Leydig cells

Franz von Leydig discovered Leydig cells in 1850, which are microscopic testosterone producers located between the sperm-producing tubules of the testis. Despite their small size, they drive important changes such as puberty, muscle growth, and sperm production. Under a microscope, they seem polygonal and pink, with mitochondria, smooth ER, and the mysterious Reinke crystals. Fetal Leydig cells begin functioning prior to birth, whereas adult cells come into action at puberty—structurally distinct, but in agreement in their function to synthesize testosterone. When things go out of control, such as with tumors or Klinefelter syndrome, their function is halted. Nonetheless, these little cells have a huge impact on male biology.