288 days, respectively), whereas bulls in the high nutrition group had greater paired‐testes weight at 70 weeks of age (655 g) than bulls in the low and medium nutrition groups (520 and 549 g).
Source: From [2], © 2007, Elsevier.
Figure 5.4 Mean (± SEM) number of LH pulses and serum testosterone concentrations in Angus and Angus × Charolais bulls receiving medium (control) or high nutrition from 10 to 30 weeks of age and the same medium nutrition from 31 to 74 weeks. N, A, and N*A indicate nutrition, age, and nutrition‐by‐age interaction effects, respectively. Superscript a and b indicate differences (P = 0.09) between groups within age. Bulls in the high nutrition group were younger at puberty (314 days) and had greater paired‐testes weight at 74 weeks of age (610 g) than bulls in the medium group (327 days and 531 g, respectively).
Source: From [3], © 2007, Elsevier.
Figure 5.5 Mean (± SEM) number of LH pulses and serum testosterone concentrations in Angus and Angus × Charolais bulls receiving medium (control) from 10 to 70 weeks of age or low nutrition from 10 to 26 weeks of age and either medium or high nutrition from 27 to 70 weeks of age. N, A, and N*A indicate nutrition, age, and nutrition‐by‐age interaction effects, respectively. Bulls in the medium/medium nutrition group were younger at puberty and had greater paired‐testes weight at 70 weeks of age than those in the low/medium nutrition group (293 vs 331 days and 600 vs 528 g, respectively). Age at puberty and paired‐testes weight for bulls in the low/high nutrition group were intermediate (313 days and 553 g, respectively).
Source: From [4], © 2007, Society for Reproduction and Fertility.
During the prepubertal period there is a progressive increase in the proportion of testicular parenchyma occupied by seminiferous tubules, and seminiferous tubule diameter increases to approximately 125 μm at six months of age [20, 36]. FSH‐binding sites can be observed in seminiferous tubules of bull calves at birth and at four months age, and decreased inhibin concentrations, coupled with increased FSH concentrations, stimulate the proliferation of undifferentiated Sertoli cells [6, 22]. Although there is considerable evidence that FSH is essential for normal Sertoli cell function, the period of Sertoli cell differentiation coincides with the initiation of testosterone secretion by the Leydig cells, indicating that testosterone may also be involved in promoting maturation of undifferentiated Sertoli cells. At approximately four months of age, undifferentiated Sertoli cells enter the G0 phase of the cell cycle for the rest of the bull's life. With the end of the proliferative phase, undifferentiated Sertoli cells begin to transform into adult‐type Sertoli cells. Opposing cell membranes of adjacent undifferentiated Sertoli cells start to develop extended junctional complexes above the spermatogonia and in the basal portion of the tubules; “cracking” of the tubular cytoplasm is first detected around six months of age [1,15–17]. As immature Sertoli cells begin to differentiate they cease to secrete AMH and circulating concentrations of this hormone decrease after four months of age [1]. FSH secretion, maturation of Sertoli cells, and increased testosterone secretion are probably also involved in the differentiation of gonocytes into spermatogonia. Gonocytes are gradually displaced to a position close to the basal lamina and divide by mitosis, originating A‐spermatogonia. Differentiation and degeneration result in the complete disappearance of gonocytes from the seminiferous tubules by five months of age. A‐spermatogonia divide mitotically to form In‐spermatogonia and B‐spermatogonia, which in turn enter meiosis around four to five months of age [15, 17, 20, 36, 37].
Pubertal Period
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
