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)
Increasing evidence emerging in the literature indicate that relaxin and relaxin‐like peptides play an important role in male reproductive function. Insulin‐like peptide 3 (INSL3; formerly known as relaxin‐like factor 3) is a peptide hormone belonging to the relaxin–insulin family of peptide hormones [86–88] and binds to the RXFP2 receptor complex [89] to initiate physiological responses. It was first identified in the testes of pigs [90] and is now thought to be an important factor in regulating normal testicular decent [87], particularly during the second phase where it acts on the gubernaculum, as demonstrated using knockout mice. The peptide hormone has been identified in male and female tissues of other species, including human, marmoset monkey, sheep, goat, bovine, as well as deer, dog, and other species [88]. It is produced in large quantities by the Leydig cells of both the fetal and adult testes, and circulating INSL3 concentrations have been measured in the blood of adult male mammals including the rat (5 ng/ml) [91], mouse (2 ng/ml) [91], and human (0.8–2.5 ng/ml) [92, 93]. Although INSL3 has been successfully extracted from bovine testis [94] and found to be present in amniotic fluid from human male fetuses [95], it is only recently that Anand‐Ivell et al. [96] using new time‐resolved fluorescence immunoassay to directly detect INSL3 in the blood and body fluids of ruminants reported that mid‐gestation (day 153) cows carrying a male fetus showed significantly higher maternal blood concentrations of INSL3 compared with cows carrying a female fetus. The authors speculate that INSL3 provides the first example of a gender‐specific fetal hormone with the potential to influence both placental and maternal physiology. In a recent review, it is reported that INSL3 is a major secreted product of the interstitial Leydig cells of the mature testes in all mammalian males [97]. Moreover, current evidence points to autocrine, paracrine, and endocrine roles, acting through the G‐protein coupled relaxin family receptor 2 (RXFP2), although more research is required to characterize these functions in detail. Indeed, recent studies have provided evidence of the presence of both RXFP1 and RXFP2 receptors in porcine spermatozoa [98], which would suggest that relaxin may play an important role in sperm production motility and [99]. Disruption of the INSL3 binding to its receptor complex RXFP2 demonstrated the importance of this hormone in promoting and maintaining normal sperm production in the boar testis [100]. Pitia and colleagues [101] investigated the functional INSL3 hormone‐receptor system in the testes and spermatozoa of several domestic ruminant species including bulls, rams, and goats, and explored the potential of this hormone‐receptor to evaluate the fertility of sires. In all ruminants examined, the study identified the presence of RXFP2 in the Leydig cells, testicular germ cells, and sperm (equatorial segment) of fertile males, but in sub‐fertile bulls, the expression pattern in these tissues was significantly reduced. Moreover, the study observed a significant reduction in spermatozoa INSL3 binding in semen recovered from sub‐fertile bulls [101]. Consequently, the authors proposed that poor INSL3 binding by spermatozoa may have potential in predicting sire fertility.
Relaxin
The physiological role of relaxin has been intensively studied in many species over the years, including domestic ruminants, with most of the emphasis on the role in pregnancy. Experimental evidence suggests that a number of species in the Bovidae family, both domestic and non‐domestic species, do synthesize relaxin, but apparently respond to it in a physiological manner [102–104]. Specific studies performed by Bagna et al. [102] and Musah et al. [105] explored the efficacy of purified porcine relaxin on parturition in beef and dairy cattle, indicating the possible presence of relaxin RXFP receptors in the bovid female. Observing the experimental evidence suggesting that cattle may not synthesize relaxin, but appear capable of responding to the hormone physiologically, Malone et al. [106] undertook an investigation to characterize the genomic locus of the relaxin gene (RLN1) in Laurasiatherian mammals to understand how cattle may have lost the ability to synthesize this peptide hormone. The outcome of these studies revealed that both domestic and wild ruminant species, including the cow, giraffe, Tibetan antelope, sheep, and goat, lack a functional RLN1 gene. Moreover, the study documented the progressive loss of RLN1 in the evolutionary lineage leading to cattle (Figure 2.3) and that the cow genome has lost all trace of the gene [106]. Interestingly, the same study confirmed that all bovids examined possess copies of the relaxin receptors, RXFP1 and RXFP2, which may explain why in some studies, dairy and beef cattle respond to relaxin despite the inability to synthesize the hormone [106].
Figure 2.3 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. Note that the cow has completely lost the RLN gene, while it has remained as a truncated pseudogene in sheep, goat, and antelope.
Source:[106].
The importance of relaxin is less well studied in male mammals. The presence