Figure 217

Electron micrograph of Leydig cells. This electron micrograph shows portions of several Leydig cells. The cytoplasm contains an abundance of sER, a characteristic of Leydig cells. Other features characteristic of the Leydig cell seen in the lower-power micrograph are the numerous lipid droplets (L), the segmented profiles of the Golgi apparatus (G), and the presence of variable numbers of lysosomes (Ly). Occasional profiles of rER are also seen. Note also the presence of microvilli along portions of the cell surface (arrows). M, cytoplasm of adjacent macrophage. xl0,000. Inset. sER at higher magnification. The very dense particles are glycogen, x 60,000

Like other steroid-secreting cells, Leydig cells have an elaborate smooth endoplasmic reticulum (sER), a feature that accounts for their eosinophilia (see Fig. 21.7). 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:

• 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.

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