Role of the SRY gene, testosterone, AMH, and 5-alpha-reductase in masculinization

Genetic activation of the SRY gene and gonadal determination

During the early phase of embryonic development, the reproductive anatomy remains in a bipotential and undifferentiated state.

Until the sixth week of gestation, all embryos share a common architectural blueprint consisting of the undifferentiated gonadal ridge and two pairs of parallel internal ducts.

The decisive bifurcation toward the male phenotype begins between the sixth and seventh weeks through the expression of the SRY gene, located on the short arm of the Y chromosome. This gene acts as a master switch that encodes the testis-determining factor.

Its specific expression transforms the undifferentiated gonad into the structure of the male testes, orchestrating the differentiation of Sertoli and Leydig cells.

Thus, the activation of the SRY gene breaks embryonic neutrality, setting in motion the complex hormonal cascade that will guide the remainder of somatic differentiation in males.

Internal Hormonal Action: AMH and Testosterone

Once the male gonadal structure is established, the embryonic testis takes over the endocrine regulation of internal development.

The newly differentiated Sertoli cells immediately secrete anti-Müllerian hormone.

The function of this chemical messenger is to trigger the complete involution and regression of the Müllerian ducts, preventing the somatic formation of the uterus, cervix, and fallopian tubes. At the same time, Leydig cells begin massive production of testosterone.

This androgenic hormone acts directly on the Wolffian ducts, rescuing them from their programmed atrophy and stimulating their anatomical differentiation into the seminal vesicles, the vas deferens, the epididymis, and the ejaculatory ducts.

Through this dual physiological mechanism, the coordinated presence of anti-Müllerian hormone and testosterone eliminates potential female structures and definitively establishes the entire internal male reproductive architecture in the embryo.

Conversion by the enzyme 5-alpha-reductase and genital masculinization

Beginning in the ninth week of gestation, the anatomical formation of the male external genitalia requires specific peripheral hormonal processing.

The external genital primordia—comprising the genital tubercle, the urethral folds, the urogenital sinus, and the labioscrotal protuberances—begin in a neutral state that is identical in both sexes.

For somatic masculinization to occur, circulating testosterone must be converted into an androgen with greater affinity and biological potency called dihydrotestosterone.

This chemical conversion is catalyzed locally by the enzyme 5-alpha-reductase in peripheral target tissues.

Dihydrotestosterone stimulates the growth of the genital tubercle to form the penis, promotes the fusion of the urethral folds to structure the penile urethra, and triggers the union of the labioscrotal protuberances to form the scrotum, thereby completing the differentiation of the external genital phenotype in male biological development.

Summary

Embryonic masculinization begins between the sixth and seventh weeks of gestation through the action of the SRY gene. This chromosomal gene encodes the testis-determining factor, inducing the differentiation of the neutral gonad into testes.

Subsequently, Sertoli cells secrete anti-Müllerian hormone to suppress the Müllerian ducts. At the same time, Leydig cells release testosterone, which rescues and transforms the Wolffian ducts into the internal reproductive tract.

Finally, the enzyme 5-alpha-reductase locally converts testosterone into dihydrotestosterone. This natural and potent androgen induces labioscrotal and urethral fusion, forming the penis and scrotum from the undifferentiated external genital structures.