Leydig cells (interstitial cells)

Leydig cells (interstitial cells) are large, polygonal, eosinophilic cells that typically contain lipid droplets Lipofuscin pigment is also frequently present in these cells as well as distinctive, rod-shaped cytoplasmic crystals, the crystals of Reinke.

Like other steroid-secreting cells, Leydig cells have an elaborate smooth endoplasmic reticulum (sER), a feature that accounts for their eosinophilia . The enzymes necessary for the synthesis of testosterone from cholesterol are associated with the sER. Mitochondria with tubulovesicular

cristae, another characteristic of steroid-secreting cells, are also present in Leydig cells.

Leydig cells differentiate and secrete testosterone during early fetal life. Secretion of testosterone is required during embryonic development, sexual maturation, and reproductive

function:

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• In the embryo, secretion of testosterone and other androgens is essential for the normal development of the gonads in the male fetus.

At puberty, secretion of testosterone is responsible for the initiation of sperm production, accessory sex gland secretion, and development of secondary sex characteristics.

• In the adult, secretion of testosterone is essential for the maintenance of spermatogenesis and of secondary sex characteristics, genital excurrent ducts, and accessory sex glands.

The Leydig cells are active in the early differentiation of the male fetus and then undergo a period of inactivity beginning at about 5 months of fetal life. Inactive Leydig cells are difficult to

distinguish from fibroblasts. When Leydig cells are exposed to gonadotropic stimulation at puberty, they again become androgen-secreting cells and remain active throughout life.