Effect of continuous light and darkness on the testicular histology of toad (Bufo melanostictus)

Continuous darkness decreases spermatogenesis as well as Leydig cell function whereas continuous illumination suppresses spermatogenesis along with increased Leydig cell activity.     

Luteinizing hormone receptor-mediated effects on initiation of spermatogenesis in gonadotropin-deficient (hpg) mice are replicated by testosterone

Testosterone (T) is an absolute requirement for spermatogenesis and is supplied by mature Leydig cells stimulated by LH. We previously showed in gonadotropin-deficient hpg mice that T alone initiates qualitatively complete spermatogenesis bypassing LH-dependent Leydig cell maturation and steroidogenesis. However, because maximal T effects do not restore testis weight or germ cell number to wild-type control levels, additional Leydig cell factors may be involved. We therefore examined 1). whether chronic hCG administration to restore Leydig cell maturation and steroidogenesis can restore quantitatively normal spermatogenesis and testis development and 2). whether nonandrogenic Leydig cell products are required to initiate spermatogenesis. Weanling hpg mice were administered hCG (0.1-100 IU i.p. injection three times weekly) or T (1-cm subdermal Silastic implant) for 6 weeks, after which stereological estimates of germinal cell populations, serum and testicular T content, and testis weight were evaluated. Human CG stimulated Leydig cell maturation and normalized testicular T content compared with T treatment where Leydig cells remained immature and inactive. The maximal hCG-induced increases in testis weight and serum T concentrations were similar to those for T treatment and produced complete spermatogenesis characterized by mature, basally located Sertoli cells (SCs) with tripartite nucleoli, condensed haploid sperm, and lumen development. Compared with T treatment, hCG increased spermatogonial numbers, but both hCG and T had similar effects on numbers of spermatocytes and round and elongated spermatids per testis as well as per SC. Nevertheless, testis weight and germ cell numbers per testis and per SC remained well below phenotypically normal controls, confirming the involvement of non-Leydig cell factors such as FSH for quantitative normalization of spermatogenesis. We conclude that hCG stimulation of Leydig cell maturation and steroidogenesis is not required, and that T alone mostly replicates the effects of hCG, to initiate spermatogenesis. Because T is both necessary and sufficient for initiation of spermatogenesis, it is likely that T is the main Leydig cell secretory product involved and that additional LH-dependent Leydig cell factors are not essential for induction of murine spermatogenesis.     

Effects and interactions of LH and LHRH agonist on testicular morphology and function in hypophysectomized rats

The effects of single or combined daily treatment with an LHRH agonist and low or high doses of LH upon the testes of adult hypophysectomized rats were studied for up to 2 weeks in which changes in testicular histology, particularly the interstitial tissue, were examined by morphometry and related to functional assessment of the Leydig cells in vivo and in vitro. Compared to saline-treated controls, LHRH agonist treatment did not alter testis volume or the composition of the seminiferous epithelium or any of the interstitial tissue components although serum testosterone and in-vitro testosterone production by isolated Leydig cells were significantly reduced. With 2 micrograms LH for treatment, testis volume was increased, spermatogenesis was qualitatively normal, total Leydig cell volume was increased, serum testosterone values were initially elevated but subsequently declined and in-vitro testosterone production was enhanced. Testis volume with 20 micrograms LH treatment was unchanged compared to saline treatment, the seminiferous epithelium exhibited severe disruption but total Leydig cell volume was greatly increased due to interstitial cell hyperplasia. This group showed elevated serum testosterone concentrations and major increases in testosterone production in vitro. Treatment with LHRH agonist with either dose of LH resulted in reduced testis volume, moderate to very severe focal spermatogenic disruption and increased total Leydig cell volume although serum testosterone values and in-vitro testosterone production were markedly reduced compared to control rats. It is concluded that, in the absence of the pituitary, LHRH agonist fails to disrupt spermatogenesis and the previously described antitesticular action of LHRH agonists in intact rats is therefore dependent upon the presence of LH, which alone or in combination with LHRH agonist, may focally disrupt spermatogenesis in hypophysectomized rats whereas the Leydig cells undergo hyperplasia. The findings show that impairment of spermatogenesis is accompanied by alterations of the interstitial tissue and suggest that communication between these two compartments is involved in the regulation of testicular function.     

Cell-cell interactions in the control of spermatogenesis as studied using Leydig cell destruction and testosterone replacement

This review centers around studies which have used ethane dimethane sulphonate (EDS) selectively to destroy all of the Leydig cells in the adult rat testis. With additional manipulations such as testosterone replacement and/or experimental induction of severe seminiferous tubule damage in EDS-injected rats, the following questions have been addressed: 1) What are the roles and relative importance of testosterone and other non-androgenic Leydig cell products in normal spermatogenesis and testicular function in general? 2) What are the factors controlling Leydig cell proliferation and maturation? 3) Is it the Leydig cells or the seminiferous tubules (or both) which control the testicular vasculature? The findings emphasize that in the normal adult rat testis there is a complex interaction between the Leydig cells, the Sertoli (and/or peritubular) cells, the germ cells, and the vasculature, and that testosterone, but not other Leydig cell products, plays a central role in many of these interactions. The Leydig cells drive spermatogenesis via the secretion of testosterone which acts on the Sertoli and/or peritubular cells to create an environment which enables normal progression of germ cells through stage VII of the spermatogenic cycle. In addition, testosterone is involved in the control of the vasculature, and hence the formation of testicular interstitial fluid, presumably again via effects on the Sertoli and/or peritubular cells. When Leydig cells regenerate and mature after their destruction by EDS, it can be shown that both the rate and the location of regenerating Leydig cells is determined by an interplay between endocrine (LH and perhaps FSH) and paracrine factors; the latter emanate from the seminiferous tubules and are determined by the germ cell complement. Taken together with other data on the paracrine control of Leydig cell testosterone secretion by the seminiferous tubules, these findings demonstrate that the functions of all of the cell types in the testis are interwoven in a highly organized manner. This has considerable implications with regard to the concentration of research effort on in vitro studies of the testis, and is discussed together with the need for a multidisciplinary approach if the complex control of spermatogenesis is ever to be properly understood.     

Reinitiation of spermatogenesis by exogenous gonadotropins in a seasonal breeder, the woodchuck (Marmota monax), during gonadal inactivity.

The present study was undertaken (1) to document structural and functional changes in the testes of seasonally breeding woodchuck during active and inactive states of spermatogenesis and (2) to evaluate the ability of exogenous gonadotropins to reinitiate spermatogenesis outside the breeding season. During seasonal gonadal inactivity, there were significant (P less than 0.05) reductions in volumes of several testicular features (testis, seminiferous tubules, tubular lumen, interstitial tissue, individual Leydig cells, Leydig cell nuclei, and Leydig cell cytoplasm) as compared with gonadally active animals. The diameter of the seminiferous tubules was decreased by 26%, and Leydig cell numbers also declined in the regressed testes. These changes were accompanied by a decline in testosterone (T) levels in both plasma and testis, and reduction in epithelial height of accessory reproductive organs. A hormonal regimen was developed that would reinitiate spermatogenesis in captive, sexually quiescent woodchucks. A combination of PMSG and hCG markedly stimulated testicular growth and function and restored spermatogenesis qualitatively. Quantitatively normal spermatogenesis was restored in 2 of 6 treated males. Morphometric analyses revealed substantial increases in seminiferous tubular diameter and in the volume of seminiferous tubules, tubular lumen, total Leydig cells, and individual Leydig cells in the hormone-treated animals. These increased values corresponded to 99, 75, 68, 51, and 200%, respectively, of the values measured in naturally active woodchucks. Leydig cell numbers, however, remained unchanged and approximated only 31% of the number found in naturally active testes. Hormonal stimulation also resulted in a significant rise in serum T as well as in the total content of testicular T, and a marked increase in epithelial height in various accessory reproductive glands. The most effective hormonal protocol for stimulating spermatogenesis was treatment with 12.5 IU of PMSG twice a week for 4 weeks followed by 12.5 IU of PMSG + 25 IU of hCG twice a week for 4 weeks.     

Ultrastructural analysis of a local regulation of Leydig cells in the adult monkey (Macaca fascicularis) and rat

Peritubular Leydig cells located in interstitial areas surrounded by tubules at nearly the same stage of spermatogenesis were analysed. Low-power electron micrographs were used for measurement of cell profile area and higher magnification views provided volume density of SER, Golgi stacks, mitochondria, and lipids. In the adult monkey, no cyclic changes were found in Leydig cells in their size or in the volume density of their organelles. In the adult rat (63 days of age), a comparison limited to stage VII-VIII and stage XI-XII peritubular Leydig cells demonstrated a significantly higher SER content (P less than 0.01) in the former, but no other differences. The study of subadult rats (45 days of age) showed that the full development of spermatogenesis was required to detect significant changes in Leydig cell SER content. The present results provide morphological evidence for an intratesticular control of the Leydig cells of the rat but not for those of the monkey.     

The effect of cryptorchidism and subsequent orchidopexy on testicular function in adult rats

Cryptorchidism for 28 or 10 days resulted in a severe disruption of spermatogenesis (assessed histologically or by fertility tests), Sertoli cell function (assessed by seminiferous tubule fluid production after efferent duct ligation, ABP levels, binding of 125I-labelled FSH to testis homogenates and serum FSH levels) and Leydig cell function (assessed by serum LH and testosterone levels, in-vitro testosterone production, binding of 125I-labelled hCG). Orchidopexy after 28 days of cryptorchidism resulted in a poor recovery of spermatogenesis since the majority of tubules were lined by Sertoli cells and a few spermatogonia. No recovery occurred in the indicators of Sertoli and Leydig cell function. Orchidopexy after 10 days of cryptorchidism also resulted in a poor recovery of spermatogenesis, with a few animals showing partial recovery after 6 months. No recovery occurred in seminiferous tubule fluid production but partial recovery occurred in ABP content and production rate. Serum FSH, LH levels and in-vitro testosterone production by the testis remained elevated and did not change from the values found during cryptorchidism. Fertility testing at 6 months revealed a small number of rats in which fertility was restored although the number of embryos was lower than in controls. In this group of animals there was a significant improvement in a number of indicators of Sertoli cell and Leydig cell function. These data provide further evidence to link the changes in Sertoli cell and Leydig cell function to the germ cell complement present in the testis.     

Heat shock protein 27 expression in the human testis showing normal and abnormal spermatogenesis

Heat shock proteins (HSPs) are molecular chaperones involved in protein folding, assembly and transport, and which play critical roles in the regulation of cell growth, survival and differentiation. We set out to test the hypothesis that HSP27 protein is expressed in the human testes and its expression varies with the state of spermatogenesis. HSP27 expression was examined in 30 human testicular biopsy specimens (normal spermatogenesis, maturation arrest and Sertoli cell only syndrome, 10 cases each) using immunofluorescent methods. The biopsies were obtained from patients undergoing investigations for infertility. The seminiferous epithelium of the human testes showing normal spermatogenesis had a cell type-specific expression of HSP27. HSP27 expression was strong in the cytoplasm of the Sertoli cells, spermatogonia, and Leydig cells. Alternatively, the expression was moderate in the spermatocytes, weak in the spermatids and absent in the spermatozoa. In testes showing maturation arrest, HSP27 expression was strong in the Sertoli cells, weak in the spermatogonia, and spermatocytes. It was absent in the spermatids and Leydig cells. In Sertoli cell only syndrome, HSP27 expression was strong in the Sertoli cells and absent in the Leydig cells. We report for the first time the expression patterns of HSP27 in the human testes and show differential expression during normal spermatogenesis, indicating a possible role in this process. The altered expression of this protein in testes showing abnormal spermatogenesis may be related to the pathogenesis of male infertility.     

Relationships between Leydig cell morphometry and plasma testosterone during postnatal development of the monkey, Macaca fascicularis

In neonates (0 to 3-4 months), the testis contained a mean number of 4.6 X 10(6) Leydig cells representing 4.2 % of its volume; Leydig cell cytoplasm contained 10.2 % of SER. In infants (up to 45 months), Leydig cells regressed but their number increased; their volume density did not change. Leydig cell cytoplasmic volume (454 microns3 ), which was about 2.5-fold less than in neonates (1 119 microns3 ) or adults (1 170 microns3 ), contained only 8.7% of SER. During meiosis stage (38-52 months). Leydig cell numbers and volume density did not vary but the cells reached a maximal size and an amount of SER comparable with that at birth was measured. When spermatogenesis was complete, the Leydig cells represented no more than 0.8% of testis volume, but their number and SER content were significantly increased. Except for a significant decrease when spermatogenesis was completed, Leydig cell lipid content did not change during development, and the volume density of mitochondria did not vary. The mean level of plasma testosterone was 2 ng/ml in neonates and 0.4 ng/ml in infants; it increased to 3 ng/ml during onset of meiosis and reached 10 ng/ml in adults. The profile of testosterone was positively and significantly correlated with the total volume and total number of Leydig cells (P less than 0.01 and P less than 0.02, respectively) and with changes in their cytoplasmic volume (P less than 0.001). Moreover, plasma testosterone levels were positively and significantly correlated with changes in Leydig cell SER content i.e. SER volume density and mean absolute volume per cell (P less than 0.001), total SER in the whole testis (P less than 0.01).     

The germ cell development strategy and seasonal changes in spermatogenesis and Leydig cell morphologies of the spiny lizard Sceloporus mucronatus (Squamata: Phrynosomatidae)

The viviparous lizards of the Sceloporus genus exhibit both seasonal and continuous spermatogenesis. The viviparous lizard Sceloporus mucronatus from Tecocomulco, Hidalgo, México, exhibits seasonal spermatogenesis. This study demonstrates the relationship between changes in testis volume, spermatogenesis activity, and Leydig cells during the male reproductive cycle of S. mucronatus. A recrudescence period is evident, which starts in the winter when testicular volume is reduced and climaxes in February, when the greatest mitotic activity of spermatogonia occurs. The testicular volume and Leydig cell index increase gradually during the spring with primary spermatocytes being the most abundant cell type observed within the germinal epithelium. In the summer, the secondary spermatocytes and undifferentiated round spermatids are the most abundant germinal cells. The breeding season coincides with spermiogenesis and spermiation; testicular volume also increases significantly as does the Leydig cell index where these cells increase in both cytoplasmic and nuclear volume. During fall, testicular regression begins with a significant decrease in testicular volume and germinal epithelium height, although there are remnant spermatozoa left within the lumen of the seminiferous tubules. During this time, the Leydig cell index is also reduced, and there is a decrease in cellular and nuclear volumes within these interstitial cells. Finally, during quiescence in late fall, there is reduced testicular volume smaller than during regression, and only spermatogonia and Sertoli cells are present within the seminiferous tubules. Leydig cells exhibit a low index number, their cellular and nuclear volumes are reduced, and there is a depletion in lipid inclusion cytoplasmically.     

Fine structure of leydig and sertoli cells in the testis of immature and mature spotted ray Torpedo marmorata

An ultrastructural investigation revealed the presence of true Leydig cells in the testis of sexually mature specimens of Torpedo marmorata. They showed the typical organization of steroid-hormone-producing cells, which, however, changed as spermatocysts approached maturity. In fact, they appeared as active cells among spermatocysts engaged in spermatogenesis, while in regions where spermiation occurred, they progressively regressed resuming the fibroblastic organization typically present in the testis of immature specimens. Such observations strongly suggest that these cells might be engaged in steroidogenesis and actively control spermatogenesis. Sertoli cells, too, appeared to play a role in spermatogenesis control, since, like Leydig cells, they showed the typical aspect of steroidogenic cells. In addition, the presence of gap junctions between Sertoli cells suggests that their activity might be coordinated. After sperm release, most Sertoli cells were modified and, finally, degenerated, but few of them changed into round cells (cytoplasts) or round cell remnants, which continued their steroidogenic activity within the spermatocyst and the genital duct lumen. From the present observations, it can be reasonably concluded that, in T. marmorata, spermatogenesis depends on both Leydig and Sertoli cells, and, as postulated by Callard (1991), in cartilaginous fish, the function of the Leydig cells as producers of steroids might be more recent and subsequent to that of Sertoli cells. In this regard, it is noteworthy that, in immature males, when Leydig cells showed a fibroblastic organization, Sertoli cells already displayed the typical organization of a steroidogenic cell.     

A comparison of Leydig cell function after unilateral and bilateral cryptorchidism and efferent-duct-ligation

Surgical induction of cryptorchidism or ligation of the efferent ducts disrupts spermatogenesis. The response of Leydig cells to disrupted gametogenesis was studied in vitro in tissue and collagenase dispersed Leydig cells obtained from the testes of rats that were made unilaterally or bilaterally cryptorchid or had been efferent-duct-ligated. Four wks after surgery, androgen secretion per mg of tissue or per Leydig cell in response to maximal luteinizing hormone (LH) stimulation was greater in tissue from damaged than from sham-operated testes. It was concluded that disruption of spermatogenesis resulted in Leydig cells that were hyperresponsive to LH stimulation in vitro. Unilateral lesions produced different responsiveness of Leydig cells from the testes ipsilateral and contralateral to the lesion, supporting the hypothesis that intragonadal modulation of Leydig cells function occurs when the function of seminiferous tubules is impaired. Stimulated androgen production of Leydig cells from the contralateral nonligated testis did not differ from that of the sham-operated controls. With unilateral cryptorchidism, which is accompanied by an increase in the temperature of the operated testis, Leydig cells from the scrotal testis were also hyperresponsive compared to those from sham-operated controls. This suggests a possible intergonadal influence of aspermatogenesis caused by cryptorchidism.     

Developmental appearance of the Ca2+-binding proteins parvalbumin,calbindin D-28K,S-100 proteins and calmodulin during testicular development in the rat

Summary Calcium and intracellular Ca²⁺-binding proteins are possibly involved in hormone production and spermatogenesis in rat testis. Parvalbumin, calbindin D-28K, S-100 proteins and calmodulin were localized in the Leydig cells, which are sites of testosterone synthesis. Only the appearance of parvalbumin-immunoreactivity is closely correlated to testosterone production during development of the testes. Calbindin D-28K-immunoreactivity persisted in foetal-type Leydig cells and in adult-type Leydig cells at all stages of development. S-100-immunoreactivity was low during all foetal stages, absent between birth and puberty, and increased thereafter. Calmodulin staining is most prominent in the cytoplasm of developing spermatocytes and of maturing spermatids. All four proteins co-exist in the seminiferous tubules. The distinct localization and developmental appearance of these proteins suggests different regulatory roles in Leydig cell function and spermatogenesis.     

Effects of CDB-4022 on Leydig cell function in adult male rats

CDB-4022, an indenopryridine, suppresses spermatogenesis and decreases inhibin secretion in adult male rats. In the present study, we investigated the effects of CDB-4022 on Leydig cell function. A single oral dose of CDB-4022 (2.5 mg/kg) resulted in a 2-fold decrease in serum testosterone levels after 7 days that was paralleled by a decrease in Cyp17a1 mRNA and protein levels and 17alpha hydroxylase enzymatic activity compared with vehicle-treated rats. Consistent with the lower serum testosterone levels, pituitary Lhb and Fshb mRNA levels were increased 3.2- and 2.3-fold, respectively, by CDB-4022 treatment. Ultrastructural analysis of pituitary gonadotrophs showed distended endoplasmic reticulum (ER) and fewer secretory granules in CDB-4022-treated rats, characteristic of enhanced secretory activity. Conversely, CDB-4022 increased serum progesterone levels, testicular Star mRNA and protein expression, and the number of Leydig cells per testis. Serum inhibin B levels were undetectable in CDB-4022-treated rats, while serum activin A levels were similar to controls, indicating that the CDB-4022-treated rats have an elevated activin A:inhibin B ratio. In the presence of hCG stimulation, activin A directly suppressed testosterone secretion but enhanced progesterone secretion from rat Leydig cell primary cultures. Likewise, treatment of MA-10 cells with activin A was found to enhance cAMP-stimulated progesterone secretion and STAR expression. Together, our data indicate that CDB-4022 treatment inhibits CYP17A1 and stimulates STAR expression, thereby decreasing testosterone but increasing progesterone production. We propose that unopposed actions of activin A most likely contribute to the steroid profile in rats after CDB-4022 treatment. Our findings establish CDB-4022 as a new model to examine intratesticular control mechanisms that modulate Leydig cell gene expression and function.     

Seminiferous tubules and daily sperm production in older adult men with varied numbers of Leydig cells

Previous studies of adult men have failed to reveal a relationship between numbers of Leydig cells in the testes and rates of sperm production, perhaps because of a functional excess of these cells in younger men. Hence, a possible relationship between Leydig cell numbers and sperm production was sought in 50 older men, aged 50-90 years, in whom the Leydig cell population had been depleted by age-related attrition. When these men were sorted by increasing numbers of Leydig cells per man into two, three, or five groups, no difference could be found between or within these groups when daily sperm production per man (DSP); seminiferous tubular volume, diameter, or length; or seminiferous epithelial volume was examined. Furthermore, no significant correlation could be detected between Leydig cell numbers and DSP in these 50 men. The only relationship between numbers of Leydig cells and spermatogenesis appeared to be a threshold effect, in that men with fewer than 60 million Leydig cells (4 in this study) had drastically reduced DSP. Men with few Leydig cells tended to have larger Leydig cells, and the increased size was due to more cytoplasm instead of nucleoplasm. There were weak but significant positive correlations between total Leydig cell cytoplasm per man and DSP and between average size of a Leydig cell and DSP. These findings suggest that a relationship may exist between sperm production and the amount of cytoplasm containing testosterone-producing organelles in surviving Leydig cells of older men.     

The interstitial cells of the trout testis (Oncorhynchus mykiss): ultrastructural characterization and changes throughout the reproductive cycle

The major non-vascular cell types present in the interstitial compartment in trout testes have been ultrastructurally characterized and cell changes in the course of the two first reproductive cycles have been studied. Three major cell types are always present fibroblasts, myoid cells and Leydig cells. Their structure varies with the maturational stage of the gonad. Fibroblasts are centrally located in the interstitial areas. Numerous typical myoid cells are always present near the basal lamina. However, at the end of a cycle some of them display degenerative changes, then disappear. At the beginning of the next cycle, fibroblasts appear to differentiate to new myoid cells. Leydig cells present before the start of the first spermatogenesis are replaced by new ones which probably also arise from fibroblasts. These cells progressively differentiate during spermatogenesis, so that most of the Leydig cells present during the spermiation phase are fully-differentiated steroidogenic cells. At the end of a cycle, a certain number of the Leydig cells disappear and are replaced at the beginning of the next cycle by new ones most likely derived from fibroblasts. The remaining Leydig cells dedifferentiate to cells which may redifferentiate during gonadal activity. Macrophages are present mostly at the end of a cycle and participate at this time in the removal of dead interstitial cells. In conclusion, at least some fibroblast-like cells are precursor cells, able to replace myoid cells and Leydig cells which disappear between two consecutive reproductive cycles.     

Sertoli Cells Maintain Leydig Cell Number and Peritubular Myoid Cell Activity in the Adult Mouse Testis

The Sertoli cells are critical regulators of testis differentiation and development. In the adult, however, their known function is restricted largely to maintenance of spermatogenesis. To determine whether the Sertoli cells regulate other aspects of adult testis biology we have used a novel transgenic mouse model in which Amh-Cre induces expression of the receptor for Diphtheria toxin (iDTR) specifically within Sertoli cells. This causes controlled, cell-specific and acute ablation of the Sertoli cell population in the adult animal following Diphtheria toxin injection. Results show that Sertoli cell ablation leads to rapid loss of all germ cell populations. In addition, adult Leydig cell numbers decline by 75% with the remaining cells concentrated around the rete and in the sub-capsular region. In the absence of Sertoli cells, peritubular myoid cell activity is reduced but the cells retain an ability to exclude immune cells from the seminiferous tubules. These data demonstrate that, in addition to support of spermatogenesis, Sertoli cells are required in the adult testis both for retention of the normal adult Leydig cell population and for support of normal peritubular myoid cell function. This has implications for our understanding of male reproductive disorders and wider androgen-related conditions affecting male health.     

Factors affecting spermatogenesis in the stallion

Spermatogenesis is a process of division and differentiation by which spermatozoa are produced in seminiferous tubules. Seminiferous tubules are composed of somatic cells (myoid cells and Sertoli cells) and germ cells (spermatogonia, spermatocytes, and spermatids). Activities of these three germ cells divide spermatogenesis into spermatocytogenesis, meiosis, and spermiogenesis, respectively. Spermatocytogenesis involves mitotic cell division to increase the yield of spermatogenesis and to produce stem cells and primary spermatocytes. Meiosis involves duplication and exchange of genetic material and two cell divisions that reduce the chromosome number to haploid and yield four spermatids. Spermiogenesis is the differentiation without division of spherical spermatids into mature spermatids which are released from the luminal free surface as spermatozoa. The spermatogenic cycle (12.2 days in the horse) is superimposed on the three major divisions of spermatogenesis which takes 57 days. Spermatogenesis and germ cell degeneration can be quantified from numbers of germ cells in various steps of development throughout spermatogenesis, and quantitative measures are related to number of spermatozoa in the ejaculate. Germ cell degeneration occurs throughout spermatogenesis; however, the greatest seasonal impact on horses occurs during spermatocytogenesis. Daily spermatozoan production is related to the amount of germ cell degeneration, pubertal development, season of the year, and aging. Number of Sertoli cells and amount of smooth endoplasmic reticulum of Leydig cells and Leydig cell number are related to spermatozoan production. Seminiferous epithelium is sensitive to elevated temperature, dietary deficiencies, androgenic drugs (anabolic steroids), metals (cadmium and lead), x-ray exposure, dioxin, alcohol, and infectious diseases. However, these different factors may elicit the same temporary or permanent response in that degenerating germ cells become more common, multinucleate giant germ cells form by coalescence of spermatocytes or spermatids, the ratio of germ cells to Sertoli cells is reduced, and spermatozoan production is adversely affected. In short, spermatogenesis involves both mitotic and meiotic cell divisions and an unsurpassed example of cell differentiation in the production of the spermatozoon. Several extrinsic factors can influence spermatogenesis to cause a similar degenerative response of the seminiferous epithelium and reduce fertility of stallions.