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The pubertal period is characterized by reduced gonadotropin secretion, increased testosterone secretion, initiation of spermatogenesis, and the eventual appearance of sperm in the ejaculate. This period also coincides with the start of a phase of rapid testicular growth (see Chapter 6 for testicular growth charts) and extends from approximately 6 to 12 months of age in B. taurus bulls.

The rapidly increasing testosterone secretion and possibly increased hypothalamic sensitivity to negative feedback from androgens are likely responsible for the decrease in LH secretion during the pubertal period. Although immunization with inhibin antiserum results in a marked increase in FSH concentrations in prepubertal bulls, whether inhibin produced by Sertoli cells acts on the gonadotrophs to limit FSH secretion is uncertain, since circulating inhibin decreases steadily from birth to seven months of age [6, 38, 39]. After seven months of age, Leydig cell mass increases slowly but continuously to reach about 10 g in the young adult testis at 24 months of age as a result of considerable increase in Leydig cell volume (hypertrophy); Leydig cell mitochondrial mass more than doubles from 10 to 24 months of age [14]. Testosterone pulse frequency does not increase after the peripubertal period and remains at approximately 4.5–6.8 pulses per 24 hours from 6 to 10 months of age. However, pulse amplitude increases during the pubertal period with consequent increase in testosterone mean concentrations until approximately 12 months of age. Elevated testosterone secretion is essential for initiation of spermatogenesis [12,25–27, 40].

Seminiferous tubule diameter increases to approximately 200 μm at eight months of age and reaches 240 μm by 16 months [20, 36, 41]. Total seminiferous tubule length increases from 830 m per testis at three months of age to 2010 m per testis at eight months of age in Holstein bulls [36]. Most Sertoli cells complete their morphological differentiation and attain adult structure after six to seven months of age. Junctional complexes consisting of many serially arranged points or lines of fusion involving neighboring Sertoli cell membranes can be observed. These junctions form a functional blood–testis barrier and divide the tubular epithelium into a basal compartment containing spermatogonia and an adluminal compartment containing germ cells at later stages of spermatogenesis; formation of a functional blood–testis barrier is accompanied by formation of the tubular lumen and precedes the appearance of primary spermatocytes and more advanced germ cells [1,15–17]. In Holstein bulls, the number of adult‐type Sertoli cells increases dramatically from 202 to 8862 million cells per testis between five and eight months of age, respectively [36].

Germ cell proliferation is maximal between four and eight months of age and represents the expansion of the spermatogonial stem cell. In Holstein bulls, the number of spermatogonia increases from 181 million cells per testis at four months of age to 3773 million cells per testis at eight months of age; the number of spermatogonia continues to increase until approximately 12 months of age [36]. Primary spermatocyte numbers increase slowly until eight months of age, when the numbers exceed the number of spermatogonia. Secondary spermatocytes and round spermatids first appear at approximately six to seven months of age, whereas elongated spermatids appear around eight months of age. The number of spermatids increases rapidly after 10 months of age when spermatid numbers exceed the numbers of any other germ cell. Mature sperm appear in the seminiferous tubules at approximately 8–10 months of age. Testes weighing more than 100 g in Swedish Red‐and‐White bulls or more than 80 g in Holstein bulls are likely to be producing sperm [15, 17, 20, 36, 37]. Spermatogenesis eventually reaches a level of efficiency (i.e. increasing number of more advanced germ cells resulting from individual precursor cells) that results in the production of a number of sperm sufficient for those to appear in the ejaculate, the physiological event that characterizes puberty (see Chapter 6).