Leydig cells are found in the testes of mammals and produce testosterone. In the case of the rooster, these cells produce androstenedione, which is converted into testosterone by the action of enzymes. The testosterone produced by Leydig cells is responsible for the development of male secondary sexual characteristics such as the growth of facial and body hair, deepening of the voice, and increased muscle mass. These cells also play a role in spermatogenesis, the process by which sperm are produced. Leydig cells are named after German anatomist Franz Leydig, who first described them in 1856.