INTRODUCTION
Spermatogenesis is a fundamental process in maintaining male fertility through the differentiation and self-renewal of male germ cells. Any abnormality in this process can result in subfertility/infertility. In addition to abnormal spermatogenesis, other factors, such as ill-health, disturbance in mating ability, reproductive problems, and behavioral disorders, also negatively affect stallion fertility. In the horse industry, unlike other domestic species, one primary reason for a lower fertility index than the considered standard in a stallion is that selection for breeding purposes is mainly based on performance records and conformation rather than reproductive soundness [1]. These maladies/practices demand more comprehensive research to address fertility in stallions to maximize profitability in the horse industry.
Somatic cells play a pivotal role in spermatogenesis. Different types of somatic cells are present in the lumen (Sertoli cells) and outer membrane (peritubular myoid [PM] cells) of the seminiferous tubules. Leydig cells are present within the connective tissue of the testicular stroma between the seminiferous tubules. In addition to the Leydig cells, nerve fibers, lymphatic vessels, the microvasculature, lymphocytes, fibroblasts, and macrophages surround the tubules [2]. These somatic cells, particularly Leydig and Sertoli cells, are season- and age-dependent in stallions. The number of Sertoli and Leydig cells is almost double in the breeding season compared with that in the non-breeding season [3]. Johnson and Neaves revealed age-related differences between 2- and 20-year-old stallions in the Leydig cell population. They found a five- and three-fold increase in Leydig cell volume and number/testes and a two- and three-fold increase in Leydig cell number and volume/g of parenchyma, respectively [4]. In another study of stallions aged from 6 months to 20 years, the highest number of Sertoli cell populations was recorded up to 4–5 years [5]. The endocrine system composed of complex network of organs and glands, uses the hormones to coordinate and regulate body metabolism, development, and reproduction. Paracrine pathway is cellular communication produce by cell to induce changes in nearby cells while autocrine pathway is a cell signaling originated from one cell and act on the same cell type [6]. Important paracrine-autocrine factors associated with spermatogenesis are included testosterone, estrogen, inhibin, activin, oxytocin, insulin-like peptide 3 (INSL3), proopiomelanocortin (POMC), transferrin, growth factors like transforming growth factor (TGF) alpha and beta, insulin-like growth factor-1 (IGFs). These factors are secreted from different types of somatic cells including Leydig, Sertoli, PM cells. The precise regulation of these somatic cells is required for a normal spermatogenesis. In this review, based on the emerging role of somatic cells in spermatogenesis, we summarized the biological function and development of Leydig, Sertoli, and PM cells and their roles in the regulation of spermatogenesis.
FUNCTIONS OF LEYDIG CELLS IN SPERMATOGENESIS
Leydig cells, which are important somatic cells, reside in the interstitial spaces between seminiferous tubules. The proliferation of Leydig cell precursors (stem Leydig cells), differentiation into immature Leydig cells, and final differentiation into adult Leydig cells are the three steps in the development of these somatic cells [7]. Leydig cells contribute to spermatogenesis in different ways. Leydig cells regulate spermatogenesis by affecting different growth factors. IGFs, inhibin, activin, and INSL3 are influenced by Leydig cells, thereby regulating spermatogenesis. IGF-1 plays a key role in spermatogenesis by differentiating spermatogonia [8] and regulating spermatogonial DNA synthesis [9]. Studies have also revealed the anti-apoptotic effects of IGF-1 on germ and Leydig cells in rats. In addition to that in experimental animals and humans, localization of IGF-1 has been reported in the testicular extract of stallions [10], Sertoli cells, Leydig cells of equine testicular cell culture [11], and stallion germ cells [12]. Hess and Roser found that IGF-1 levels are age- and season-dependent in stallions. They compared the IGF-1 levels in plasma and testicular extracts of stallions aged between 6 months and 23 years in breeding and non-breeding seasons. The highest level of IGF-1 was recorded in stallions less than 2 years of age (colt) compared with other age groups, and the seasonal difference was also recorded (highest level found in the breeding season) in colts. Significant differences in colts and fertility levels and no difference in other groups suggested that IFG-1 is involved in testicular development in stallions [10]. Another study conducted on the presence of IGF-1 and its receptor in stallion testicular tissue, and compared the expression patterns among different age groups found that regardless of the stage-dependent expression in germ cells, IGF-1 expression was found in the cytoplasm of Leydig cells at all stages, including pre-pubertal, pubertal, post-pubertal, and adult stallions. However, strong immunolabeling of IGF-1 and its receptor was found around puberty, suggesting that IGF-1 is involved in the proliferation of Leydig and germ cells via a paracrine/autocrine system [12]. Moreover, IGF-1 has been reported in the Leydig cells of dogs [13] and more transcription has been found in Leydig cells than in germ cells [14]. However, the presence of IGF-1 in somatic and germ cells indicates its significant role in spermatogenesis through proliferation and differentiation.
Inhibins and activins are glycoproteins and members of the TGF family. Inhibin is composed of a common α subunit primed with either a βA subunit (inhibin-A) or a βB (inhibin-B) subunit [15] considered to participate in regulating spermatogenesis. The inhibin-A isoform is detected in rams [16] and bulls [17] whereas the inhibin-B isoform is detected mainly in hamsters [18], miniature pigs [19], rats [20], and humans [15]. The localization of inhibin in Leydig [21] and Sertoli cells [22] has been reported in stallions. Reduced serum inhibin-B concentration seizes spermatogenesis [23] and is therefore used as a male fertility marker in many species [24]. In humans, it is used as a reliable marker of male fecundity [25]. Immunoreactive (ir-) inhibin in stallions increases together with estradiol, gonadotropin, and testosterone concentrations in the breeding season [21], and a decline in ir-inhibin concentration has been reported in infertile stallions [26]. In another study, ir-inhibin circulating concentration was 8.6-fold higher in stallions during the breeding season than in mares during the entire estrous cycle. Moreover, an 8.5-fold higher concentration was recorded in the testicular vein than in the jugular vein of stallions, whereas gonadectomized mares and stallions had no detectable ir-inhibin [27]. The presence of α and β subunits of inhibin in Leydig and Sertoli cells of stallions showed its possible role in regulating and maintaining spermatogenesis by secreting dimetric (bioactive) inhibin in the circulation of stallions. Furthermore, inhibin plasma concentration is season-dependent and thus can be used as a reliable indicator of testicular function in stallions during the breeding or non-breeding seasons [21]. Activity and production of activin A are necessary for the regulation of the seminiferous epithelium cycle during spermatogenesis [28] and stimulate DNA synthesis in germ cells (intermediate spermatogonia and preleptotene spermatocytes) in a dose-dependent manner [29]. Inhibin-A decreases DNA synthesis, whereas activin A increases the DNA synthesis thus participating in spermatogenesis by modulating DNA synthesis [29]. Activin is involved in the stimulation of follistatin and inhibin from Sertoli cells [30] and suppress steroidogenesis in Leydig cells of rats [31] through the paracrine/autocrine pathway. Studies have revealed the presence of activin in the follicular fluid [32] and placenta [33] of mares, but this has not yet been identified in stallion testes. Although its action in stallions is undefined, its presence in mares suggests that like in other species, it is assumed to regulate follicle-stimulating hormone release at the pituitary level [34]. Therefore, further studies are warranted in equines, especially stallions, to determine their presence and possible role in regulating spermatogenesis in stallions.
INSL3, previously called relaxin-like factor (RLF), secreted solely from Leydig cells, has been identified in several species, including stallions [35]. It has been used to recognize unambiguous Leydig cells in interstitial tissues [36] and differences in seasonal testicular functions [37] thereby acting as a typical marker for Leydig cell differentiation [38]. Recently, the expression pattern of INSL3 in Leydig cells and serum concentration levels have been reported in post-pubertal intact and cryptorchid stallions [35]. Serum INSL3 concentrations were recorded as higher to lower in intact, non-castrated unilateral, hemi-castrated unilateral, and bilateral cryptorchid stallions, respectively, and the expression level of INSL3 was positively correlated with the serum concentration [35]. A similar study by Klonisch et al. [38] revealed an upregulation and significantly strong expression of INSL3 in normally descended testes compared with that in the unilateral cryptorchid testes of stallions [39]. These results indicate a possible role of INSL3 in steroidogenesis and spermatogenesis in stallions.
Leydig cells are the main source of androgens [40]. The presence of androgen receptors (AR) in different reproductive parts of stallions, including the epididymis [41], testes [42], and prostate glands [43], and immunolabeling in all somatic cells, including Leydig cells [42], are important in spermatogenesis. Androgens are not only essential for male sexual differentiation and behavior [44] but also play an important role in maintaining spermatogenesis [45]. The possible mechanism is that androgen diffuses into the blood vessels of interstitial spaces and seminiferous tubules and binds to the AR present on Sertoli [46] and PM cells [47] thereby controlling male germ cell signaling and contributing to spermatogenesis [48]. Testosterone is the most common androgen involved in spermatogenesis maintenance. The intratesticular concentration is 10-fold higher than the serum concentration in rats, demonstrating the importance of testosterone in regulating spermatogenesis. The higher intratesticular concentration of testosterone is possibly because of its involvement in the progression of round spermatids, thereby controlling meiosis [49]. Previous studies revealed that testosterone withdrawal results in the failure of round spermatids to complete spermiogenesis [50] and high apoptosis of pachytene spermatocytes and round spermatids [51]. This theory is further strengthened by Stanton et al. [51], who found that deficiency or loss of testosterone influences changes in the expression of proteins involved in DNA repair, cell signaling, metabolism, and apoptosis [52] by facilitating meiotic progression without affecting spermatogonial proliferation [53]. 24-glutathione S-transferase A3 (GSTA3), dehydrocholesterol reductase (DHCR24), and squalene epoxidase (SQLE) are mainly expressed in Leydig cells. In humans and livestock, Δ4 -androstenedione is the immediate precursor of testosterone produced by the 3-ketosteroid isomerase activity of the GSTA3 protein. Administration of steroids (dexamethasone) in stallions lowered the biosynthesis of testosterone, resulting in a 60% decrease in the main testosterone concentration [54]. Transcriptome analysis in this study showed that the expression of genes involved in steroidogenesis (GSTA3 and aromatase), cholesterol synthesis (DHCR24 and SQLE), and hormone signaling (luteinizing hormone receptor, α-actinin 4, glucocorticoid receptor α) were sharply downregulated by dexamethasone and hence decreased testosterone levels because of the interference of the Δ5 -pathway of steroidogenesis in stallions [54]. In addition to controlling meiosis, testosterone plays an important role in regulating spermatogenesis by maintaining the blood-testes barrier [55], the release of mature sperm [56], and affects the expression level of connexin between round spermatids and Sertoli cells [57]. Oxytocin is an important autocrine, paracrine, and endocrine factor that is primarily involved in steroidogenesis and spermiation. Recently, its expression in different spermatogonia, spermatids, and Leydig cells of stallions has been studied [58]. The presence of oxytocin receptors in Leydig cells suggests that they are involved in steroidogenesis. Dose-dependent treatment with oxytocin results in increased basal testosterone production [59] and 5α-reductase activity in rat Leydig cells [60]. In vitro studies in mice and goats treated with oxytocin showed increased basal testosterone [61] and a 3.5-fold higher dihydrotestosterone concentration [62] respectively. It is suggested that the conversion of testosterone into dihydrotestosterone through 5α-reductase synthesis is regulated by the oxytocin level [63]. This hypothesis is further strengthened by the difference in oxytocin expression in the breeding and non-breeding seasons. The level of oxytocin in stallions was recorded to be higher during the breeding season [58]. However, the role of oxytocin and its exact mechanism in the Leydig cells of stallions and its involvement in stallion steroidogenesis require further studies. Aromatase P450 (aromatase), present in Sertoli and Leydig cells, is responsible for the modulation of steroidogenesis and spermatogenesis by converting testosterone to 17β-estradiol. Thus, estrogen is important for spermatogenesis and plays a key role in the development and maturation of reproductive tissues in males including stallions [64]. Leydig cell functions, controlled by follicle stimulating hormone (FSH) receptors located in Sertoli cells [65], and luteinizing hormone (LH) receptors present in Leydig cells are responsible for the production of testosterone and estrogen [64]. The expression of aromatase in Leydig cells [66], epididymis [67], Sertoli cells [66], seminiferous tubules [68], and prostate gland [67] in stallions was studied. The presence of aromatase in Sertoli cells indicates that, in stallions, estrogen production is not only controlled by Leydig cells but also by Sertoli cells via the paracrine pathway [66]. Studies have shown that immunolabeling and expression levels of aromatase are age-dependent in stallions. Hess and Roser found that the expression of aromatase in Leydig cells was more distinct as the age of stallions advanced, while immunolabeling in seminiferous tubules decreased, and no immunostaining was found in postpubertal stallions [68]. Aromatase expression in 2-year-old stallions was observed in two different types of Leydig cells, that is, Leydig cells with good reactivity (more distinct expression) and a weak reactivity (light expression) and no seasonal difference (from February and April to June) were found [66]. More studies are warranted to determine the role, estrogen plays in the stallion.
FUNCTIONS OF SERTOLI CELLS IN SPERMATOGENESIS
Sertoli cells provide structural, nutritional, and immunological support to spermatogonial stem cells (SSCs) and are often named the “nurse” or “mother” cells for SSCs. These are important parts of the testicular niche that play key roles in the development of SSCs and the formation of functional testes [69]. Sertoli cells in the seminiferous tubules can be recognized by their biochemical phenotypes and morphology. There are several distinctive features of Sertoli cells regarding the phenotype, such as FSHR, GATA4, Wilms’ tumor suppressor gene1 (WT1), SRY-box transcription factor 9 (SoX9), and vimentin. Morphologically, Sertoli cells have a smooth endoplasmic reticulum, abundant lipid droplets, well-developed nucleoli, intercellular junction complexes, mitochondria, and irregularly shaped nuclei [70]. SSC destiny is highly dependent and regulated by Sertoli cells in various manners.
Sertoli cells are involved in the self-renewal and differentiation of SSCs [71]. Stallion spermatogonia are usually classified into eight subtypes: As, Apr, Aal, A1, A2, A3, B1, and B2 spermatogonia [72]. Symmetric or asymmetric division of SSCs occurs during the infancy of spermatogenesis. The dominance of differentiation or self-renewal of SSCs may result in Sertoli cell-only syndrome (SCOR) or tumors, respectively [73]. Therefore, understanding the regulation mechanism of the precise equilibrium between the differentiation and self-renewal of SSCs is highly important for diagnosing male infertility. As a “mother” and important part of the niche, Sertoli cells play a significant role in modulating differentiation and self-renewal of SSCs by secreting different cytokines and growth factors. Retinoid acid (RA), a molecule derived from vitamin A in Sertoli cells, plays an important role in SSC differentiation. Primordial germ cells failed to enter meiosis in the absence of RA within the seminiferous cords of fetal testes [74] and increased meiotic activity and upregulation of receptor tyrosine kinase (KIT) expression (a marker of the switch from undifferentiated to differentiating spermatogonia) were observed in vitro after treatment with RA [75]. It has also been observed that Vitamin A-deficient rats or -mice have only type A spermatogonia and Sertoli cells with spermatogenesis arrest [76]. The consequences of aging result in different deleterious alterations, including developmental and genetic defects, stochastic, environmental, and epigenetic events, as well as innate aging processes [77]. Lower Sertoli cell ratios (germ cells / Sertoli cells), germ cell number, and volume [78] along with a reduction in sperm quality and fertility [79] are consequences of testicular aging. Recently, it was revealed that the lack of retinoic acid receptor-related orphan receptor-alpha (ROR-α) increases testicular aging [80] and abnormal Sertoli cells, reduction in Leydig cell number, vacuolation of seminiferous tubule epithelium, hypo-spermatogenesis, and poorly developed sperm were noted in 3-month-old RORα-/- mice [81]. It is suggested that during different stages of seminiferous epithelial development, bone morphogenetic protein 4 (BMP4), an important member of the TGF-β family released from Sertoli cells and spermatogonia in adult testes, helps in the differentiation of SSCs through autocrine and paracrine pathways [82]. BMP4 receptors (BMPRIa and BMPRII) promote RA signaling in mouse SSCs in vitro, and Noggin, a BMP4 antagonist, prevents the RA-induced expression of KIT and Stra8 [83]. KIT, an important molecular marker, is a receptor for stem cell factor (SCF). KIT localization in differentiating SSCs in all stages of stallions, including pre-pubertal, pubertal, post-pubertal, and adult stages, indicates the role of this protein in spermatogenesis and makes it a potential candidate for the isolation and identification of differentiating germ cells from stallions [84]. KIT and SCF are believed to regulate the differentiation or proliferation of SSCs by activating the PI3K/Akt pathway via type A spermatogonial DNA synthesis [85]. In vitro spermiation is achieved by the induction of CSF from telomerase-immortalized mouse type A spermatogonia [86]. A reduced apoptosis by up to 21.80%, 37.45%, and 44.40% in a short-term stallion testicular culture treated with SCF combined with granulocyte–macrophage colony-stimulating factor (GM-CSF), leukemia-inhibiting factor (LIF) + GM-CSF, and estradiol (E2) + LIF + GM-CSF, respectively [87]. Reduction in apoptosis in this study demonstrating the key role of SCF in the differentiation of SSCs. The expression levels and localization of RA and BMP4 and their in vitro studies in stallion testes are needed to determine their actions and species-specific characteristics for a better understanding of spermatogenesis in stallions.
Sertoli cells are involved in SSC self-renewal. Among TGF-β family members, glial cell line-derived neurotrophic factor (GDNF) is a major growth factor produced by Sertoli cells. Previous studies have revealed a connection between GDNF and SSC self-renewal [88]. Aggregation of undifferentiated spermatogonia was observed in GDNF-overexpressing mice, in contrast to a reduced percentage of spermatogonial proliferation in an age-dependent manner in Gdnf+/− mice [89]. GDNF family receptor alpha-1 (GFRA1) expressed in undifferentiated spermatogonia type A of stallions [72] is considered to be important for SSC self-renewal in stallions, as in mice [90]. More than 90% prevalence of GFRA1 labeling of undifferentiated stallion spermatogonial cells suggests that almost all subtypes of undifferentiated spermatogonia (As, Apr, and Aal) express the GFRA1 receptor in stallions, but further studies such as whole-mount analysis are warranted to confirm this finding [72]. In another study, TGF-α and vascular endothelial growth factor (VEGF) and its receptor VEGF-R2 were investigated in a dose-dependent treatment of peripubertal stallions with Durateston [91]. The expression of TGF-α and VEGF-R2 in peripubertal stallions castrated after four weeks of treatment was significantly higher than that in the control group. Similarly, significantly higher expression of VEGF-R2 was observed in Sertoli cells of stallions castrated after 12 weeks of treatment than after four weeks of treatment and the control group [91]. These studies showed that TGF is more important than other factors, such as IGF, LIF, and fibroblast growth factor 2 (FGF2), for SSCs self-renewal [92]. Transferrin, a glycoprotein is largely secreted from Sertoli cells and involved in the meiotic progression and responsible for the import of iron to the seminiferous tubules necessary for spermatogenesis [93]. In a study, a decrease in transferrin receptors in mice results in decrease of pachytene spermatocytes and accumulation of zygotene and leptotene spermatocytes, denoting early meiotic arrest [94].
Sertoli cells act as phagocytic agents of SSCs. Death or expansion of germ cells is necessary to maintain the ratio of germ cell-Sertoli cells to regulate spermatogenesis [73]. Over 50% of differentiating spermatogonia undergo apoptosis, a programmed death of SSCs. However, an increase in apoptosis may lead to subfertility/infertility, resulting in considerable financial loss in the equine industry. Normally, the number of differentiated A1 spermatogonia decreases to approximately 25% because of A2, A3, and A4 spermatogonia apoptosis [95]. For normal and efficient regulation of spermatogenesis, dead spermatogonia and residual bodies are degraded and phagocytosed by Sertoli cells through a phospholipid phosphatidylserine (PS)-dependent pathway [96]. In addition to the involvement of Sertoli cells in differentiation, self-renewal, and apoptosis, they play a key role in the de- [97] and trans-differentiation [98] of spermatogonial germ cells. Furthermore, alterations in the number or maturation [99], damage to cell junctions [100], and alteration of hormone receptors on [101] of Sertoli cells result in impaired spermatogenesis.
FUNCTIONS OF PERITUBULAR MYOID CELLS IN SPERMATOGENESIS
PM cells, thin smooth-muscle-like cells disseminate over the peripheral surface of the basal lamina of seminiferous tubules. Compared with other somatic cells, relatively little is known about their role in spermatogenesis [102]. These are an important part of the testicular niche and are considered to be involved in tubule contraction, which helps in spermiation [103]. They also help to form the basal lamina of the tubules by facilitating the secretion of extracellular matrix materials such as type I and IV collagens, fibronectin, and proteoglycans [104]. Decreases in testicular size, number of germ cells, and sperm production have been noted in a conditional knockout of the Ar gene in PM cells [47]. Furthermore, PM cells have recently been shown to play an important role in postnatal testicular growth by affecting the proliferation of Sertoli cells [105]. Based on these findings, we will discuss the possible and prominent role of PM cells in the structural integrity of seminiferous tubules and their participation in regulating steroidogenesis, spermatogenesis, and testicular function. The interactions of factors from all somatic cells including Leydig, Sertoli and PM cells in controlling spermatogenesis are illustrated in Fig. 1.
Fig. 1. A schematic diagram illustrating the paracrine-autocrine-endocrine interactions of factors from Leydig, Sertoli, and peritubular myoid cells. FSH, follicle stimulating hormone; LH, luteinizing hormone; INH, inhibin; IGF-1, insulin-like growth factor-1; E2, estradiol; T, testosterone; β-EP, beta-endorphin; Trans, transferrin; INSL3, insulin like peptide 3; PMOC, proopiomelanocortin; ACT, activing; TGF, transforming growth factor; OT, oxytocin; EST, estrogen; IL, interleukin; NGF, nerve growth factor; bFGF, basic fibroblast growth factor; PmodS, peritubular modifying substance; CSF1, colony stimulating factor 1; GDNF, glial cell line-derived neurotrophic factor; AR, androgen receptor; GATA4, GATA binding protein 4; LIF, leukemia inhibitory factor. The direction of arrowhead indicates the action on the respective site (e.g., FSH release from pituitary and act on Sertoli cell).
PM cells contribute to SSC proliferation. Previously, it was considered that GDNF produced by Sertoli cells secretes auxin-binding protein (ABP) and transferrin (TRF) and manipulates the differentiation of SSCs. However, testosterone treatment of PM cells has recently been shown to provoke GDNF secretion and regulate SSC development [106]. Reduced sperm count in the cauda epididymis [106] and a significantly low number of sperm and testicular volume [107] were noted in Gdnf-defective and SM cell AR knockout (PTM-ARKO) mice, respectively. Androgen is necessary for the manipulation of spermatogenesis and its receptors found in all somatic cells, including PM cells, at all ages of stallions, including pre-, peri-, and post-pubertal [42]. No age-specific differences in AR immunolabeling were recorded in stallion testes, suggesting that steroidogenesis and spermatogenesis are initiated and regulated by androgen via somatic cells, including PM cells, by the paracrine/autocrine pathway system, but not via germ cells. It has been hypothesized that androgen deposition is maintained by the interaction of PM cells with Leydig Cells because aberrant spermatogenesis is prominent in abnormal PM cells [108]. Reduction in the Steroidogenic factor (SF)-1 gene, which is essential for the differentiation of stem Leydig cells [109], and a decrease in the proportion of germ cell attachment to Sertoli cells [47] in PTM-ARKO mice indicated that PM cells play a role in the regulation of Leydig and Sertoli cells. In another study [42], presence of estrogen receptor-alpha (ESR-α) in PM cells in post-pubertal stallions but absent in pre-and peri-pubertal stallions suggesting that perhaps, in post-pubertal, the paracrine factors like peritubular myoid substances (Pmods) produced by PM cells regulate the functions of Sertoli cells [110]. Furthermore, ESR- β was not identified in PM cells of all age groups; therefore, it is hypothesized that in pre-and peri-pubertal stallions, estrogen actions are carried out in Leydig and Sertoli cells through ESR- β [42]. LIF produced mainly from PM cells regulate the proliferation and survival of SSCs [111]. The structural integrity of seminiferous tubules and the formation of the blood-testes-barrier are regulated by the paracrine effect of LIF [112]. Stallion testicular cultures treated for 6 h with GM-CSF, LIF, SCF, and E2 reduced apoptosis by 44.40% [87]. PM cells participate in SSC self-renewal. CSF1 found in PM cells, in combination with other growth factors, such as GDNF and FGF2, promotes SSC self-renewal. Oatley et al. [110] found 2.1-and 3.2-fold higher SSC numbers in CSF- treated testicular cell culture at 35 and 63 days, respectively [113]. Intermediate filaments composed of vimentin are used to calibrate the functional state of individual cells found in all somatic cells, including PM cells [93]. In stallion testicular tissue, strong immunoexpression was observed in somatic cells, including PM cells, in normal seminiferous tubules, whereas weak immunoexpression was observed in abnormal seminiferous tubules with low or no spermatogenesis [114]. Vimentin filaments are believed to be involved in the maturation of spermatogenesis via gap junction intercellular communication and maintenance of cell integrity therefore, their roles in stallion testes require further study for a better understanding of the regulation of spermatogenesis.
CONCLUSION AND FUTURE DIRECTION
Fertility is maintained by overlapping functions of different cell types. Although these functions are responsible for regulating spermatogenesis by different correcting or compensating factors when environmental or genetic factors impact the normal testicular structure. This overlap makes it difficult for researchers to understand the individual roles of different somatic cells in the self-renewal, proliferation, and apoptosis of SSCs. In vivo and in vitro cell/organ culture systems in laboratory animals are helping researchers understand the mechanisms of spermatogenesis; however, this type of research is limited to domestic animals, especially stallions. Culture studies will enable researchers to highlight and help diagnose the important cascade and cell type-specific factors involved in the fate of SSCs and provide a more conclusive understanding of the role of an individual cell or the contribution of distinct signaling molecules during spermatogenesis in domestic animals.
Most studies have been conducted within the seminiferous tubule (Sertoli cells-germ cells interaction) or interstitial (role of Leydig cells) compartments, compared with studies conducted to understand the influence and interaction of different somatic cells across the two compartments. More focus on the seminiferous tubule compartment and assumption of the “nurse-like” quantities of Sertoli cells results in more research focus on Sertoli cell-germ cell interaction pathways and less on other somatic cells, particularly PM cells. However, with the advancement of cell/tissue culture studies in laboratory animals, it is evident that the SSC niche is regulated by signals received from cells residing in interstitial and peritubular compartments. In addition, recent studies have revealed that the paracrine, autocrine, and endocrine signals provided by all testicular cells, including macrophages and somatic cells, influence germ cells directly or indirectly through Sertoli cells, thereby influencing spermatogenesis. Therefore, a comprehensive study of somatic cells, particularly PM cells, is required to understand spermatogenesis.
Several different growth factors, hormones, receptors, and cytokines procured from all three somatic cells, including Sertoli, PM, and Leydig cells, involved in the regulation and manipulation of spermatogenesis are illustrated in Table 1; however, the mechanisms by which these signals interact within somatic and germ cells are largely unknown in domestic animals, especially stallions. Therefore, thorough studies are warranted to understand these factors in-order to establish treatment regimens for infertility/subfertility in domestic animals including stallions. It is critical to maintain the fertility of stallion, therefore it is necessary to consider the reproductive soundness during selection of stallions for breeding purpose than the performance records and conformation. Moreover, establishment of a comprehensive protocol for stallion cell culture studies to explore different paracrine/autocrine factors produced from somatic cells, and treatment of somatic and germ cells (whole cell study) with different growth factors are necessary to improve the fertility in stallion.
Table 1. Factors/receptors produced or present in testes regulate steroidogenesis and spermatogenesis in stallions
+++, identified in stallions; ---, not identified in stallions.
IGF-1, insulin-like growth factor-1; FSH, follicle stimulating hormone; INSL3, insulin like peptide 3; AR, androgen receptor; PM, peritubular myoid; GSTA3, glutathione S-transferase A3; DHCR24, dehydrocholesterol reductase; SQLE, squalene epoxidase; LH, luteinizing hormone; ESR, estrogen receptor; KIT, receptor tyrosine kinase; SSCs, spermatogonial stem cells; PI3K, phosphoinositide 3-kinases; Akt, Ak strain transforming; BMP4, bone morphogenetic protein 4; VEGF, vascular endothelial growth factor; LIF, leukemia inhibitory factor; GDNF, glial cell line-derived neurotrophic factor; GFRA1, GDNF family receptor alpha-1; FSHR, follicle stimulating hormone receptor.
참고문헌
- Varner DD, Gibb Z, Aitken RJ. Stallion fertility: a focus on the spermatozoon. Equine Vet J. 2015;47:16-24. https://doi.org/10.1111/evj.12308
- Zhou R, Wu J, Liu B, Jiang Y, Chen W, Li J, et al. The roles and mechanisms of Leydig cells and myoid cells in regulating spermatogenesis. Cell Mol Life Sci. 2019;76:2681-95. https://doi.org/10.1007/s00018-019-03101-9
- Johnson L, Tatum ME. Temporal appearance of seasonal changes in numbers of Sertoli cells, Leydig cells, and germ cells in stallions. Biol Reprod. 1989;40:994-9. https://doi.org/10.1095/biolreprod40.5.994
- Johnson L. Efficiency of spermatogenesis. Microsc Res Tech. 1995;32:385-422. https://doi.org/10.1002/jemt.1070320504
- Johnson L, Thompson DL Jr. Age-related and seasonal variation in the Sertoli cell population, daily sperm production and serum concentrations of follicle-stimulating hormone, luteinizing hormone and testosterone in stallions. Biol Reprod. 1983;29:777-89. https://doi.org/10.1095/biolreprod29.3.777
- Chamindrani Mendis-Handagama SML, Siril Ariyaratne HB. Differentiation of the adult Leydig cell population in the postnatal testis. Biol Reprod. 2001;65:660-71. https://doi.org/10.1095/biolreprod65.3.660
- Tajima Y, Watanabe D, Koshimizu U, Matsuzawa T, Nishimune Y. Insulin-like growth factor-I and transforming growth factor-1α stimulate differentiation of type A spermatogonia in organ culture of adult mouse cryptorchid testes. Int J Androl. 1995;18:8-12. https://doi.org/10.1111/j.1365-2605.1995.tb00928.x
- Soder O, Bang P, Wahab A, Parvinen M. Insulin-like growth factors selectively stimulate spermatogonial, but not meiotic, deoxyribonucleic acid synthesis during rat spermatogenesis. Endocrinology. 1992;131:2344-50. https://doi.org/10.1210/endo.131.5.1425434
- Hess MF, Roser JF. The effects of age, season and fertility status on plasma and intratesticular insulin-like growth factor I concentration in stallions. Theriogenology. 2001;56:723-33. https://doi.org/10.1016/S0093-691X(01)00602-1
- Roser JF. Endocrine and paracrine control of sperm production in stallions. Anim Reprod Sci. 2001;68:139-51. https://doi.org/10.1016/S0378-4320(01)00151-8
- Yoon MJ, Berger T, Roser JF. Localization of insulin-like growth factor-I (IGF-I) and IGF-I receptor (IGF-IR) in equine testes. Reprod Domest Anim. 2011;46:221-8. https://doi.org/10.1111/j.1439-0531.2010.01643.x
- Muller L, Kowalewski MP, Reichler IM, Kollar E, Balogh O. Different expression of leptin and IGF1 in the adult and prepubertal testis in dogs. Reprod Domest Anim. 2017;52:187-92. https://doi.org/10.1111/rda.12896
- Yuan C, Chen K, Zhu Y, Yuan Y, Li M. Medaka igf1 identifies somatic cells and meiotic germ cells of both sexes. Gene. 2018;642:423-9. https://doi.org/10.1016/j.gene.2017.11.037
- Makanji Y, Zhu J, Mishra R, Holmquist C, Wong WPS, Schwartz NB, et al. Inhibin at 90: from discovery to clinical application, a historical review. Endocr Rev. 2014;35:747-94. https://doi.org/10.1210/er.2014-1003
- McNeilly AS, Souza CJ, Baird DT, Swanston IA, McVerry J, Crawford J, et al. Production of inhibin A not B in rams: changes in plasma inhibin A during testis growth, and expression of inhibin/activin subunit mRNA and protein in adult testis. Reproduction. 2002;123:827-35. https://doi.org/10.1530/rep.0.1230827
- Kaneko H, Noguchi J, Kikuchi K, Hasegawa Y. Molecular weight forms of inhibin A and inhibin B in the bovine testis change with age. Biol Reprod. 2003;68:1918-25. https://doi.org/10.1095/biolreprod.102.012856
- Jin W, Wada S, Arai KY, Kishi H, Herath CB, Watanabe G, et al. Testicular secretion of inhibin in the male golden hamster: (Mesocricetus auratus). J Androl. 2001;22:207-11. https://doi.org/10.1002/j.1939-4640.2001.tb02173.x
- Weng Q, Medan MS, Watanabe G, Tsubota T, Tanioka Y, Taya K. Immunolocalization of steroidogenic enzymes P450scc, 3βHSD, P450c17, and P450arom in Gottingen miniature pig testes. J Reprod Dev. 2005;51:299-304. https://doi.org/10.1262/jrd.16077
- Woodruff TK, Besecke LM, Groome N, Draper LB, Schwartz NB, Weiss J. Inhibin A and inhibin B are inversely correlated to follicle-stimulating hormone, yet are discordant during the follicular phase of the rat estrous cycle, and inhibin A is expressed in a sexually dimorphic manner. Endocrinology. 1996;137:5463-7. https://doi.org/10.1210/endo.137.12.8940372
- Nagata S, Tsunoda N, Nagamine N, Tanaka Y, Taniyama H, Nambo Y, et al. Testicular inhibin in the stallion: cellular source and seasonal changes in its secretion. Biol Reprod. 1998;59:62-8. https://doi.org/10.1095/biolreprod59.1.62
- Taya K, Nagata S, Tsunoda N, Nagamine N, Tanaka Y, Nagaoka K, et al. Testicular secretion of inhibin in stallions. J Reprod Fertil Suppl. 2000;56:43-50.
- Pierik FH, Vreeburg JTM, Stijnen T, de Jong FH, Weber RFA. Serum inhibin B as a marker of spermatogenesis. J Clin Endocrinol Metab. 1998;83:3110-4. https://doi.org/10.1210/jcem.83.9.5121
- Kumanov P, Nandipati K, Tomova A, Agarwal A. Inhibin B is a better marker of spermatogenesis than other hormones in the evaluation of male factor infertility. Fertil Steril. 2006;86:332-8. https://doi.org/10.1016/j.fertnstert.2006.01.022
- Mabeck LM, Jensen MS, Toft G, Thulstrup M, Andersson M, Jensen TK, et al. Fecundability according to male serum inhibin B-a prospective study among first pregnancy planners. Hum Reprod. 2005;20:2909-15. https://doi.org/10.1093/humrep/dei141
- Stewart BL, Roser JF. Effects of age, season, and fertility status on plasma and intratesticular immunoreactive (IR) inhibin concentrations in stallions. Domest Anim Endocrinol. 1998;15:129-39. https://doi.org/10.1016/S0739-7240(97)00083-0
- Roser JF, McCue PM, Hoye E. Inhibin activity in the mare and stallion. Domest Anim Endocrinol. 1994;11:87-100. https://doi.org/10.1016/0739-7240(94)90037-X
- Hedger MP, Winnall WR. Regulation of activin and inhibin in the adult testis and the evidence for functional roles in spermatogenesis and immunoregulation. Mol Cell Endocrinol. 2012;359:30-42. https://doi.org/10.1016/j.mce.2011.09.031
- Hakovirta H, Kaipia A, Soder O, Parvinen M. Effects of activin-A, inhibin-A, and transforming growth factor-beta 1 on stage-specific deoxyribonucleic acid synthesis during rat seminiferous epithelial cycle. Endocrinology. 1993;133:1664-8. https://doi.org/10.1210/endo.133.4.8404607
- de Kretser DM, Buzzard JJ, Okuma Y, O'Connor AE, Hayashi T, Lin SY, et al. The role of activin, follistatin and inhibin in testicular physiology. Mol Cell Endocrinol. 2004;225:57-64. https://doi.org/10.1016/j.mce.2004.07.008
- Lejeune H, Chuzel F, Sanchez P, Durand P, Mather JP, Saez JM. Stimulating effect of both human recombinant inhibin A and activin A on immature porcine Leydig cell functions in vitro. Endocrinology. 1997;138:4783-91. https://doi.org/10.1210/endo.138.11.5542
- Beg MA, Ginther OJ. Follicle selection in cattle and horses: role of intrafollicular factors. Reproduction. 2006;132:365-77. https://doi.org/10.1530/rep.1.01233
- Arai KY, Tanaka Y, Taniyama H, Tsunoda N, Nambo Y, Nagamine N, et al. Expression of inhibins, activins, insulin-like growth factor-I and steroidogenic enzymes in the equine placenta. Domest Anim Endocrinol. 2006;31:19-34. https://doi.org/10.1016/j.domaniend.2005.09.005
- Roser JF. Regulation of testicular function in the stallion: an intricate network of endocrine, paracrine and autocrine systems. Anim Reprod Sci. 2008;107:179-96. https://doi.org/10.1016/j.anireprosci.2008.05.004
- Tsogtgerel M, Komyo N, Murase H, Hannan MA, Watanabe K, Ohtaki T, et al. Serum concentrations and testicular expressions of insulin-like peptide 3 and anti-Mullerian hormone in normal and cryptorchid male horses. Theriogenology. 2020;154:135-42. https://doi.org/10.1016/j.theriogenology.2020.05.026
- Caprio M, Fabbrini E, Ricci G, Basciani S, Gnessi L, Arizzi M, et al. Ontogenesis of leptin receptor in rat Leydig cells. Biol Reprod. 2003;68:1199-207. https://doi.org/10.1095/biolreprod.102.007831
- Hombach-Klonisch S, Schon J, Kehlen A, Blottner S, Klonisch T. Seasonal expression of INSL3 and Lgr8/Insl3 receptor transcripts indicates variable differentiation of Leydig cells in the roe deer testis. Biol Reprod. 2004;71:1079-87. https://doi.org/10.1095/biolreprod.103.024752
- Sadeghian H, Anand-Ivell R, Balvers M, Relan V, Ivell R. Constitutive regulation of the Insl3 gene in rat Leydig cells. Mol Cell Endocrinol. 2005;241:10-20. https://doi.org/10.1016/j.mce.2005.03.017
- Klonisch T, Steger K, Kehlen A, Allen WR, Froehlich C, Kauffold J, et al. INSL3 ligandreceptor system in the equine testis. Biol Reprod. 2003;68:1975-81. https://doi.org/10.1095/biolreprod.102.008466
- Shima Y, Miyabayashi K, Haraguchi S, Arakawa T, Otake H, Baba T, et al. Contribution of Leydig and Sertoli cells to testosterone production in mouse fetal testes. Mol Endocrinol. 2013;27:63-73. https://doi.org/10.1210/me.2012-1256
- Parlevliet JM, Pearl CA, Hess MF, Famula TR, Roser JF. Immunolocalization of estrogen and androgen receptors and steroid concentrations in the stallion epididymis. Theriogenology. 2006;66:755-65. https://doi.org/10.1016/j.theriogenology.2005.12.013
- Pearl CA, Mason H, Roser JF. Immunolocalization of estrogen receptor alpha, estrogen receptor beta and androgen receptor in the pre-, peri- and post-pubertal stallion testis. Anim Reprod Sci. 2011;125:103-11. https://doi.org/10.1016/j.anireprosci.2011.03.007
- Bilinska B, Hejmej A, Gancarczyk M, Sadowska J. Immunoexpression of androgen receptors in the reproductive tract of the stallion. Ann NY Acad Sci. 2005;1040:227-9. https://doi.org/10.1196/annals.1327.030
- Antonio-Cabrera E, Paredes RG. Effects of chronic estradiol or testosterone treatment upon sexual behavior in sexually sluggish male rats. Pharmacol Biochem Behav. 2012;101:336-41. https://doi.org/10.1016/j.pbb.2012.01.021
- Jarow JP, Zirkin BR. The androgen microenvironment of the human testis and hormonal control of spermatogenesis. Ann NY Acad Sci. 2005;1061:208-20. https://doi.org/10.1196/annals.1336.023
- Willems A, Roesl C, Mitchell RT, Milne L, Jeffery N, Smith S, et al. Sertoli cell androgen receptor signalling in adulthood is essential for post-meiotic germ cell development. Mol Reprod Dev. 2015;82:626-7. https://doi.org/10.1002/mrd.22506
- Welsh M, Saunders PTK, Atanassova N, Sharpe RM, Smith LB. Androgen action via testicular peritubular myoid cells is essential for male fertility. FASEB J. 2009;23:4218-30. https://doi.org/10.1096/fj.09-138347
- Haider SG. Cell biology of Leydig cells in the testis. Int Rev Cytol. 2004;233:181-241. https://doi.org/10.1016/S0074-7696(04)33005-6
- O'Donnell L, McLachlan RI, Wreford NG, Robertson DM. Testosterone promotes the conversion of round spermatids between stages VII and VIII of the rat spermatogenic cycle. Endocrinology. 1994;135:2608-14. https://doi.org/10.1210/endo.135.6.7988449
- O'Donnell L, McLachlan RI, Wreford NG, de Kretser DM. Testosterone withdrawal promotes stage-specific detachment of round spermatids from the rat seminiferous epithelium. Biol Reprod. 1996;55:895-901. https://doi.org/10.1095/biolreprod55.4.895
- Bartlett JMS, Kerr JB, Sharpe RM. The effect of selective destruction and regeneration of rat Leydig cells on the intratesticular distribution of testosterone and morphology of the seminiferous epithelium. J Androl. 1986;7:240-53. https://doi.org/10.1002/j.1939-4640.1986.tb00924.x
- Stanton PG, Sluka P, Foo CFH, Stephens AN, Smith AI, McLachlan RI, et al. Proteomic changes in rat spermatogenesis in response to in vivo androgen manipulation; impact on meiotic cells. PLOS ONE. 2012;7:e41718. https://doi.org/10.1371/journal.pone.0041718
- Haywood M, Spaliviero J, Jimemez M, King NJC, Handelsman DJ, Allan CM. Sertoli and germ cell development in hypogonadal (hpg) mice expressing transgenic follicle-stimulating hormone alone or in combination with testosterone. Endocrinology. 2003;144:509-17. https://doi.org/10.1210/en.2002-220710
- Ing NH, Forrest DW, Riggs PK, Loux S, Love CC, Brinsko SP, et al. Dexamethasone acutely down-regulates genes involved in steroidogenesis in stallion testes. J Steroid Biochem Mol Biol. 2014;143:451-9. https://doi.org/10.1016/j.jsbmb.2014.07.003
- Pelletier RM. The blood-testis barrier: the junctional permeability, the proteins and the lipids. Prog Histochem Cytochem. 2011;46:49-127. https://doi.org/10.1016/j.proghi.2011.05.001
- Holdcraft RW, Braun RE. Androgen receptor function is required in Sertoli cells for the terminal differentiation of haploid spermatids. Development. 2004;131:459-67. https://doi.org/10.1242/dev.00957
- Kopera IA, Bilinska B, Cheng CY, Mruk DD. Sertoli-germ cell junctions in the testis: a review of recent data. Philos Trans R Soc B Biol Sci. 2010;365:1593-605. https://doi.org/10.1098/rstb.2009.0251
- Jung Y, Yoon M. Oxytocin receptor expression in stallion testes and epididymides. Domest Anim Endocrinol. 2021;74:106562. https://doi.org/10.1016/j.domaniend.2020.106562
- Frayne J, Nicholson HD. Effect of oxytocin on testosterone production by isolated rat Leydig cells is mediated via a specific oxytocin receptor. Biol Reprod. 1995;52:1268-73. https://doi.org/10.1095/biolreprod52.6.1268
- Nicholson HD, Jenkin L. 5α-Reductase activity increased by oxytocin in the rat testis. In: Bartke A, editor. Function of somatic cells in the testis. New York, NY: Springer; 1994. p. 278-85.
- Tahri-Joutei A, Pointis G. Time-related effects of arginine vasopressin on steroidogenesis in cultured mouse Leydig cells. J Reprod Fertil. 1988;82:247-54. https://doi.org/10.1530/jrf.0.0820247
- Inaba T, Nakayama Y, Tani H, Tamada H, Kawate N, Sawada T. Oxytocin gene expression and action in goat testis. Theriogenology. 1999;52:425-34. https://doi.org/10.1016/S0093-691X(99)00140-5
- Assinder SJ, Johnson C, King K, Nicholson HD. Regulation of 5α-reductase isoforms by oxytocin in the rat ventral prostate. Endocrinology. 2004;145:5767-73. https://doi.org/10.1210/en.2004-0711
- Hess MF, Roser JF. A comparison of the effects of equine luteinizing hormone (eLH), equine growth hormone (eGH) and human recombinant insulin-like growth factor (hrIGF-I) on steroid production in cultured equine Leydig cells during sexual maturation. Anim Reprod Sci. 2005;89:7-19. https://doi.org/10.1016/j.anireprosci.2005.06.014
- O'Shaughnessy PJ, Bennett MK, Scott IS, Charlton HM. Effects of FSH on Leydig cell morphology and function in the hypogonadal mouse. J Endocrinol. 1992;135:517-25. https://doi.org/10.1677/joe.0.1350517
- Sipahutar H, Sourdaine P, Moslemi S, Plainfosse B, Seralini GE. Immunolocalization of aromatase in stallion Leydig cells and seminiferous tubules. J Histochem Cytochem. 2003;51:311-8. https://doi.org/10.1177/002215540305100306
- Hejmej A, Gorazd M, Kosiniak-Kamysz K, Wiszniewska B, Sadowska J, Bilinska B. Expression of aromatase and oestrogen receptors in reproductive tissues of the stallion and a single cryptorchid visualised by means of immunohistochemistry. Domest Anim Endocrinol. 2005;29:534-47. https://doi.org/10.1016/j.domaniend.2005.03.002
- Hess MF, Roser JF. Immunocytochemical localization of cytochrome P450 aromatase in the testis of prepubertal, pubertal, and postpubertal horses. Theriogenology. 2004;61:293-9. https://doi.org/10.1016/S0093-691X(03)00237-1
- Oatley JM, Brinster RL. The germline stem cell niche unit in mammalian testes. Physiol Rev. 2012;92:577-95. https://doi.org/10.1152/physrev.00025.2011
- Chui K, Trivedi A, Cheng CY, Cherbavaz DB, Dazin PF, Huynh ALT, et al. Characterization and functionality of proliferative human Sertoli cells. Cell Transplant. 2011;20:619-35. https://doi.org/10.3727/096368910X536563
- Li Y, Wu Q, Li X, Von Tungeln LS, Beland FA, Petibone D, et al. In vitro effects of cannabidiol and its main metabolites in mouse and human Sertoli cells. Food Chem Toxicol. 2022;159:112722. https://doi.org/10.1016/j.fct.2021.112722
- Costa GMJ, Avelar GF, Rezende-Neto JV, Campos-Junior PHA, Lacerda SMSN, Andrade BSC, et al. Spermatogonial stem cell markers and niche in equids. PLOS ONE. 2012;7:e44091. https://doi.org/10.1371/journal.pone.0044091
- Hai Y, Hou J, Liu Y, Liu Y, Yang H, Li Z, et al. The roles and regulation of Sertoli cells in fate determinations of spermatogonial stem cells and spermatogenesis. Semin Cell Dev Biol. 2014;29:66-75. https://doi.org/10.1016/j.semcdb.2014.04.007
- Koubova J, Menke DB, Zhou Q, Capel B, Griswold MD, Page DC. Retinoic acid regulates sex-specific timing of meiotic initiation in mice. Proc Natl Acad Sci USA. 2006;103:2474-9. https://doi.org/10.1073/pnas.0510813103
- Pellegrini M, Filipponi D, Gori M, Barrios F, Lolicato F, Grimaldi P, et al. ATRA and KL promote differentiation toward the meiotic program of male germ cells. Cell Cycle. 2008;7:3878-88. https://doi.org/10.4161/cc.7.24.7262
- van Pelt AMM, de Rooij DG. Synchronization of the seminiferous epithelium after vitamin A replacement in vitamin A-deficient mice. Biol Reprod. 1990;43:363-7. https://doi.org/10.1095/biolreprod43.3.363
- Harman D. Aging: phenomena and theories. Ann NY Acad Sci. 1998;854:1-7. https://doi.org/10.1111/j.1749-6632.1998.tb09886.x
- Jiang H, Zhu WJ, Li J, Chen QJ, Liang WB, Gu YQ. Quantitative histological analysis and ultrastructure of the aging human testis. Int Urol Nephrol. 2014;46:879-85. https://doi.org/10.1007/s11255-013-0610-0
- Hellstrom WJG, Overstreet JW, Sikka SC, Denne J, Ahuja S, Hoover AM, et al. Semen and sperm reference ranges for men 45 years of age and older. J Androl. 2006;27:421-8. https://doi.org/10.2164/jandrol.05156
- Sayed RKA, Mokhtar DM, Fernandez-Ortiz M, Fernandez-Martinez J, Aranda-Martinez P, Escames G, et al. Lack of retinoid acid receptor-related orphan receptor alpha accelerates and melatonin supplementation prevents testicular aging. Aging. 2020;12:12648-68. https://doi.org/10.18632/aging.103654
- Sayed RKA, Mokhtar DM, Fernandez-Ortiz M, Escames G, Acuna-Castroviejo D. Retinoid-related orphan nuclear receptor alpha (RORα)-deficient mice display morphological testicular defects. Lab Invest. 2019;99:1835-49. https://doi.org/10.1038/s41374-019-0299-5
- Pellegrini M, Grimaldi P, Rossi P, Geremia R, Dolci S. Developmental expression of BMP4/ALK3/SMAD5 signaling pathway in the mouse testis: a potential role of BMP4 in spermatogonia differentiation. J Cell Sci. 2003;116:3363-72. https://doi.org/10.1242/jcs.00650
- Yang Y, Feng Y, Feng X, Liao S, Wang X, Gan H, et al. BMP4 cooperates with retinoic acid to induce the expression of differentiation markers in cultured mouse spermatogonia. Stem Cells Int. 2016;2016:9536192. https://doi.org/10.1155/2016/9536192
- Jung H, Song H, Yoon M. The KIT is a putative marker for differentiating spermatogonia in stallions. Anim Reprod Sci. 2015;152:39-46. https://doi.org/10.1016/j.anireprosci.2014.11.004
- Feng LX, Ravindranath N, Dym M. Stem cell factor/c-kit up-regulates cyclin D3 and promotes cell cycle progression via the phosphoinositide 3-kinase/p70 S6 kinase pathway in spermatogonia. J Biol Chem. 2000;275:25572-6. https://doi.org/10.1074/jbc.M002218200
- Feng LX, Chen Y, Dettin L, Pera RAR, Herr JC, Goldberg E, et al. Generation and in vitro differentiation of a spermatogonial cell line. Science. 2002;297:392-5. https://doi.org/10.1126/science.1073162
- Donnelly CL, Staub C, Varner D, Blanchard T, Johnson L, Forrest DW. The effects of growth factor on testicular germ cell apoptosis in the stallion. J Equine Vet Sci. 2007;27:212-6. https://doi.org/10.1016/j.jevs.2007.04.003
- Meng X, Lindahl M, Hyvonen ME, Parvinen M, de Rooij DG, Hess MW, et al. Regulation of cell fate decision of undifferentiated spermatogonia by GDNF. Science. 2000;287:1489-93. https://doi.org/10.1126/science.287.5457.1489
- He Z, Jiang J, Kokkinaki M, Golestaneh N, Hofmann MC, Dym M. Gdnf upregulates c-Fos transcription via the Ras/Erk1/2 pathway to promote mouse spermatogonial stem cell proliferation. Stem Cells. 2008;26:266-78. https://doi.org/10.1634/stemcells.2007-0436
- Hofmann MC, Braydich-Stolle L, Dym M. Isolation of male germ-line stem cells; influence of GDNF. Dev Biol. 2005;279:114-24. https://doi.org/10.1016/j.ydbio.2004.12.006
- Teubner A, Muller K, Bartmann CP, Sieme H, Klug E, Zingrebe B, et al. Effects of an anabolic steroid (Durateston) on testicular angiogenesis in peripubertal stallions. Theriogenology. 2015;84:323-32. https://doi.org/10.1016/j.theriogenology.2015.03.022
- Kubota H, Avarbock MR, Brinster RL. Growth factors essential for self-renewal and expansion of mouse spermatogonial stem cells. Proc Natl Acad Sci USA. 2004;101:16489-94. https://doi.org/10.1073/pnas.0407063101
- Allan DJ, Harmon BV, Roberts SA. Spermatogonial apoptosis has three morphologically recognizable phases and shows no circadian rhythm during normal spermatogenesis in the rat. Cell Prolif. 1992;25:241-50. https://doi.org/10.1111/j.1365-2184.1992.tb01399.x
- Welt C, Sidis Y, Keutmann H, Schneyer A. Activins, inhibins, and follistatins: from endocrinology to signaling. A paradigm for the new millennium. Exp Biol Med. 2002;227:724-52. https://doi.org/10.1177/153537020222700905
- Gao T, Lin M, Wu Y, Li K, Liu C, Zhou Q, et al. Transferrin receptor (TFRC) is essential for meiotic progression during mouse spermatogenesis. Zygote. 2021;29:169-75. https://doi.org/10.1017/S0967199420000659
- Zhang Z, Gong Y, Guo Y, Hai Y, Yang H, Yang S, et al. Direct transdifferentiation of spermatogonial stem cells to morphological, phenotypic and functional hepatocyte-like cells via the ERK1/2 and Smad2/3 signaling pathways and the inactivation of cyclin A, cyclin B and cyclin E. Cell Commun Signal. 2013;11:67. https://doi.org/10.1186/1478-811X-11-67
- Sharpe RM, McKinnell C, Kivlin C, Fisher JS. Proliferation and functional maturation of Sertoli cells, and their relevance to disorders of testis function in adulthood. Reproduction. 2003;125:769-84. https://doi.org/10.1530/rep.0.1250769
- Salian S, Doshi T, Vanage G. Neonatal exposure of male rats to bisphenol A impairs fertility and expression of sertoli cell junctional proteins in the testis. Toxicology. 2009;265:56-67. https://doi.org/10.1016/j.tox.2009.09.012
- Abel MH, Baker PJ, Charlton HM, Monteiro A, Verhoeven G, De Gendt K, et al. Spermatogenesis and sertoli cell activity in mice lacking sertoli cell receptors for folliclestimulating hormone and androgen. Endocrinology. 2008;149:3279-85. https://doi.org/10.1210/en.2008-0086
- Skinner MK, Griswold MD. Sertoli cell biology. Burlington, NJ: Elsevier; 2004
- Losinno AD, Morales A, Fernandez D, Lopez LA. Peritubular myoid cells from rat seminiferous tubules contain actin and myosin filaments distributed in two independent layers. Biol Reprod. 2012;86:150, 1-8. https://doi.org/10.1095/biolreprod.111.095158
- Skinner MK, Tung PS, Fritz IB. Cooperativity between Sertoli cells and testicular peritubular cells in the production and deposition of extracellular matrix components. J Cell Biol. 1985;100:1941-7. https://doi.org/10.1083/jcb.100.6.1941
- Nurmio M, Kallio J, Adam M, Mayerhofer A, Toppari J, Jahnukainen K. Peritubular myoid cells have a role in postnatal testicular growth. Spermatogenesis. 2012;2:79-87. https://doi.org/10.4161/spmg.20067
- Chen LY, Willis WD, Eddy EM. Targeting the Gdnf gene in peritubular myoid cells disrupts undifferentiated spermatogonial cell development. Proc Natl Acad Sci USA. 2016;113:1829-34. https://doi.org/10.1073/pnas.1517994113
- Chen LY, Brown PR, Willis WB, Eddy EM. Peritubular myoid cells participate in male mouse spermatogonial stem cell maintenance. Endocrinology. 2014;155:4964-74. https://doi.org/10.1210/en.2014-1406
- Quigley CA, de Bellis A, Marschke KB, El-Awady MK, Wilson EM, French FS. Androgen receptor defects: historical, clinical, and molecular perspectives. Endocr Rev. 1995;16:271-321. https://doi.org/10.1210/edrv-16-3-271
- Welsh M, Moffat L, Belling K, de Franca LR, Segatelli TM, Saunders PTK, et al. Androgen receptor signalling in peritubular myoid cells is essential for normal differentiation and function of adult Leydig cells. Int J Androl. 2012;35:25-40. https://doi.org/10.1111/j.1365-2605.2011.01150.x
- Skinner MK, Fritz IB. Identification of a non-mitogenic paracrine factor involved in mesenchymal-epithelial cell interactions between testicular peritubular cells and Sertoli cells. Mol Cell Endocrinol. 1986;44:85-97. https://doi.org/10.1016/0303-7207(86)90109-7
- Piquet-Pellorce C, Dorval-Coiffec I, Pham MD, Jegou B. Leukemia inhibitory factor expression and regulation within the testis. Endocrinology. 2000;141:1136-41. https://doi.org/10.1210/endo.141.3.7399
- Maekawa M, Kamimura K, Nagano T. Peritubular myoid cells in the testis: their structure and function. Arch Histol Cytol. 1996;59:1-13. https://doi.org/10.1679/aohc.59.1
- Oatley JM, Oatley MJ, Avarbock MR, Tobias JW, Brinster RL. Colony stimulating factor 1 is an extrinsic stimulator of mouse spermatogonial stem cell self-renewal. Development. 2009;136:1191-9. https://doi.org/10.1242/dev.032243
- Virtanen I, Kallajoki M, Naurvaunen O, Paranko J, Thornell LE, Miettinen M, et al. Peritubular myoid cells of human and rat testis are smooth muscle cells that contain desmin-type intermediate filaments. Anat Rec. 1986;215:10-20. https://doi.org/10.1002/ar.1092150103
- Lydka M, Kotula-Balak M, Kopera-Sobota I, Tischner M, Bilinska B. Vimentin expression in testes of Arabian stallions. Equine Vet J. 2011;43:184-9. https://doi.org/10.1111/j.2042-3306.2010.00135.x
- Show MD, Anway MD, Folmer JS, Zirkin BR. Reduced intratesticular testosterone concentration alters the polymerization state of the Sertoli cell intermediate filament cytoskeleton by degradation of vimentin. Endocrinology. 2003;144:5530-6. https://doi.org/10.1210/en.2003-0735