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 andro gens 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 secre tion, and development of secondary sex characteristics. In the adult, secretion of testosterone is essential for the maintenance of spermatogenesis and of secondary sex char acteristics, 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.
Leydig cell tumors represents predominately benign tumors, which occur during two distinct periods (in childhood and in adults between 20 and 60 years old). They are hormonally active and secrete androgens or combination of androgens and estrogens. Commonly, they are composed of uniform cells with all characteristics of steroid hormone-secreting cells containing Reinke crystals. The frst symptom of these benign tumors, besides testicular enlargement, usually is related to abnormal level of hormone production. In prepubertal boys,
FIGURE 22.5 • Photomicrographs of human testis. a. This low-magnification photomicrograph of an H&E–stained section of a human testis shows seminiferous tubules and the tunica albuginea. The larger blood vessels are present in the inner aspect of the tunica albuginea. The seminiferous tubules are highly convoluted; thus the profiles that they present in the section are variable in appearance. 30. b. A higher magnification of the previous specimen shows several seminiferous tubules. Note the population of
Leydig (interstitial) cells that occur in small clusters in the space between adjoining tubules. 250.
FIGURE 22.6 • Schematic drawing of human seminiferous epithelium. This drawing shows the relationship of the Sertoli cells to the spermatogenic cells. The seminiferous epithelium rests on a basal lamina, and a layer of peritubular cells surrounds the seminiferous tubule. The spermatogonia—type A pale, type A dark, and type B pale— and preleptotene spermatocytes are located in the basal compartment of the seminiferous epithelium below the junctional complex, between adjacent Sertoli cells. Pachytene primary spermatocytes, early spermatids, and late spermatids, with partitioning residual cytoplasm that becomes the residual body, are seen above the junctional complex in the abluminal compartment. (Redrawn from Clermont Y. The cycle of the seminiferous epithelium in man. Am J Anat 1963;112:35.) this leads to sexual precocity (unexpected pubertal changes in early age), whereas in adults it may be observed as feminization (development of female sexual characteristics) and gynecomastia (development of breast in males).
Spermatogenesis is the process by which spermatogonia develop into sperm.
Spermatogenesis, the process by which sperm are produced, involves a complex and unique series of events. It begins shortly before puberty, under the influence of rising levels of pituitary gonadotropins, and continues throughout life. For descriptive purposes, spermatogenesis is divided into three distinct phases:  Spermatogonial phase, in which spermatogonia divide by mitosis to replace themselves as well as provide a popu lation of committed spermatogonia that will eventually