**1.Introduction**

118 Steroids – Basic Science

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The steroid hormones are lipophilic compounds with low molecular weight, derived from cholesterol, which play a crucial role in differentiation, development and physiological functions of many tissues. They are synthesized primarily by endocrine glands, such as the gonads, the adrenal glands and the feto-placental unit during pregnancy. In addition, the central nervous system (CNS) seems to be able to synthesize a number of biologically active steroids, termed "neurosteroids", with autocrine or paracrine functions (Baulieu, 1991). The circulating steroid hormones act both on peripheral target tissues and on the CNS, coordinating physiological and behavioral responses with specific biological purposes, e.g. reproduction. Thus, they influence the sexual differentiation of the genitalia and their functional state in adulthood, the development of secondary sexual characteristics, and sexual behavior. Unlike the lower mammals in which the ovaries and testes are the exclusive source of androgens and estrogens, in humans the adrenals cortex secretes large amount of inactive steroid precursors. These adrenal steroid precursors exert their functions in target tissues after conversion into active estrogens and/or androgens. This phenomenon which describes the conversion and action of steroid hormones within peripheral target tissues has been called "intracrinology" (Labrie, 1991, 2000).

The rate of formation of each sex steroid hormone depends on the level of expression of the specific enzymes that synthesize androgens and estrogens in each cell of each tissue (Labrie et al., 1998; Stewart § Sheppard, 1992).

The final step in the biosynthesis of active steroid hormones is catalyzed by members of the family of 17hydroxysteroid dehydrogenase (17HSD), which comprises different enzymes involved in steroidogenesis.
