性の決定機構―H-Y抗原説その後―

抄録

The sex of an individual is determined at fertilization by the combination of sex chromosomes, e.g., XX or XY in mammals. Morphological sex differences become evident during the organization of gonadal primordia into ovaries or testes. Consequently, fetal testes produce hormones which are responsible for the development of the male phenotype. The absence of male hormones results in the development of the females phenotype. It has been proposed that a male dominant, histocompatibility-Y (H-Y) antigen
determines testicular differentiation, based on the finding that individuals with testest are H-Y antigen positive regardless of their sex karyotype. Recently, this hypothesis has been challenged bý the finding of several exceptions, e.g., development of ovaries in the presence of H-Y antigen, and testes in its absence.
A testis-determining gene (Tdy or TDF) has been thought to occur on the Y chromosome. Over 100 Y-specific DNA fragments have been examined with only one possible candidate for the Tdy gene. In addition to the Tdy gene, at least two autosomal genes (tda-1 and Tas) appear to be involved in testis determination. It has been suggested that gonadal sex determination may also be influenced by environmental factors.
Fetal rat and mouse ovaries develop testicular structures (ovotestes) after transplantation into various sites of adult host animals. This finding suggests that XX gonadal primordial cells can differentiate into testicular cells. Electron microscopic examinations have revealed that testicular structures of mouse ovotestes are comparable to those of the genetic male. Furthermore, it has been shown that ovotestes produce hormones and glycoproteins characteristic of normal neontal testes. These results provide strong evidence for sex reversal by transplantation. The study of the factors involved in the induction of ovotestis development should aid better understanding of the mechanism of gonadal sex determination.