Excerpt from Genital Anomalies

Synonyms, Key Words, and Related Terms: genital anomalies, abnormalities of the male and female external genitalia, penile agenesis, aphallia, penile duplication, diphallia, microphallus, micropenis, penile torsion, lateral penile curvature, penile deviation, penoscrotal transposition, webbed penis, buried penis, hidden penis, agenesis of the scrotum, congenital absence of the scrotum, ectopic scrotum, accessory scrotum, accessory perineal scrotum, splenogonadal fusion, labial adhesion, ectopic labium, ectopic labium majus, clitoral duplication, clitoral hypertrophy, interlabial masses, urethral prolapse, prolapsed ectopic ureterocele, hydrocolpos, hydrometrocolpos, sarcoma botryoides, rhabdomyosarcoma, periurethral cyst, persistent urogenital sinus, persistent cloaca

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Disorders of the external genitalia are especially troubling for parents because of the unconscious emotional significance of these reproductive structures and, probably, the consequent impact of deformities on future generations.

Approximately 50 years ago, researchers demonstrated that castrated rabbit embryos of both sexes developed as females, thus proving that testes are required for development of the male phenotype in mammals. Differentiation of the primitive gonad into testis is controlled by a multitude of genes, beginning with the sex-determining region on the Y chromosome (SRY), which is believed to represent the testis-determining factor.

Until the eighth week of embryonic development, the fetus is undifferentiated sexually and contains both male (wolffian) and female (müllerian) genital ducts. Wolffian structures differentiate into the vas deferens, epididymis, and seminal vesicles. Müllerian ducts develop into the fallopian tubes, uterus, and upper one third of the vagina.

In the male fetus, the genital tubercle enlarges to form the penis; the genital folds become the shaft of the penis; and the labioscrotal folds fuse to form the scrotum. Differentiation occurs during 12-16 weeks of gestation and is the result of testicular hormones acting upon the undifferentiated genitalia in the following ways:

Testicular secretion of antimüllerian hormone (AMH), also known as müllerian inhibiting substance (MIS), leads to regression of the female müllerian structures.
Testosterone and its active metabolite, dihydrotestosterone, determine full differentiation and stabilization of internal and external genitalia.

In the female fetus, without the influence of the AMH, the müllerian ducts complete their differentiation, whereas the wolffian structures involute. In the absence of testosterone and dihydrotestosterone, the genital tubercle develops into the clitoris, and the labioscrotal folds do not fuse, leaving labia minora and majora.

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