Leydig cells. Transmission electron micrograph (TEM) of a section through a group of Leydig cells in the intertubular tissue of a testis (testicle). Leydig cells are the principle source of androgens (male sex hormones), specifically testosterone, which stimulates spermatogenesis (sperm development), and by entering the circulation acts upon androgen-dependent tissues elsewhere in the body. The cells contain many mitochondria (dark grey, oval), smooth endoplasmic reticulum and numerous lipid droplets (light grey circles). Magnification: x5500, when printed 10 centimetres wide.