Origin and regulation of stem Leydig cells in the adult testis

Testosterone, produced by testicular Leydig cells, is essential for the development of the male reproductive system and the maintenance of male reproductive function throughout life. Testosterone produced by the fetal Leydig cells is critical for the differentiation of the male reproductive system and for the masculinization of the fetal brain. Testosterone produced by adult Leydig cells plays a major role in the development of the male reproductive system during puberty and the maintenance of reproductive function in the adult. Fetal Leydig cells undergo regression beginning in fetal life and continuing thereafter. Adult Leydig cells begin to develop from stem cells (referred to as stem Leydig cells, SLCs) during 2–3 postnatal weeks. SLCs also are present in the adult testis, associated with both seminiferous tubules and blood vessels. These cells can form new adult Leydig cells in the event of loss of the existing adult cells. Considerable recent progress has been made in the identification, isolation, and culture of adult SLCs and in the identification of factors involved in regulating their proliferation and differentiation.