Origin, development and regulation of human Leydig cells.

Sex steroids are crucial regulators of sexual differentiation and the proper development of secondary sex characteristics and patterns of sexual behavior. Since Leydig cells are the primary major producers of these steroid hormones, maintenance of the normal functions of these cells determines the reproductive capacity and fertility of males. The present minireview discusses recent findings concerning endocrine and paracrine regulation of the proliferation, differentiation and involution of human Leydig cells. The physiology and function of the two distinct fetal and adult populations of human Leydig cells are described, with particular focus on the paracrine environment that triggers their differentiation and functional maturation. The roles of established and more recently discovered paracrine regulators of this maturation, including insulin-like factor 3, platelet-derived growth factor-alpha, desert hedgehog, ghrelin and leptin are considered. A brief description of the origin, ontogenesis and functional markers of human fetal and adult Leydig cells is presented.