Bovine Reproduction. Группа авторов

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Bovine Reproduction - Группа авторов


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may be related to the fact that both species secrete significant amounts of estrogens [75], but the significance of the vast quantities of estrogens produced by these two species is unexplained. It has been suggested that estrogens act synergistically with testosterone to enhance both secretory activity of accessory sex organs and sexual behavior in boars castrated after puberty [76]. Estrogens are C18 steroids and are formed by the conversion of androgens by the aromatase enzyme system to produce estrone and estradiol from androstenedione and testosterone, respectively. Of interest in the boar are the musk‐smelling Δ16‐androstene steroids (pheromones) that are regarded quantitatively as the most abundant steroids produced by the boar testis and contribute to the familiar “boar taint” of pork [77]. However, there is insufficient evidence to demonstrate that bull testis produces estrogens in the quantities found in the boar and stallion, nor is there evidence that the bull secretes much in the way of the Δ16‐androstene steroids. However, what is now well documented is that testosterone is the most potent androgen produced by Leydig cells in mammalian testes, and the site of action is primarily on seminiferous tubule target cells, thus influencing the reduction division of the spermatogenic cells [78]. Androgens stimulate production of androgen‐binding protein (ABP) by the Sertoli cells [79], and this acts as an intracellular carrier of testosterone and DHT within the Sertoli cells. Testosterone is also the most important determinant of the rate of formation of fructose by the seminal vesicles and of citric acid by the prostate and seminal vesicle glands of the bull, ram, and human [78].

      Oxytocin is a nine amino acid neuropeptide hormone normally associated with the hypothalamic–posterior pituitary system and the regulation of parturition and lactation in the female, but has also been shown to have an endocrine and paracrine role in male reproduction [19, 80]. There is evidence reported in the literature that oxytocin is produced and secreted by the male reproductive tract including the testis [81, 82]. Moreover, there is now evidence to show that oxytocin is produced locally by the testis and that it has a paracrine role in modulating testicular steroidogenesis and contractility of the male reproductive tract [83]. In addition, it has been shown that the Leydig cells are the testicular site of production of this hormone, and that oxytocin acts as a paracrine hormone influencing the contractility of the peritubular myoid cells [19]. The contraction of myoid cells in the seminiferous tubule epithelium is thought to facilitate sperm transport through the testicular parenchyma emptying into the rete testis and on into the epididymal system. It has been shown that, within the prostate, testosterone is converted by 5α‐reductase to DHT, which stimulates growth of the prostate gland. Nicholson [83] has postulated that oxytocin increases the activity of 5α‐reductase, resulting in increased concentrations of DHT and growth of the prostate, but that androgen feedback reduces oxytocin concentrations in the prostate, thereby modulating prostate gland growth. Definitive evidence of oxytocin synthesis within the bovine testis has come from studies on oxytocin gene expression in the seminiferous tubules [84, 85].

      Relaxin and Insulin‐like Peptide (INSL‐3)

      Relaxin

Photo depicts genomic context of the relaxin gene in selected mammals. The cluster containing the RLN, INSL4, and INSL6 genes is flanked by the PLGRKT gene on one side and the JAK2 on the other side.

      Source:[106].


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