Although the chromosomal and genetic sex of an embryo is determined at fertilization, male and female morphological sexual characteristics do not differ until the seventh week of gestation (Moore 1988). This initial period of early genital development is referred to as the indifferent stage of sexual development. During the fifth week, proliferation of the mesothelial cells and of the underlying mesenchyme produces a bulge on the medial side of the mesonephros, known as the gonadal ridge. Next, finger-like epithelial cords grow into the underlying mesenchyme. The indifferent gonad now consists of an outer cortex and inner medulla (Moore 1988). During the sixth week, the primordial germ cells enter the underlying mesenchyme and incorporate into the primary sex cords. In embryos with an XY sex chromosome complex, testis organizing factor (H-Y antigen) regulated by the Y chromosome determines testicular differentiation (Moore 1988); the medulla differentiates into a testis and the cortex regresses. The gonads can be recognized as testes 7–9 weeks post-fertilization (Figs. 2 and 3a, b).

The first stage of testis differentiation is the formation of testicular cords consisting of Sertoli precursor cells packed tightly around germ cells. The diploid germ cells, the prespermatogoonia, undergo meiosis in the fetal testis and remain in meiotic arrest until puberty. Sertoli cells, which provide a location for support and proliferation of spermatogonia are derived from the mesonephros and proliferate only during fetal life and in the neonatal period (Styne 1982). After the 8th week of fetal life, the Leydig cell of the fetal testis secretes testosterone resulting in masculinization of the external genitalia and urogenital sinus. By the third month, the penis and prostate form (Conte and Grumbach 1990). Normal testes descend by the 7th month of gestation with little likelihood of continuing spontaneous descent after 9 months (Czeizel et al. 1981).

The adult testis is an oblong organ, approximately 4.5 cm in length, weighing 34–45 g (Steinberger 1989) (Fig. 2). The testis is composed of three principal cell types: germ cells that develop into sperm, Sertoli cells that support and nurture developing germ cells and are also the site of production of the glycoprotein hormone inhibin, and Leydig cells that are responsible for testosterone synthesis (Sklar 1999). Seminiferous tubules, the sites of spermatogenesis, are formed by germ cells and Sertoli cells (Fig. 3a). The Leydig cells that are responsible for the production of testosterone lie near the basal compartment of the seminiferous tubules, enabling them to deliver high concentrations of testosterone necessary for normal spermatogenesis and male secondary sexual characteristics (Morris 1996; Sklar 1999) (Fig. 2).

The seminiferous tubules are embedded in a connective tissue matrix containing interspersed Leydig cells, blood vessels, and lymphatics and are surrounded by a basement membrane (tunica propria) upon which the seminiferous epithelium rests. Spermatogenesis takes place in the seminiferous epithelium. The least differentiated germ cells, the spermatogonia, divide to form spermatocytes that immediately undergo meiosis (Fig. 3b). The haploid cells (the spermatids) that are formed develop into flagellate motile spermatozoa. This process requires up to 74 days (Smith and Rodgriguez-Rigau 1989). Since spermatozoa are continuously produced in adult men, a constant supply of germ cell precursors is necessary. The newly formed spermatozoa are transported through the lumen of the seminiferous tubules into the epididymis where they are stored.

The primary regulators of testicular function are the anterior pituitary hormones, luteinizing hormone (LH) and follicle-stimulating hormone (FSH), both of which are released in response to the hypothalamic releasing factor, GnRH (Fig. 4). GnRH is secreted from the median eminence into the hypophyseal portal system in a pulsatile manner and acts on the gonadotropes of the pituitary gland to stimulate secretion of LH and FSH (Morris 1996). LH regulates Leydig cell function by binding to specific LH receptors on the plasma membrane of Leydig cells. This leads to the formation of the cAMP that drives testosterone biosynthesis via a complex cascade starting with cholesterol (Morris 1996; Sklar 1999). Testosterone is transported from Leydig cells to the seminiferous tubules, where it acts to enhance spermatogenesis (Sklar 1999). Testosterone is also a prohormone for two different and metabolically active hormones, dihydroxytestosterone (DHT) and estradiol. DHT mediates male sexual differentiation and virilization, whereas estradiol mediates bone maturation, mineralization, and epiphyseal fusion (Smith et al. 1994). Testosterone controls pituitary LH secretion by a negative feedback mechanism; LH levels will rise when the Leydig cells are unable to produce testosterone (Figs. 4, 5a, b, Tables 1, 2, 3).

Sertoli cells are the only cell type of the testis that possess FSH receptors (Sklar 1999). FSH is delivered to the interstitial area of the testis by way of the arterial system. It then passes through the basement membrane of the seminiferous tubule to reach the Sertoli cells that line this membrane. FSH binds to the FSH receptors of the Sertoli cell and then mediates the synthesis of a variety of proteins and enzymes including the inhibins (Morris 1996). Inhibin is a major feedback regulator of FSH and may thus be useful as a marker of spermatogenesis (Hayes et al. 1998; Schmiegelow et al. 2001). The production of androgen transport protein by Sertoli cells can be induced and maintained by FSH.

FSH triggers an event in the immature testis that is essential for the completion of spermatogenesis. Thereafter, spermatogenesis will proceed continuously as long as an adequate and uninterrupted supply of testosterone is available. The lack of negative feedback from germinal epithelium results in an elevated FSH level.

A neonatal surge in testosterone secretion is caused by high LH and FSH concentrations that occur during the first few months of life. From the age of about 3 months until the onset of puberty, plasma concentrations of LH and FSH are quite low and the testes are relatively quiescent (Sklar 1999). At the onset of puberty, pulsatile secretion of LH (and to a lesser extent, FSH) occurs during sleep that is associated with increased nighttime plasma concentrations of testosterone (Sklar 1999). As puberty progresses, the increased pulsatile release of gonadotropins and the high concentrations of testosterone are maintained throughout the day and night (Sklar 1999) (Tables 1, 2).

In a normal male the first sign of puberty is enlargement of the testis to greater than 2.5 cm (Styne 1982). This is due to seminiferous tubule growth, although Leydig cell enlargement contributes as well. Androgens synthesized in the testes are the driving force behind secondary sexual development, although adrenal androgens also play a role in normal puberty. The range of onset of normal male puberty extends from 9 to 14 years of age. Boys complete pubertal development in 2–4.5 years (mean 3.2 years) (Styne 1982). The development of the external genitalia and pubic hair has been described in stages by Marshall and Tanner (1970) (Tables 1 and 2). The first appearance of spermatozoa in early morning urinary specimens (spermarche) occurs at a mean age of 13.4 years, at gonadal stages 3 and 4 (pubic hair stages 2 through 4) simultaneous with the pubertal growth spurt.