Bovine Reproduction. Группа авторов

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Bovine Reproduction - Группа авторов


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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].

Schematic illustration of mean (± SEM) serum IGF-I, insulin, GH, and leptin concentrations during sexual development in Angus and Angus times Charolais bulls receiving adequate nutrition.

      Sources: Data from [2–5].

      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


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