Endocrine Abstracts

Sexual dimorphism in fetal testosterone output in the silver fox
LV Osadchuk; Institute of Cytology and Genetics, Novosibirsk, Russia
In mammals physiological mechanisms regulating reproduction are sexually differentiated. A basic mechanism of sexual differentiation in mammals is that testosterone produced by the testes organise development of the genital tract, external genitals, gonadotrophin secretion and the capacity to display sex specific behaviour in the male direction. Sexual dimorphism in the gonadal testosterone secretion in canine species has hardly been studied. This study was designed to characterise the pattern of testosterone output in male (n=101) and female (n=91) fetuses of the silver fox. Plasma concentrations, gonadal and adrenal contents of testosterone, and also body, adrenal and gonadal weights were measured in the fetuses during the last trimester of gestation (on days 31, 35, 40, 45 and 50 of pregnancy, term on day 52). Testosterone in plasma samples and homogenates of the gonads and adrenals was measured by RIA. The study demonstrated that testosterone can be detected in the fetal fox gonads as early as on day 31 of pregnancy, when the first signs of sex differentiation are conspicuous in the gonadal structure. The gonadal testosterone contents in males exceeded those in females from day 35 onwards (p<0.05), while the same sex differences in the plasma testosterone concentrations were observed only from day 40 onwards (p<0.05). Negligible amounts of testosterone were detected in the fetal adrenals and there were no sex-related differences in the adrenal testosterone content. The gonadal weights were higher in female than in male fetuses (p<0.05) from day 45 onwards. In conclusion, the quantitative aspects of gonadal steroidogenesis differ greatly between male and female fetuses in the silver fox. In this species the fetal testes produce much higher testosterone than the ovaries directing sexual differentiation on the male way.