Читать книгу Bovine Reproduction - Группа авторов - Страница 41

Since the beginning of the twentieth century, Leydig cells have been considered the probable source of testicular androgens [65]. Berthold [66] was the first to observe from experimentation with the rooster that the testes produced a substance that influenced secondary sex organ development and maintenance [26]. The first isolation of an androgen, androsterone, from human urine [67] and the crystallization of testosterone from bull testes [11] established the major site of testosterone production as the testis. Extensive literature has emerged over the past 50 years on the function of the Leydig cell. These studies, employing a variety of techniques, have identified and confirmed the Leydig cells as the primary source of testicular androgens and elucidated the important pathways in androgen biosynthesis [5, 26, 52, 68]. LH has also been confirmed as the major pituitary hormonal stimulus on the Leydig cells [46]. Ewing et al. [68] have suggested that because Leydig cells are concentrated in clusters in the interstitial tissue of the testis, they must influence seminiferous tubular and peripheral androgen‐dependent functions (accessory sex glands) by hormonal signals rather than by cell‐to‐cell interaction.

The primary steroids produced and secreted by the bull testis are shown in Table 2.1. The conversion of the precursor cholesterol to androgens and estrogen is facilitated by a series of enzymatic reactions involving hydroxylation, dehydrogenation, isomerization, C–C side‐chain cleavage (lyase), and aromatase activity. Testosterone is now recognized as the principal steroid responsible for the endocrine functions of the testis since it is synthesized in copious amounts by mammalian Leydig cells. The Leydig cell has also been identified as a major source of other androgens and estrogens [5, 69]. In addition, the Leydig cells of the boar testis produce large amounts of the musk‐smelling steroids, Δ¹⁶‐androstenes [70]. It has become apparent that the primary function of the Leydig cell is the provision of the androgen stimulus required for the initiation and maintenance of spermatogenesis in the germinal epithelium within the seminiferous tubules [71]. Some of the earliest functions of the Leydig cell are associated with regulation of the male reproductive organs, the organization of parts of the brain, pituitary secretion of gonadotropins, and accessory sex organ development of the neonate to ensure the appropriate response by these tissues to testicular steroids in the adult male [69]. The dependence of male sex accessory organs on testicular hormones is not restricted to the early fetal period but occurs also during puberty and throughout male adult life [72].

Table 2.1 Primary steroid and peptide hormones synthesized and/or secreted by the bull testis.

Hormone family	Hormone	Site of synthesis
Steroid family
Cholesterol (27 carbons)	Cholesterol22‐hydroxycholesterol20,22,‐dihydroxycholesterol	De novo biosynthesis, fat deposits, or from blood
Progestins (21 carbons)	∆5‐Pregnenolone17α‐Hydroxypregnenolone17α‐HydroxyprogesteroneProgesterone	Leydig cells (mitochondria)
Androgens (19 carbons)	Dehydroepiandrosterone∆4‐Androstenedione∆5‐AndrostenediolTestosteroneDihydrostestosterone	Leydig cells (microsomal compartments)
Estrogens (18 carbons)	EstroneEstradiol‐17β	Leydig cells
Peptide family
Relaxin‐like peptides	Relaxin/insulin‐like peptide‐3	Leydig cells
Neuropeptides	OxytocinGlial cell‐derived factor	Leydig cellsSeroli cells
Cytokine family
	ActivinInhibin	Sertoli cellsSertoli cells
Glycoproteins
	Androgen binding proteinTesticular transferrin	Sertoli cellsSertoli cells

Cholesterol has been described as an obligatory intermediate in testosterone synthesis [5]. Testosterone is synthesized from a pool of metabolically active cholesterol, which is derived from either de novo biosynthesis of cholesterol, cholesterol esters stored as lipid droplets in the cell cytoplasm, or from blood plasma [68]. The conversion of cholesterol to testosterone involves five main enzymatic steps that include 20,22‐lyase, 3β‐dehydrogenase isomerase, 17α‐hydroxylase, 17,20‐lyase, and 17β‐hydroxysteroid dehydrogenase, and in some species the conversion of androgens to estrogens via the aromatase enzyme system [73]. Conversion of cholesterol to pregnenolone is the initial step in the pathway and is catalyzed by the cholesterol side‐chain cleavage enzyme complex, a three‐step process that takes place in the mitochondria of the cell [45]. Briefly, pregnenolone is formed from cholesterol (C27 sterol) by cleavage of the bond between C20 and C22 catalyzed by the multienzyme complex of the side‐chain cleavage system in the mitochondria, and metabolized in the microsomes by the microsomal enzyme complex 3β‐hydroxysteroid dehydrogenase/isomerase. Pregnenolone is also an obligatory intermediate in Leydig cell steroid synthesis. Van der Mollen and Rommerts [52] have indicated that the conversion of C21 steroids (i.e. pregnenolone) to C19 steroids (testosterone) may occur in the mammalian testis through two biosynthetic pathways. The crucial step in the biosynthesis of pregnenolone to androgens is the cleavage of the two‐carbon side‐chain of 17α‐hydroxyprogesterone or 17α‐hydroxypregnenolone by the 17,21‐lyase enzyme complex [74]. This is regarded as an irreversible reaction producing “weak” androgens (androstenedione and dehydroepiandrosterone, respectively). Through the action of 17β‐hydroxysteroid dehydrogenase, androstenedione is converted to the more potent androgen testosterone, which occurs in the microsomal compartments of the cell. However, testosterone is converted to dihydrotestosterone (DHT) by the 5α‐reductase enzyme system, and is regarded as the more biologically active androgen produced by the testis. A more detailed account of the mechanisms involved in testosterone biosynthesis can be found in the review by Hall [45], but a schematic overview of steroid synthesis in the bovine testis is presented in Figure 2.2.

Figure 2.2 Synthetic pathway of testosterone and conversion to active androgen and estrogen metabolites in the bull testis. Relevant enzyme systems involved in the synthesis are shown; in some instances, the enzyme reactions are reversible. Color code: blue, C₂₇ cholesterol steroid precursors; purple, C₂₁ progestin steroids; green, C₁₉ androgen steroids; red, C₁₈ estrogen steroids.

The unusual abundance of Leydig cells in domestic boars and stallions promoted the hypothesis that this 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].