We have measured steroid hormone biosynthesis from pregnenolone in ovaries and testes, aromatization of testosterone in gonads and peripheral tissues, and 5α-reduction of testosterone in peripheral tissues of developing opossum pouch young. Sex of the newborn opossums 15 discernible grossly with development of the pouch in females and the scrotum in males approximately 10 days after birth. Differentiated endocrine function of ovaries and testes was demonstrable as soon as development of the pouch or scrotum was apparent. The testes synthesized testosterone, and ovaries aromatized androgens to estrogens as assessed by the conversion of [1β-3H]testosterone to 3H2O. This endocrine differentiation of the goriads occurs days or weeks before differentiation of the male and female urogenital tracts. As in other species, 5a-reduction of [lβ-3H] testosterone was high in urogenital sinus and urogenital tubercle. However, 5α-reductase activity was highest in mesonephros and structures derived from the mesonephros. In wolffian and mullerian ducts of pouch young less than 10 days old, 5α-reductase activity was greater than 50 pmol/h · mg protein and decreased by 19 days of age to approximately 3 pmol/h·mg protein, a pattern different than in eutherian mammals in which testosterone itself appears to mediate virilization of the wolffian ducts. These studies provide the framework for studies of the endocrine control of phenotypic sexual differentiation in the opossum.
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