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).
Metabolic Hormones During the Prepubertal and Pubertal Periods
The mechanisms controlling reproduction and energy balance are intrinsically related and have evolved to confer reproductive advantages and guarantee the survival of species. The neural apparatus designed to gauge metabolic rate and energy balance has been denominated the body “metabolic sensor.” This sensor translates signals provided by circulating (peripheral) concentrations of specific hormones into neuronal signals that ultimately regulate the GnRH pulse generator and control the reproductive process. Metabolic indicator hormones may serve as signs to the hypothalamus–pituitary–gonadal axis and affect sexual development. The patterns of some of these hormones have been studied in growing beef bulls (Figure 5.6). In contrast to species in which circulating growth hormone (GH) concentrations continue to increase until after puberty, GH concentrations decrease during the pubertal period in bulls [2, 5]. Differences in the stage of body development at which each species attains puberty are likely responsible for the different GH profiles among species. Accordingly, the GH profile in bulls seems to indicate that a relatively advanced stage of body development must be attained before the gonads are efficiently producing sperm. The differences in GH secretion among species may be due to the regulatory role of steroids on GH secretion. In other species, steroids stimulate GH secretion, but GH concentrations do not differ between intact bulls and castrated steers [42]. Moreover, decreasing GH concentrations during sexual development are observed along with increasing testosterone concentrations, indicating that steroids do not have a positive feedback on GH secretion in bulls as in other species [2, 5].
Figure 5.6 Mean (± SEM) serum IGF‐I, insulin, GH, and leptin concentrations during sexual development in Angus and Angus × Charolais bulls receiving adequate nutrition.
Sources: Data from [2–5].
Circulating insulin‐like growth factor (IGF)‐I concentrations in bulls increase continuously and only reach a plateau (or decrease slightly) after sexual development is mostly completed after 12–14 months of age; increasing circulating concentrations of IGF‐binding protein 3 and decreasing concentrations of IGF‐binding protein 2 are also observed during sexual development [2–5,43–45]. The concomitant decrease in circulating GH concentrations with the increase in IGF‐I concentrations during sexual development in bulls indicates that either there are drastic changes in liver sensitivity to GH or other sources are responsible for IGF‐I production. A possible IGF‐I source might be the testes, since Leydig cells are capable of secreting this hormone in other species. Observations that intact bulls tend to have greater IGF‐I concentrations than castrated steers at 12 months of age further support the hypothesis that the testes might contribute substantially to circulating IGF‐I concentrations during the prepubertal and pubertal periods in bulls [42]. Close temporal associations observed in a series of nutrition studies strongly suggest that circulating IGF‐I might be involved in regulating the GnRH pulse generator and the magnitude and duration of the early gonadotropin rise in beef bulls [2–4, 33].
A possible effect of IGF‐I on testicular steroidogenesis in bulls has also been suggested. Leydig and Sertoli cells produce IGF‐I, indicating the existence of paracrine/autocrine mechanisms of testicular regulation involving IGF‐I [46, 47]. It is assumed that most of the IGF‐I in the testes is produced locally and that circulating IGF‐I may play a secondary role in regulating testicular development and function. However, the temporal patterns and strong associations among circulating IGF‐I concentrations, testicular size, and testosterone secretion observed in bulls receiving different nutrition argue for a primary role for this hormone [2–4, 33, 35]. The primary role of increased circulating IGF‐I during the pubertal period may be to promote the increase in testosterone concentrations by regulating Leydig cell multiplication, differentiation, and maturation. Since testosterone upregulates IGF‐I production and IGF‐I receptor expression by Leydig and Sertoli cells [48], the establishment of a positive feedback loop between IGF‐I secretion and testosterone production may be important for sexual development.
Circulating leptin and insulin concentrations also increase during the pubertal period in bulls. However, developmental and nutritional differences in LH pulse frequency are not related to differences in leptin or insulin concentrations in beef or dairy bulls [2, 3, 35]. Other studies have also demonstrated that leptin does not stimulate in vitro GnRH secretion