The syndrome of mixed gonadal dysgenesis (MGD) is characterized by a unilateral testis, usually intra-abdominal, a streak gonad on the contralateral side, and persistent Mullerian structures. The external genitalia are always masculinized to some extent, on occasion achieveing a normal male phenotype: the somatic signs of Turner's syndrome are frequently present. These patients are always chromatin-negative, and appear to represent the commonest expression of the mosaicism of XO and XY cells, probably resulting from a cytogenetic era very early in embryogenesis. Patients with Turner's syndrome who are slightly masculinized because of "hilus cells" in their streak gonads appear on the one hand to represent a clinical and cytogenetic stage intermediate between MGD and XO Turner's syndrome; while on the other, certain male pseudohermaphrodites with bilateral dysgenetic testes are probably intermediate between MGD and the normal XY male. Testicular androgenic function appears to be quantitatively and qualitatively normal at puberty in MGD, despite complete lack of germ-cell proliferation. The discrepancy between normal Leydigcell function at puberty and the evidence of incomplete genital masculinization during fetal life may possibly be explained by delayed and asynchronous fetal testicular development. Although patients with unilateral gonadoblastoma and contralateral streak gonad were included in the original description of the syndrome of MGD, most such patients differ clinically and cytogenetically from patients with a testis and probably should be considered more akin to patients with the syndrome of pure gonadal dysgenesis, than to MGD.

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