Aaamavvw2

Time of Day

• Plasma testosterone Plasma LH

Fig. 2. Change in the patterns of pulsatile release of gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH) for 24 h during sexual maturation.

Table 1

Pubertal Stages (According to Tanner) With Respective Testis Volumes and Plasma Testosterone Concentrations

Table 1

Pubertal Stages (According to Tanner) With Respective Testis Volumes and Plasma Testosterone Concentrations

Pubertal stage

Testicular

Plasma testosterone

(Tanner)

volume (mL)

(ng/dL)

1

>4

<10

2

4-8

12-69

3

8-10

60-275

4

10-20

142-515

5

20-25

319-775

Note. Values adapted from ref. 1.

Note. Values adapted from ref. 1.

Leydig cells. LH may also stimulate Leydig cell proliferation and differentiation (9). During puberty, plasma testosterone levels increase dramatically. Table 1 summarizes the levels of testosterone at various developmental stages, with respective testis sizes. The pubertal increase in testis size results primarily from an increased number of proliferating and differentiating germ cells and, to a lesser extent, an increase in Sertoli cells (10). The adult testes produce approx 6-7 mg of testosterone/d, with approx 5% synthesized in extratesticular tissues via peripheral conversion of adrenal steroids.

Inhibins

Inhibin is a heterodimeric glycoprotein, which consists of an a-subunit linked either to a PA subunit (inhibin-A) or to a PB subunit (inhibin-B) (see Chapter 1). At least six different molecular weight isoforms of inhibin-B exist in the circulation (11). The biological activity of these isoforms in not known, and they cannot be distinguished by current immunoassays. Several studies show that serum inhibin-B levels in children change in concert with the secretion of gonadotropins (12-15). Between 1 wk and 7 mo of age, when the hypothalamic-pituitary-testicular axis is transiently activated (16,17), serum inhibin-B levels increase to similar (15,18) or higher (13) levels than those observed in adolescent boys and adult men. This early inhibin-B secretion is sustained until the age of 18-24 mo; thereafter, serum concentrations decline to lower but readily measurable levels (13). Early in puberty, between Tanner stages G1 and G2, serum inhibin-B concentrations again increase to reach peak levels at the Tanner stage G2, but then the levels plateau (12,14) (see Fig. 3). In contrast to inhibin-B, inhibin-A levels in human males are below the detection limit of the corresponding immunoassay (18,19).

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