The Etiology and Pathophysiology of Testicular Dysfunction in Man*

Vol. 29, No.5, May 1978 Printed in U.SA.

FERTILITY AND STERILITY Copyright • 1978 The American Fertility Society

THE ETIOLOGY AND PATHOPHYSIOLOGY OF TESTICULAR DYSFUNCTION IN MAN*

EMIL STEINBERGER, M.D.t

Department of Reproductive Medicine and Biology, The University of Texas Medical School at Houston, Houston, Texas 77025

As it will become apparent throughout this review, a peculiar phenomenon is occurring in the area of medicine dealing with the study of testicular disorders. More is being learned about pathophysiologic mechanisms of testicular disease than about its etiology. In other words, we are recognizing and uncovering physiologic mechanisms which, when disturbed, may easily serve as the basis of testicular disease; however, in many ifnot most instances, we'are unable to find the etiologic factors leading to the disturbance. Consequently, we are hampered in establishing specific diagnoses, and as a result therapy is frequently unsuccessful. In this brief review an attempt is made to correlate the available information concerning testicular pathophysiology with the possible etiologic factors involved in testicular dysfunction. The etiologic factors are arbitrarily divided into the following categories: (1) environmental, (2) systemic, (3) intratesticular, and (4) iatrogenic. ENVIRONMENTAL ETIOLOGY OF TESTICULAR DYSFUNCTION

Exposure to Noxious Substances The pathophysiology of testicular disorders resulting from exposure to noxious substances depends entirely on the type of agent causing the disturbance. Workers employed in the manufacture of stilbestrol (a synthetic estrogen capable of suppressing the hypothalami~-pituitary axis)

may inadvertently be exposed to sufficient quantities of this estrogen to develop impotence and testicular atrophy. Thus, the pathophysiology of testicular damage in these cases is hypogonadotropism, resulting in atrophy of both the Leydig cells and seminiferous epithelium. Numerous other· chemical agents-deuterium oxide, cadmium, fiuoroacetamide, various cytotoxic agents (e.g., nitrofurans,. dinitropyrroles, diamines, a-chlorohydrin, various insecticides, and rodenticides) (for review, see references 1 and 2)can produce testicular damage. The pathophysiology of the damage resulting from exposure to these agents varies depending upon the agent employed. Some cause microvascular damage with subsequent ischemia and necrosis of the seminiferous tubule and of the interstitial area (e.g., cadmium), while others produce arrest of spermatogenesis by direct action on the seminiferous tubules (e.g., nitrofurans, dinitropyrroles). Some still unidentified agents discharged into or created in the environment by modern technology may have similarly deleterious effects on the testes. Unfortunately, it is very difficult to establish such a relationship in a patient with testicular dysfunction. In most instances the affected individual may not even be aware of having been exposed to the offending agent. If the damage is due to a chronic low-dose exposure, the morphologic change in the testes subsequent to such a . long-term effect may be nonspecific and easily classified as "adult seminiferous tubule failure."

Received November 17, 1977. cited in this review, the experimental studies by the author were supporteq. by National Insitutes of Health Grants 5 P50 HD 08338 and HD 06316 and by grants from The Ford Foundation and The Clayton Foundation. tReprint requests: Emil Steinberger, M.D.; Department of Reproductive Medicine and Biology, University of Texas Medical School, P.O. Box 20708, Houston, Tex. 77025.

Exposure to Radiation

*As

The deleterious effects of various forms of irradiation on spermatogenesis have been clearly demonstrated both in experimental animals and in man (for review, see reference 3). The pathophysiology of testicular damage following ex-

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posure to radiation, however, is still not clearly understood. Damage to chromatin material resulting from ionizations at the target sites may cause a variety of mutations, including "lethal mutation" which results in the death of the affected germ cell. If the "stem cells" of the germ cell line are affected, irreversible destruction of the seminiferous epithelium takes place, and no recovery will occur; At lower rates of radiation the stem cells may not be affected but the more mature cells may, and a maturation depletion of the seminiferous epithelium will occur; followed in time by repopulation of the seminiferous epithelium resulting from mitotic activity of the stem cells. Although the resulting gametes may exhibit normal fertilizing capacity, in some instances the resulting embryo dies and is resorbed in utero. In other instances an apparently normal offspring may result. The possibility that such an offspring may carry mutant genes as an effect of the exposure of the stem cell to radiation is ever-present, as has been demonstrated in experiments in lower species (for review, see reference 4). Emotional Stress Some authors have suggested the possibility that emotional stress may trigger testicular dysfunction. Most of the information in man, particularly as related to the effect of emotional stress on sperm production, is essentially anecdotal in nature. However, several well-documented recent studies have suggested that emotional stress may adversely affect androgen production by the testes. 5·9 It is not known whether the diminution of androgen production in these individuals is also associated with diminished sperm production. Experiments in animals have suggested that "social stress" related to excessive population densities may result in testicular dysfunction and diminished fertility, most likely via the adrenalpituitary-hypothalamic feedback mechanism. 1°-12 Consequently, if emotional factors are indeed involved in causing testicular dysfunction, the pathophysiology of this process most likely involves inadequate gonadotropin stimulation of testicular functions. In general, there is considerable evidence that environmental factors, whether chemical, physical, or emotional, may adversely affect testicular function in men. Unfortunately, in most instances it is extremely difficult to detect and/or to relate a specific environmental factor to the testicular dysfunction in a specific patient. On the other hand, in most patients with testicular disease character-

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ized by inadequate sperm production, neither can a specific diagnosis be established nor the etiologic factor uncovered. Most of these patients are placed in the diagnostic wastebasket of "adult seminiferous tubule failure." Environmental factors, so difficult to pinpoint, may indeed be the underlying etiologic factors in a segment of a patient population with the diagnosis of "adult seminiferous tubule failure." SYSTEMIC ETIOLOGY OF TESTICULAR DYSFUNCTION

Metabolic No overt and direct relationships have been established between well-defined metabolic-endocrine disorders (e.g., diabetes, hypo- or hyperthyroidism, hypo- or hyperadrenocorticism, disorders of calcium metabolism or lipid metabolism) and testicular dysfunction; however, numerous papers have been written suggesting deleterious effects of metabolic derangements on testicular function in lower species and men (for review, see reference 1). Both hypoglycemia13 and hyperglycemia l4 • 15 have been reported to cause seminiferous epithelium damage in rodents. No definite relationship has been shown between diabetes (whether uncontrolled with the hyperglycemic state prevailing or associated with frequent hypoglycemic episodes) and oligospermia. The possible negative effect of adrenal dysfunction on sperm production has been suggested, but no definite data are available to substantiate this possibility. Considerable attention has been devoted to the possible association of hypothyroidism and diminished sperm production. Although patients with severe hypothyroidism may also show testicular dysfunction, a definite relationship between levels of thyroid hormone and spermatogenesis has been difficult to establish. Again it must be emphasized that in the vast majority of patients with oligo- or azoospermia no serious metabolic disturbance can be detected. Infectious Mumps orchitis is the classic example of testicular dysfunction secondary to a specific infection. During the acute stage, a severe inflammatory response of the gonad associated with increased temperature of the organ, marked swelling, and pain is noted. Subsequently, variable degrees of testicular atrophy may occur,t6 primarily due to .destruction. of a portion or all of theseminiferous epithelium; usually the Leydig cells remain

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functional. Characteristically the damage to the seminiferous tubules is patchy. Some crosssections of seminiferous tubules show complete spermatogenesis while others show varying degrees of spermatogenic arrest or complete destruction of the seminiferous epithelium. Depending on the severity of damage, the sperm output in these patients will be affected to varying degrees. In most instances, only one testis is severely affected, and sperm production by the other may be sufficient to result in an ejaculate with adequate sperm density. Although this condition is relatively common, the pathophysiology of the damage to the seminiferous epithelium is not clearly understood. There is no evidence that the seminiferous epithelium is directly affected by the virus. Most likely the infectious process is limited to the interstitial area of the testes, and the damage to the seminiferous tubules is due to increased temperature and intratesticular pressure secondary to the edema in the interstitial area. Possibly as a result of the inflammatory process, anatomical and physiologic changes occur in the interstitial tissue, resulting in disruption of normal relationships between the interstitial area and the seminiferous tubules. Although common systemic infections have not been demonstrated to produce clinical manifestations of orchitis or damage to the seminiferous epithelium, evidence in the clinical literature suggests that systemic infections as well as infections in the sex accessories (prostate, seminal vesicles) may be associated with transient (or at times even permanent) diminution of sperm output, possibly associated with subclinical testicular disease. 17-21 It is a distinct possibility that some cases of "adult seminiferous tubule failure" represent the end result of a series of subclinical orchidites secondary to systemic infections.

Vascular Ischemic. Interference with the blood supply to the testis results in severe damage, primarily to the seminiferous epithelium. It should be remembered, however, that the seminiferous epithelium has the potential for adequate recovery even after relatively prolonged periods of ischemia (60 to 90 minutes), but chronic ischemia causes irreversible damage. 22 , 23 Evidence for the possibility that arteriosclerotic changes affecting the intratesticular arterioles may cause seminiferous epithelium damage and gradual diminution of testicular function associated with aging has been reported. 24

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Vascular Abnormalities. The most common vascular abnormality is the varicocele, sometimes associated with diminution of sperm production and sperm motility. At this time neither the pathophysiology of the spontaneously occurring varicocele nor the mechanism by which it produces testicular damage is clear. The dilatation of the pampiniform plexus of veins is due to the backflow of blood secondary to incompetent valves in the spermatic vein. 25- 29 The mechanism responsible for the valvular incompetence is unclear. The fact that the spermatic vein empties into the renal vein at a right angle has been suggested as a contributing factor, acting in concert with increased orthostatic pressure in the spermatic vein secondary to man's upright position. 28 While these suggestions are plausible, they have not been proved experimentally, and no explanation has been given for the fact that only a small segment of the population develops this problem in spite of the fact that these anatomical and postural relationships exist in most men. The mechanism by which varicocele exerts its postulated effect on the testes is similarly unclear. Originally it was suggested that the dilated veins simply insulate the testis, causing elevation of intratesticular temperature which in turn produces thermal damage to the testes. 30 ,31 This rather plausible suggestion has not been borne out in studies involving direct measurements of intratesticular temperature in patients with varicocele. 32 , 33 It has also been suggested that, since the spermatic vein empties into the renal vein in close proximity to the adrenal vein, the backflow of blood rich in adrenal secretions may expose the testes to high concentrations of adrenal hormones, particularly cortisol, which may be deleterious to testicular function. 27 Measurement of cortisol levels in the blood of the pampiniform plexus resulted in the accumulation of controversial data which at this time are difficult to interpret. 29 Clearly, products of adrenal secretion other than cortisol could also find their way to the testes, and this possibility awaits further study. Finally, the possibility that the pathophysiology of testicular damage in patients with varicocele may be based on physical rather than biochemical phenomena must be considered. Since the testis is surrounded by a relatively inelastic capsule and the testicular parenchyma is under slight pressure, inadequate venous drainage from the testes may induce an increase in intratesticular pressure. There is considerable

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evidence suggesting that increased intratesticular pressure (as, for example, secondary to an inflammatory process) may induce seminiferous epithelium damage. This mechanism conceivably may be involved in the damage produced by a varicocele (for review, see reference 34). Neurologic The role of the nervous system in testicular function (both endocrine and exocrine) is not known. Although anatomical aspects of the nerve supply to the testes have been studied extensively and a relationship of the testes to the autonomic nervous system has been established, there is still controversy over whether the seminiferous epithelium has a nerve supply.35.36 Furthermore, no experimental studies have been conducted to demonstrate whether interruption of the nerve adversely affects the testis. Damage to the seminiferous epithelium and to the Leydig cells has been noted in patients with paraplegia. 37 . 38 Whether the damage is caused by the nerve lesions or other factors is unclear. Suggestions have been made that the anatomical relationships of the lower extremities in these patients cause elevation of temperature and that the lesions in the seminiferous epithelium are secondary to the relative hyperthermia. Developmental Testicular ectopia is the most common type of developmental abnormality affecting the testes. A discussion of the pathophysiology of this condition requires consideration of two factors: the mechanisms responsible for abnormal testicular descent and those responsible for damage to the seminiferous epithelium in the affected testis. Arrest of descent (abdominal, inguinal, or "high riding" testes) and inappropriate descent (crural, suprapubic, or perineal testes) have been considered to be the result of a developmental anatomical abnormality in the path of testicular descent. 39 The damage to the seminiferous epithelium is thought to be secondary to elevated ambient temperature. However, some authors have suggested that both faulty descent and germinal epithelium pathology are due to a congenital testicular abnormality.4°-42 One line of evidence frequently cited in support of this hypothesis is the presence of abnormalities of the seminiferous epithelium in the contralateral scrotal testis in patients with unilateral testicular ectopia. The other evidence to support a congenital etiology is

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the singularly limited success in preventing the development of abnormalities of the seminiferous epithelium in ectopic testes replaced surgically (orchidopexy) into the scrotums of young (4- to 6year-old) boys. Although thermal damage to the germinal epithelium of abdominal testes must be considered one of the pathophysiologic mechanisms responsible for spermatogenesis, underlying congenital or genetic defects may also be contributory in a least some instances. Genetic

Chromosomal. The classic example of this type of disorder is Klinefelter's syndrome. 43 The abnormality is characterized by the presence of supernumerary sex chromosomes, resulting most commonly in a 47,XXY karyotype. The testicular abnormality in these patients is expressed primarily by defective function of the seminiferous tubules, although a degree of interference with normal androgen production has also been noted. 44 Originally it was thought that the abnormalities in the seminiferous tubules were due to lack of male germ cell development in the embryo because of the "feminizing" effect of an X chromosome. However, studies of testes from prepuberal boys with the diagnosis of Klinefelter's syndrome revealed the presence of gonocytes (stem cells), which gradually diminished in numbers with age. 45 Furthermore, in the testes of adults with Klinefelter's syndrome, patchy areas of spermatogenesis have been noted. 46 . 47 These findings demonstrate that aspermatogenesis in Klinefelter's syndrome is not due to a lack of germ stem cells, but to the inability of most of these cells during testicular development to enter the spermatogenic process and form a mature seminiferous epithelium. Consequently, an abnormality of the spermatogenic process associated with degeneration of the stem cells is the underlying pathophysiology of aspermatogenesis in patients with Klinefelter's syndrome. The possibility that this defect may be related to local insufficiency of testosterone is unlikely: the capacity of testicular tissue from patients with Klinefelter's syndrome to form testosterone in vitro is actually greater than that of normal tissue,48 and the intratesticular concentration of testosterone is higher (although the production rate and ability to respond to gonadotropins is diminished48 . 49). No information is available concerning the biochemical basis for the defective development of the stem cells in testes from patients with Klinefelter's syndrome. Available

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evidence suggests that in fetal gonads the supporting cells (which, in the male, are the precursors of the Sertoli cells) may play a determinant role in the development of gonadal sex. A possibility thus exists that in patients with an abnormal complement of sex chromosomes a resulting defect in the Sertoli cells may be the underlying pathology of the aspermatogenic state. Testicular pathology associated with other forms of chromosomal abnormalities has been reported. The pathophysiology in these states is understood even less than in Klinefelter's syndrome, and a direct relationship between the chromosomal abnormality and testicular damage has not been firmly established. Biochemical. The testicular feminization syndrome may fall under this category of genetically based systemic biochemical abnormalities associated with dysfunction of the seminiferous tubules. Patients with this syndrome are genetic males and their testes produce testosterone,50, 51 but the target tissues are unable to respond to it because of lack of androgen receptors. 52 These individuals have the physical appearance offemales but have male levels of circulating testosterone. The lack of appropriate development of the seminiferous epithelium may be a reflection of the inability of testosterone to exert its action on testicular tissue and could also be due to the ectopic position of the testes in patients with this syndrome.

Nutritional Testicular damage resulting from specific nutritional deficiencies has been demonstrated experimentally in a variety of mammals. In man there is definite evidence for testicular dysfunction only in cases of chronic starvation and malnutrition. Deleterious effects of vitamin (A and E) deficiency on the seminiferous epithelium have been shown in the rat, but in man no adequate evidence exists to suggest similar relationships.53 Hypothalamic-Pituitary No specific testicular lesions have been described in patients with failure of thyroid-stimulating hormone or adrenocorticotropic hormone. However, inadequate levels of circulating gonadotropins are associated with abnormalities of testicular function in patients with hypothalamic or pituitary disease. Gonadotropins are necessary

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for both androgen production and formation of spermatozoa. The deficiency syndrome associated with hypogonadotropism is characterized in an adult male by atrophy of Leydig cells with the resulting lack of androgen production and arrest of spermatogenesis. Replacement therapy with both gonadotropins (interstitial cell-stimulating hormone [lCSH] and follicle-stimulating hormone [FSH]) results in stimulation of growth of the Leydig cells, stimulation of androgen production, and stimulation of spermatogenesis in both hypog~nadotropic eunuchs and in patients with hypogonadism secondary to hypophysectomy. 54, 55 Consequently, the pathophysiology of this disorder is simple hormonal (gonadotropin) deficiency. The best available data suggest that ICSH exerts its effect primarily if not exclusively on the Leydig cells. FSH, on the other hand, is apparently responsible for supporting the process of spermiogenesis in the developing immature testis and the process of reinitiation of spermatogenesis in adult testes. Testosterone maintains spermatogenesis in hypophysectomized animals. The administration of testosterone to hypophysectomized men or to hypogonadotropic eunuchs fails to maintain or initiate spermatogenesis. Species differences have been considered as a possible explanation for this phenomenon. However, this may not necessarily be the only explanation, or even a correct one. Maintenance of spermatogenesis in the hypophysectomized rat requires approximately 4 to 8 mg/kg/day of testosterone. Extrapolation to a 70-kg man suggests a dose of 300 to 600 mg/man! day. In no instance has such a high daily dose been administered to a human for a prolonged period of time. Recently it has been shown that the intratesticular levels of testosterone in man are 10- to 100-fold the circulating levels. 48 A similar relationship has been demonstrated in lower species. 56 The minimal intratesticular concentration of testosterone essential for maintaining spermatogenesis is considerably higher than the normal levels in circulation. These findings suggest that a relatively high intratesticular concentration of androgens is required for the spermatogenic process to proceed, and that below a minimal threshold level the spermatogenic process breaks down. Furthermore, the possibility exists that the threshold varies in different individuals. If this turns out to be the case and if methods are developed to determine the threshold, therapeutic approaches are available to increase androgens to the necessary levels (for review, see reference 57).

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STEINBERGER INTRATESTICULAR ETIOLOGY OF TESTICULAR DYSFUNCTION

Environmental and systemic factors probably play an important etiologic role in only a small percentage of patients with oligo-azoospermia and/or infertility. This leads one to believe that primary intratesticular abnormalities could be responsible for defective spermatogenesis. In the past 5 to 10 years numerous possible sites for biochemically based pathologic processes capable of causing abnormalities of spermatogenesis have been identified in the dual testicular functions of steroidogenesis and spermatogenesis. Steroidogenesis The principal sites of androgen production in the testis are the Leydig cells. The embryonal origin of these cells and the details of their development are not clearly understood, particularly in man. They are probably of mesenchymal origin, 58. 59 and, if one can extrapolate from the lower species, undergo a complicated process of differentiation from both a morphologic60• 61 and a steroidogenic (for review, see reference 62) viewpoint. After attaining maturity they apparently do not divide actively, if at all. Their multiplication, maturation, and steroid biosynthetic activity are under direct control of ICSH, although some evidence has been presented suggesting that FSH may also playa role in this process (for review, see reference 63). Before examining the possible sites where pathophysiology may occur, one must first have a clear understanding of normal Leydig cell function. Although many gaps still exist in our knowledge, considerable advances have been made recently, and a simplified scheme of these processes can be proposed. A great deal of this information has been derived from lower species, although some of the key points have also been observed directly or indirectly to occur in human testes. The first molecular 'event in the chain of events leading to formation of androgens is the interaction of the gonadotropin horinone ICSH with a specific membrane recept-or. on a cell in the interstitial area of the testis, which ultimately will become a mature Leydig cell. The receptor is composed of two parts, .thereceptor site· and a transducer, adenyl cyclase. The message received by the receptor site activates adenyl cyclase to. stimulate production of cyclic adenosine 3':5'-monophosphate(cAMP). The end result of the increased production of this nucl!,!otjde Js

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to induce multiplication and differentiation of the Leydig cells. Mechanisms involved in this process are unknown, except for the fact that they are probably mediated via a nuclear transcriptional and translation mechanism leading to de novo RNA and protein synthesis. It is doubtful that a substantial proportion of patients with seminiferous tubule abnormalities has a disorder related to inappropriate somatic differentiation of Leydig cells, because in the vast majority of patients the Leydig cell can be seen and no morphologic abnormality in growth and differentiation can be detected. On the other hand, evidence has begun to accumulate that the biochemical events involved in the biogenesis of androgens may indeed be abnormal in some of these patients. Since androgens are essential for both initiation and maintenance of spermatogenesis, an abnormality in this process may be an etiologic factor in some of the cases of "idiopathic adult seminiferous tubule failure." The mechanism ICSH~receptor~cAMP stimulation is also involved in steroidogenesis. ICSH must be bound to the receptor before a steroidogenic effect is induced. Furthermore, a steroidogenic effect can be induced in the absence of ICSH by cAMP, implicating this nucleotide in the mechanism of ICSH action on steroidogenesis. The steps between the formation of cAMP and initiation of steroidogenesis, i.e., conversion of cholesterol to pregnenolone, are still unclear. However, it is fairly well established that the rate-limiting steps in the steroidogenic process under ICSH control involve the conversion of cholesterol to pregnenolone (for review, see reference 64). There are two major steroidogenic pathways leading to formation of androgens, the a4 and the a5 pathways. In some lower species (e.g., the rat) the a4 pathway predominates. In man, the a5 pathway appears to be predoininant. In a number of somatic androgen target tissues (e.g., prostate, seminal vesicles, and skin) a 5a-reduction of testosterone to dihydrotestosterone takes place. The 5a-reduced androgen is the active hormone. Whether the seminiferous epithelium also requires dihydrotestosterone is still unclear. Th~ available information, all of which has been ac~ quired in species other than man, suggests that the seminiferous epithelium may rl:)spond to t~stos­ terone as well as to 5a-reduced androgens. Nevertheless, Ii possibility exists that 5a-reductase deficiency may .be .responsible, for abnormal· spermatogenesis. A numbet:of enzymes are involved

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in the process of androgen biogenesis. Consequently, deficiency of anyone enzyme can serve as the pathophysiologic basis of seminiferous epithelium abnormality. The maintenance of relatively normal peripheral levels of androgens could conceivably be achieved in the face of diminished androgen concentration in the testes. The range of circulating androgen levels is wide (250 to 1200 ng/100 ml) and depends not only on testicular production rates but also on the rates of peripheral metabolism and renal clearance. It is conceivable that a testis with an enzymic lesion in the steroidogenic pathway may not produce androgens in amounts sufficient to maintain the extremely high intratesticular concentrations, but the peripheral levels could remain within the normal range. Examples of specific enzymic deficiency have been published. Goebelsmann et al. 65 reported patients with 17,a-dehydrogenase deficiency and with deficiency of 17ahydroxylase. The deficiencies of these key enzymes produced an extremely severe deficiency of androgen production and resulted in pseudohermaphroditism. An excess of an enzyme involved in androgen biogenesis may also result·in inadequate androgen production. An excess of5a-reductase activity has been reported resulting in 5a-reduction of early intermediates in the steroidogenic pathway associated with a diminished testosterone production rate. 66 Under normal conditions, testicular tissue is relatively poor in 20a-reductase; 20a-dihydroprogesterone and 20a-dihydropregnenolone are normally only minor metabolites. This situation is in contradistinction to that in the ovaries, which produce great amounts of 20a-dihydroprogesterone. Evidence from in vitro studies suggests that a hydroxyl group in the 20a position interferes with further metabolism of the steroid, particularly with 17a-hydroxylation and with 17-20 lyase activity. Consequently, an excess of 20a-hydrogenase may .interfere .with· the normal progression .of. the androgen biogenetic pathway and thus with·the production of androgens. Recent studies . have demonstrated increased rates· of in vitro formation of 2Oa-hy- . droxylated C 21 steroids, in testicular tissue of some. patients with· severe oligospermia.67

Spermatogenesis Fundamental studies dealing with the phys.iology of Leydig ,cell function and with androgen biosynthesis have already led to palpable clinical

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benefits. Elucidation of the pathophysiologic principles led to identification of specific pathologic states, thus to· diagnosis, and in some instances to selection of appropriate and successful therapy. Knowledge of the intricacies of the hormonal control of the seminiferous tubule and of the spermatogenic process has been· gained only in the past 5 to 10 years. These studies resulted· in the discovery of a series of basic phenomena dealing with the molecular aspects of hormone action. In analogy with ICSH, it has been demonstrated that FSH also requires specific receptors in the target cell for expression of its action (for review see reference 68). Although FSH has always been considered to be the "germinal cell" hormone, i.e., a hormone that specifically acts on the germinal cells (for review, see reference 69), .the specific receptors for FSH were found in the Sertoli cells. 7G-72 This finding made the Sertoli cell the center of attention 100 years after its discovery and only after numerous . attempts in the course of these years to find a role for FSH in the spermatogenic process. While studies on the binding of· FSH were in progress, several other molecular phenomena were discovered. A specific protein, androgenbinding protein (ABP), was shown to be pro-. duced by the testicular tissue in response to FSH stimulation:73 • 74 An FSH-dependent increase in the formation of cAMP was also demonstrated. 75- 77 . Both events were shown to occur in the Sertoli cell (for review, see reference 78). Androgen-binding protein was shown to be secre- . ted by the Sertoli cells into the lumen of the seminiferous tubules and transported into the epididymis, where it accumulates in concentrations higher than those found in the testes and is thought to be an important factor in providing a high concentration of androgens. locally in the epididymis. 79 This high concentration of androgens . is thought to be essential to the maturation of epididymal spermatozoa. Although no direct evidence has been given for the role of ABP in the testes, a hypothesis for its function there has been proposed and is discussed below. While data have accumulated demonstrating ,a dramatic effect ofFSH on cAMP production, RNA, and general and specific (ABP)·protein.synthesis, no direct evidence has' emerged to link these phenomena with the process of spermatogeBeSis. The ,data suggest that the various.· molecular events under FSH control can be. demonstrated more readily in developing testes or in posthypophysectomy-regressed testes.

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The fact that testosterone maintains spermatogenesis in adult animals in the absence of gonadotropins 80 led to the investigation of its effects on the various biochemical parameters discussed above. These studies demonstrated that testosterone, like FSH, will maintain and (under certain conditions) reinitiate ABP production in the testes of hypophysectomized rats. 81 Furthermore,specific testosterone receptors have been demonstrated in cytosol obtained from testicular tissue,82 and evidence has been provided for the translocation of the receptor-steroid complex into the nucleus. 83 It has recently been demonstrated that receptors are present in Sertoli cells in culture. 84 ,85 Furthermore, receptors for testosterone may be present in several types of germinal cells. 86·88 The demonstration of a chromatin acceptor for the receptor-testosterone complex in testicular tissue 89 ,90 added another crucial fragment to the evidence that spermatogenesis is controlled by the hormones through the molecular mechanisms postulated for other systems. It must be emphasized that most of the studies discussed above were conducted in lower species and should be extrapolated to man with caution. The extremely limited information derived from studies on human testicular tissue suggests that the androphilic protein isolated from the human testis may possess physicochemical characteristics identical with those of testosterone-binding globulin,91 a testosterone-transport protein found in blood. Thus, the human testis may not produce a specific testicular androgen-binding protein as does the rat testis. It appears that in the human testis mechanisms at least similar to, if not identical with, those found in lower species will be uncovered. If so, it is quite plausible that a biochemical lesion at any point in the complicated molecular mechanism governing spermatogenesis may result in abnormality of this process. Thus, the pathophysiology of testicular disease resulting in what we now call "adult seminiferous tubule failure" may be the result of fine changes or disturbances in molecular mechanisms governing spermatogenesis. Since some of these mechanisms are now amenable to laboratory scrutiny, appropriate tests will have to be devised to probe these mechanisms under clinical conditions. It is quite likely that as a result of such studies Sertoli cell dysfunction will emerge as a major pathophysiologic mechanism underlying the etiology of "adult seminiferous tubule failure."

The above discussion has centered primarily around the pathophysiology of two testicular components, the Leydig cells and the seminiferous epithelium. Examination of testicular biopsies from patients with disturbed sperm production frequently reveals abnormalities not only in the seminiferous epithelium but also in the limiting membrane of the seminiferous tubules. The two striking changes most easily detected in routine histologic preparations of testicular biopsies are "hyalinization" of the limiting membrane and hyperplasia of the fibroblastic elements, "peritubular fibrosis."

It is not known whether the etiologic factors responsible for the abnormality of the seminiferous epithelium also induce pathologic changes in the limiting membrane or whether the abnormality of the limiting membrane is a primary process which may actually be the cause of damage to the seminiferous epithelium. The available information concerning this question is controversial and confusing. Some investigators have published observations suggesting that treatment with testosterone may induce pathoologic changes in the limiting membrane; however, lack of gonadotropins has also been suggested as a cause for this abnormality, and apparently gonadotropin treatment reverses the lesion. It appears that, since the clinical approach utilized until now to resolve this question may not be adequate, an animal model may have to be developed to study this problem. Unfortunately, the most commonly used experimental animals--rats, mice, and guinea pigs-do not show the characteristic pathologic changes in the limiting membrane under any ofthe experimental conditions studied thus far.

IATROGENIC ETIOLOGY OF TESTICULAR DYSFUNCTION

Surgery Although rare, interference with the blood supply to the testes can occur in the course of hernia repair. Cases of testicular damage secondary to interference with blood vessels in the spermatic cord are occasionally seen. They probably are not as common as might be expected because of the collateral blood supply to the testes. However, if a testicular lesion is produced as a result of blood supply disturbance, it is ischemic in nature and after a period of time irreversible.

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REFERENCES

Heat The application of heat to the scrotum in patients with various forms of orchidites is still an occasionally practiced form of palliative therapy. Similarly, heavy and tight dressings elevating the scrotum and testes close to the· body are sometimes employed. Such dressings result in elevation of intratesticular temperature and direct injury to the seminiferous epithelium. Although short-term exposure to heat produces reversible changes, chronic exposure will induce irreversible changes, the extent of which will depend on the duration of the exposure and the amount of heat (for review, see reference 92).

Chemotherapy Experimental studies in lower species clearly demonstrated the detrimental effects ofalkylating agents,antimetabolites, and a variety of cytotoxic substances on the seminiferous epithelium. The degree of damage and its reversibility depend greatly on the type of agent used and the dose and duration of treatment. Reports of studies in man are scarce; however, those which are available reveal results similar to those reported in lower species. The damage varies from complete and permanent sterility through subfertility to return of fertility with production of phenotypically normal offspring. The extent of reported data, however, is too limited to allow a definitive evaluation of the relationship between specific agents, dose, and duration of therapy on one hand and the severity and permanence of the damage on the other. Possible genetic abnormalities, not detectable by physical examination of the offspring, have been considered, but no data are available concerning this distinct possibility (for review, see reference 1).

Androgen Therapy The administration of androgens, whether for the purpose of treating impotence or the male climacteric or. for improvement of athletic performance in men. engaged in competitive sports, most likely causes only temporary interference with spermatogenesis. However, therapy with androgens in immature males for the purpose of induction of puberty or stimulation of growth may be detrimental to the normal development ofthe seminiferous epithelium. However, no adequate data are available to permit a definite conclusion about the safety of androgen therapy during puberty.

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