scrotal circumference, testes weight, seminiferous tubules diameter, and number of germ and Sertoli cells in tubular cross‐sections at 12 months of age [29–31]. The LH secretion pattern during the prepubertal period is also associated with age at puberty in bulls raised in contemporary groups, suggesting that this is the physiological mechanism by which genetics affect sexual development. Studies have shown that LH pulse frequency was greater around 2.5–5 months of age and that mean LH concentrations increased earlier and reached greater maximum levels in early‐ than in late‐maturing Hereford bulls (age at puberty 9.5 and 11 months, respectively) [19, 21].
Additional support for the crucial role that LH secretion pattern during the early gonadotropin rise plays in sexual development in bulls has been provided by studies demonstrating that nutrition during the prepubertal period affects LH secretion pattern, age at puberty, and testicular development. In studies with beef and dairy bulls receiving different nutrition from 2 to 16 months of age, reduced LH pulse frequency during the prepubertal period resulted in delayed puberty in bulls receiving low nutrition, while a more sustained increase in LH pulse frequency in bulls receiving high nutrition was associated with hastened testosterone production and greater testes weight at 16–19 months of age when compared with bulls receiving low and medium (control) nutrition (results in beef bulls are shown in Figure 5.3). These observations were corroborated by additional studies designed to investigate the effects of nutrition specifically during the prepubertal period. Bulls that received high nutrition only from two to seven months of age also had a more sustained increase in LH pulse frequency, had greater testosterone secretion, were approximately two to four weeks younger at puberty, and had greater testes weight at 16–19 months of age when compared with bulls receiving control nutrition (results in beef bulls are shown in Figure 5.4). On the other hand, reduced LH secretion resulting from low nutrition from two to six months of age was associated with increased age at puberty and smaller testes weight at 16–19 months of age even when these bulls received control or high nutrition after seven months of age and LH and testosterone secretion were not different after the change in nutrition (results in beef bulls are shown in Figure 5.5) [2–4,32–35].
Figure 5.3 Mean (± SEM) number of LH pulses and serum testosterone concentrations in Angus and Angus × Charolais bulls receiving low, medium (control), or high nutrition from 10 to 70 weeks of age. N, A, and N*A indicate nutrition, age, and nutrition‐by‐age interaction effects, respectively. Superscripts indicate differences (P < 0.05): a,b, group differences within age; *, age differences within group. Bulls in the low nutrition group were older at puberty (321 days) than bulls in the medium and high nutrition groups (299 and 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