The Interaction of Circulating Oestrogens and Androgens in Regulating Mammalian Sexual Differentiation

Abstract

In mammals males are the heterogametic sex, and the Y chromosome codes for differentiation of the gonads into testes. With the exception of the primary sexual characteristic, gonadal sex, all other differences between males and females are thought to be mediated hormonally via secretions from the testes. In the absence of gonads, mammalian foetuses develop into phenotypic females (Jost 1972). The ovaries of female foetuses differentiate significantly later in development than do the testes, and it appears that the ovaries of female foetuses are not steroidogenic (cf., Gibori and Sridaran 1981). In mammalian species in which the period of gestation is relatively long, sexual differentiation is usually completed by birth. In short-gestation species, such as rats and mice, however, sexual differentiation commences during the last third of pregnancy and continues during the first week to 10 days of postnatal life. Previously, it was erroneously assumed that the differentiation of sexual behaviour in mice and rats began shortly after birth (Barraclough and Leathem 1954, Young et al. 1964). There is a critical coupling of the timing of the secretion of testosterone, the primary androgen secreted by the testes of foetuses and adults, and the development of both neural and peripheral androgen-target tissues. Extrinsic factors that interfere with this normal coupling (such as maternal stress or exposure to exogenous hormones) can radically alter the course of sexual differentiation.