. In the presence of testosterone, the wolffian ducts develop into the vas deferens, seminiferoustubules, and prostate.
The female phenotype develops unless specific male influences alter development. In the absence of SRY, an ovary spontaneously develops from the bipotential, primitive gonad. Inthe absence of fetal testicular secretion of müllerian-inhibitorysubstance, a normal uterus, fallopian tubes, and posterior third of the vagina develop out of the müllerian ducts, and the wolffian ducts degenerate. In the total absence of androgens, the external genitalia appear female.
The terms intersex, hermaphroditism, and pseudohermaphroditism are not useful. DSDs are categorized under three main subgroups according to karyotype (XX, XY, and sex chromosome for mosaic karyotypes).
Figure 177-2 A diagrammatic scheme of male sex determination and differentiation. SRY is the master gene controlling male sex differentiation, but there are many other genes and their products that control male and female sexual differentiation. DHT, Dihydrotestosterone; SRY, the gene for the testis-determining factor. (From Wilson JD, Foster DW, editors: Williams Textbook of Endocrinology, ed 8, Philadelphia, 1992, WB Saunders, p 918.) 46,XX Disorders of Sexual Development
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Hypertension Amenorrhea Precocious Puberty in the Male Precocious Puberty in the Female Ambiguous Genitalia Alkalemia