Chapter 6 - Primary Testicular Failure Hermann M. Behre, MD, Martin Bergmann, PhD, Manuela Simoni, MD, PhD TO OBTAIN A DOWNLOAD OF THIS CHAPTER IN WORD OR PDF FORMAT, CLICK HERE	Index Contributors Search

INTRODUCTION

The testis has an endocrine as well as an exocrine function. Endocrine testicular failure results in testosterone deficiency. In primary endocrine testicular failure, a decline in testosterone secretion (resulting in a condition termed hypoandrogenism) is caused by a deficiency or absence of Leydig cell function. Clinically relevant diseases described in this chapter are anorchia, Leydig cell hypoplasia and numerical chromosome abnormalities. Testicular dysgenesis is another cause for primary testicular failure that is described in depth in Endotext.com, Pediatric Endocrinology, Chapter 7: Sexual Differentiation. In contrast to primary endocrine testicular failure, secondary endocrine testicular failure is caused by absent or insufficient bioactivity of GnRH or LH (see Endotext.com, Endocrinology of Male Reproduction, Chapter 5: Hypogonadotropic hypogonadism and gonadotropin therapy).

The phenotype of primary exocrine testicular failure is male infertility. A comprehensive review on causes and treatment of male infertility is given in Endotext.com, Endocrinology of Male Reproduction, Chapter 7: Clinical management of male infertility. Cryptorchidism as a clinical relevant cause for primary exocrine testicular failure is discussed in Endotext.com, Endocrinology of Male Reproduction, Chapter 19: Cryptorchidism and hypospadias; testicular tumors as a cause and/or sequelae of testicular failure in Endotext.com, Endocrinology of Male Reproduction, Chapter 13: Testicular cancer pathogenesis, diagnosis and endocrine aspects.

This chapter focuses on anorchia, germ cell aplasia, spermatogenetic arrest, hypospermatogenesis, numerical chromosome abnormalities, structural chromosomal abnormalities, as well as Y chromosome microdeletions causing primary exocrine testicular failure.

ANORCHIA

Bilateral anorchia is defined as complete absence of testicular tissue in genetically and phenotypically male patients. In unilateral anorchia testicular tissue is still present on the contralateral side.

Pure anorchia has to be differentiated from conditions with ambiguous and intersex genitalia (see Endotext.com, Pediatric Endocrinology, Chapter 7: Sexual Differentiation). A clinically important differential diagnosis is cryptorchidism and testicular atrophy where testicular tissue is still detectable (see Endotext.com, Endocrinology of Male Reproduction, Chapter 19: Cryptorchidism and hypospadias).

Congenital Anorchia

Bilateral congenital anorchia is rare, the incidence appears to be 1:20,000 males. Unilateral congenital anorchia is about 4 times as frequent.

As male differentiation of the genital tract and development of the penis and scrotum is dependent on the production of anti-Mullerian hormone (AMH) and androgens, the testis must have disappeared after initial activity in cases of bilateral anorchia. For the development of Wolffian duct structures, an ipsilateral testis must be present at least up to the 16th week of gestation ("the vanishing testis syndrome") (1). Intrauterine infarction of a maldescended testis or testicular torsion appears to be the major contributor to anorchia (2).

In patients with congenital bilateral anorchia serum gonadotropins are already elevated in childhood and rise to very high levels from the age of puberty onwards. Testosterone levels remain within the castrate range. In patients with suspected bilateral anorchia it is mandatory to rule out cryptorchidism, as cryptorchidism is associated with an increased risk for testicular cancer and should definitively not be overlooked. (see Endotext.com, Endocrinology of Male Reproduction, Chapter 13: Testicular cancer pathogenesis, diagnosis and endocrine aspects). Both the hCG stimulation test, that examines testosterone secretory capacity, and serum AMH measurement can be used for differential diagnosis. During hCG administration testosterone levels remain unchanged in patients with bilateral anorchia even after a 7-day period of stimulation while a rise can be detected in patients with cryptorchidism (3). In comparison to the hCG test, measurement of AMH, which is undetectable in anorchia, has a higher sensitivity, but equal specificity for differentiation of bilateral anorchia from bilateral cryptorchidism (4, 5). Endocrine tests are not useful for differential diagnosis of unilateral anorchia. In these cases imaging techniques such as computer tomography or MRT and finally exploratory surgery or laparoscopy have to be applied.

Unilateral anorchia does not require therapy. In phenotypically male patients with bilateral congenital anorchia testosterone substitution has to be implemented at the time of expected puberty. For psychological or cosmetic reasons implantation of testicular protheses could be offered to the patient although these are often expensive. To date, there is no treatment of infertility in bilateral anorchia.

Acquired Anorchia

Surgical removal of both testes in patients with androgen-dependent prostate carcinoma is the most prevalent cause for bilateral acquired anorchia. Other reasons include unintended removal or devascularisation during herniotomy or orchidopexy, testicular infarction, severe trauma and very rarely self-mutilation. If only one testis is lost then fertility and testosterone production will normally be maintained by the remaining testis and no specific therapy is required. However, patients with a single testis require careful management when surgery is planned on the remaining testis.

The clinical appearance in patients with bilateral acquired anorchia depends on the time when testicular loss occurred. Acquired anorchia before puberty leads to the characteristic phenotype of male eunuchoidism and after puberty to the phenotype of post-pubertal testosterone deficiency (see Endotext.com, Endocrinology of Male Reproduction, Chapter 2: Androgens).

Untreated acquired bilateral anorchia seems to have no effect on life expectancy, but clearly has an adverse effect on the quality of life (6). If both testes have been removed for therapeutic purposes, e.g. in a patient with prostate carcinoma, androgen supplementation is contraindicated. All other patients have to receive permanent testosterone substitution from the time of the expected onset of puberty in order to induce pubertal development, and in an adult immediately after testicular loss to maintain the various androgen-dependent functions.

LEYDIG CELL HYPOPLASIA

Leydig cell hypoplasia is a rare disease with an autosomal recessive pattern of inheritance and estimated incidence of 1:1,000,000. The Leydig cells are unable to develop because of inactivating mutations of the LH receptor that fails to provide the necessary stimulation of intracellular pathways. The underlying gene defect in Leydig cell hypoplasia was first described by Kremer (7) and various other defects have since been described (8 - 21). Men with Leydig cell hypoplasia present with very low serum testosterone and high LH levels. Leydig cell hypoplasia belongs to the group of the disorders of sex differentiation (DSD) and is currently classified as 46,XY DSD.

The phenotype is dependent on the extent of intrauterine testosterone secretion. Two types of Leydig cell hypoplasia have been described. Type I is the most severe form, resulting in a female phenotype of the external genitalia with blind ending vagina, primary amenorrhea, and absence of secondary sex differentiation at puberty. It is caused by inactivating mutations in the LH receptor that completely prevent LH and hCG signal transduction and thus testosterone production. Leydig cell hypoplasia type II is characterized by milder signs of androgen deficiency with a predominantly male habitus but signs of hypogonadism with micropenis and/or hypospadia. This milder form is derived from mutations of the LH receptor, which only partially inactivate signal transduction and retain some responsiveness to LH (16). Testicular histology reveals seminiferous tubules, whereas Leydig cells are not present or appear only as immature forms. Epididymides and deferent ducts are usually present, whereas the uterus, tubes or upper vagina are not found. Recently, a patient with Leydig cell hypoplasia type II lacking exon 10 of the LH receptor has been described (22). In this patient, maternal hCG synthesized during pregnancy probably led to the development of a normal male phenotype, whereas LH was unable to stimulate the mutant receptor at the time of puberty (22, 23). HCG treatment of this patient was capable of inducing testosterone biosynthesis and complete spermatogenesis (22). This case, however, represents an exception. Therapy of 46,XY DSD with complete feminization requires both orchidectomy because cryptorchid gonads are prone to malignant degeneration, and estrogen substitution therapy.

Spermatogenic failure

Whereas endocrine testicular failure causes hypogonadism, spermatogenic failure - defined as exocrine testicular failure - leads to male infertility. Spermatogenic failure might be caused by hypothalamic, pituitary, testicular or post-testicular disorders. A comprehensive review on causes and treatments of male infertility is given in Endotext.com, Endocrinology of Male Reproduction, Chapter 7: Clinical management of male infertility. Various testicular etiologies of spermatogenic failure may lead to the same histopathological pattern. In this sense, germ cell aplasia, spermatogenic arrest, and hypospermatogenesis as described below have to be considered as a description of certain histopathologic phenotypes of spermatogenetic failure, and not as manifestations of single disease entities.

GERM CELL APLASIA (SERTOLI CELL ONLY SYNDROME)

Germ cell aplasia or Sertoli cell only (SCO) syndrome is a histopathologic phenotype that was first described by Del Castillo et al. in 1947 (24). In complete germ cell aplasia the tubules are reduced in diameter, and contain only Sertoli cells but no other cells involved in spermatogenesis [Figure 1]. Germ cell aplasia can also be focal with a variable percentage of tubules containing germ cells, but in these tubules spermatogenesis is often limited in both quantitative and qualitative terms (25), and such cases should be referred to as hypospermatogenesis (see below). Germ cell aplasia or SCO syndrome is one common cause of non-obstructive azoospermia.

In congenital germ cell aplasia the primordial germ cells do not migrate from the yolk sac into the future gonads or do not survive in the epithelium of the seminiferous tubule. Chromosomal abnormalities, especially microdeletions of the Y chromosome, are important genetic causes for complete germ cell aplasia (26). Anti-neoplastic therapy with radiation or chemotherapy may cause complete loss of germ cells. Other reasons include viral infections of the testes such as mumps orchitis. Germ cell aplasia can occur in maldescended testes.

Diagnosis of germ cell aplasia can only be made by testicular biopsy. However, the testicular biopsy may not be representative in certain patients, as testicular sperm have been retrieved by testicular sperm extraction (TESE) in patients with apparently "complete germ cell aplasia" following a diligent review of the testicular histology (27). In addition, it has been demonstrated in a large consecutive series of bilateral biopsies from 534 infertile men that a marked discordance of spermatogenic phenotype pattern between both testes can be detected in about 28% of patients (28). Therefore, multiple testicular biopsies of both testes must be scrupulously screened before a diagnosis of complete germ cell aplasia can be made.

Patients with the complete form of germ cell aplasia are always azoospermic. Currently, there is no therapy for exocrine testicular failure of patients with complete germ cell aplasia. In general, testosterone production in the Leydig cells is not affected and patients are normally androgenized, and only few patients have hypoandrogenism requiring treatment.

SPERMATOGENIC ARREST

Spermatogenic arrest is also not a specific diagnosis for primary exocrine testicular failure, but a histopathological description of the interruption of normal germ cell maturation [Figure 2] at the level of a specific cell type including that of spermatogonial arrest [Figure 3], spermatocyte arrest [Figure 4], and spermatid arrest [Figure 5]). A definite diagnosis can only be made by multiple testicular biopsies. Testicular volume, FSH and inhibin B may lie in their normal ranges, but may also be elevated or decreased, respectively.

The arrest may be caused by genetic or by secondary influences. Genetic aetiologies include trisomy, balanced-autosomal anomalies (translocations, inversions) or deletions in the Y chromosome (Yq11). It is likely that many genetic factors exist but have not yet been identified. In some patients with predominant round spermatid maturation arrest, the expression of cAMP Responsive Element Modulator (CREM) is significantly reduced or undetectable (29). Study of the numerous mouse knock out models that display as spermatogenic phenotype, including sperm cell arrest, has contributed little of clinical relevance to the large number of men with idiopathic infertility. The possible role of several gene mutations and polymorphisms has been extensively investigated but no clear-cut genetic factor could be identified so far (30, 31). An association between increased androgen receptor CAG repeat length (which reduces the transcriptional activity of the androgen receptor) and idiopathic male infertility is suggested by a recent, comprehensive meta-analysis (32). However, no gene mutation or polymorphism analysis has the necessary sensitivity and specificity to be recommended as a diagnostic tool in the workup of male infertility.

Secondary factors for spermatogenetic arrest are toxic substances (radiotherapy, chemotherapy, antibiotics), heat or general diseases (liver or kidney insufficiency, sickle cell anaemia) (33).

Complete arrest of spermatogenesis results in azoospermia. To date, there is no known therapy for uniform spermatogenic arrest (34).

HYPOSPERMATOGENESIS

This is the third major histological grouping in primary exocrine testicular failure in which all germ cell types, including mature spermatids, is present in some or all tubules, but are mildly, moderately or severely reduced in number. Some patients have the appearance of complete germinal cell aplasia in some tubules but with complete spermatogenesis in adjacent tubules (sometimes called 'focal' germinal cell aplasia) while others have the appearance of an excess number of precursor germ cell in relation to the number of mature spermatids in the epithelium. Such cases have been described as incomplete or focal germinal cell aplasia which implies, perhaps falsely, a commonality between these disorders and those with complete germinal cell aplasia in all tubules.

All these variants may be seen originating from the same causes outlined under germinal cell aplasia. From a practical clinical perspective, the differentiation is important as patients with hypospermatogenesis may have azoospermia or varying degrees of oligoasthenoteratozoospermia, or sperm may be retrieved from testicular biopsies (TESE) (27).

In most patients with hypospermatogenesis testicular volume is reduced. FSH is elevated in most, but not all patients, with serum levels correlating positively with the proportion of tubules with germ cell aplasia (35). Several studies have demonstrated that inhibin B is a more sensitive and specific endocrine marker of hypospermatogenesis (36, 37). However, even the combined measurement of inhibin B and FSH provides no certainty concerning the presence or absence of sperm in multiple testicular biopsies (38, 39).

In azoospermic men with severe hypospermatogenesis, pregnancies can be achieved with testicular sperm extracted from testicular biopsies (TESE) that are injected into mature oocytes by intracytoplasmic sperm injection (ICSI). It has been suggested that residual sperm production could be improved by FSH therapy in incomplete germ cell aplasia. However, randomized studies performed so far have demonstrated some increase in total sperm count in the ejaculate but failed to prove a significant effect on fertility (40).

NUMERICAL CHROMOSOME ABNORMALITIES

Klinefelter Syndrome

This syndrome was first described by Harry Klinefelter in 1942 as a clinical condition with small testes, azoospermia, gynecomastia and an elevated serum FSH (41). Only in 1959 was the chromosomal basis of the disorder described. Subsequently the diagnosis of Klinefelter syndrome has required the demonstration of the 47,XXY karyotype or one of its rarer variants.

The prevalence of Klinefelter syndrome appears to be approximately 1 in 660 males, and recent data suggest a rising incidence over the last decades (42, 43). It is the most frequent form of primary testicular dysfunction affecting spermatogenesis as well as hormone production and is found in about 12% of men presenting with azoospermia. It appears that at least half of the cases remain undiagnosed and untreated throughout life (42).

A non-mosaic 47,XXY karyotype is found in 80 - 90 percent of Klinefelter patients. A mosaic is seen in another 5 - 10 percent of patients. The 47,XXY/46,XY mosaicism is most common. The 48,XXXY, 48,XXYY and 49,XXXXY karyotypes constitute 4 - 5 percent of all Klinefelter syndrome karyotypes, structurally abnormal extra X chromosomes are found in less than one percent of patients. Apart for karyotype analysis, molecular genetics methods can be used to quantify the number of X chromosomes, for example by quantitative PCR analysis of the androgen receptor gene located on the X chromosome (44).

The numerical aberration in non-mosaic 47,XXY is derived with equal likelihood from maternal or paternal meiotic error (45). Most cases are caused by meiosis without X/Y or X/X recombination. Advanced maternal age seems to be a risk factor (42). It is not known whether the 47,XXY karyotype is slightly over-represented among spontaneous abortions and stillbirths. However, in contrast to many other aneuploidies, Klinefelter syndrome seems to be only a minor risk factor and most pregnancies result in a live-birth.

Patients with Klinefelter syndrome are usually inconspicuous until puberty. Interestingly the velocity of height gain can be increased in the pre-pubertal years. Men with Klinefelter syndrome tend to be tall (mean adult height is about the 80th percentile for the population) and to have relatively long legs compared to their overall height.

In most patients, early stages of puberty proceed normally. Post-pubertally the syndrome is characterized by the small testes with firm consistency remaining in the range of 1 - 4 ml. Most patients with Klinefelter syndrome are infertile because of azoospermia. Testicular histopathology in adult men with Klinefelter syndrome displays various patterns. Classically, germ cell aplasia, total tubular atrophy or hyalinizing fibrosis and relative hyperplasia of Leydig cells are found. However, in some adult Klinefelter patients, foci of spermatogenesis up to the stage of mature testicular sperm can be detected (46, see below).

The degree of virilization varies widely. In early puberty, LH and FSH increase while serum levels of testosterone plateaus at or just below the lower limit of the normal range. After the age of 25, about 80% of patients have reduced serum testosterone levels and complain of decreasing libido and potency. On average serum estradiol levels are high normal or may exceed the normal range. LH and especially FSH levels are exceedingly high, serum levels of inhibin B are very low or undetectable (47, 48).

During puberty bilateral painless gynecomastia of varying degrees develops in about half of the patients. In a large Danish study covering 696 men with Klinefelter syndrome no evidence for a substantial increase in the overall cancer rate was found (49). The risk of developing mammary carcinoma may be increased relative to normal men but remains a rare occurrence and routine surveillances is not recommended (49, 50). A significantly increased risk was found for the rare mediastinal malignant germ cell tumors, which occur preferentially at the age of 14 to 29 years (49).

The intelligence of Klinefelter patients is very variable. The group difference between boys with Klinefelter syndrome and controls amounts to 11 points in full scale IQ (92 versus 103), and deficits are observed primarily in verbal and cognitive abilities (51). Some of the young patients attract attention because of learning difficulties and school problems. In general, they fail to reach the level of achievement or professional expectations of their families (52, 53). Compared with their classmates certain abnormal physical and psychological characteristics of the patients become obvious and they may become socially alienated. Higher-grade aneuploidy of the sex-chromosomes (48,XXXY, 48,XXYY and 49,XXXXY) is associated with mild mental retardation. Klinefelter patients with chromosome mosaics (47,XXY/46,XY) may show very few clinical symptoms.

In general, the variability of the clinical features in patients with Klinefelter syndrome is related to degree of androgenisation, which, in turn, depends on the pattern of inactivation of one copy of the androgen receptor gene. In particular, a significant genotype-phenotype association exists in Klinefelter patients and androgen effects on appearance and social characteristics are modulated by the androgen receptor CAGn polymorphism (54).

Regarding infertility treatment, it should be noted that in rare cases sperm could be found in the ejaculate and, exceptionally, spontaneous paternity has been described (55). The rate of diploidy of sperm as well as disomy for gonosomes and autosomes seems to be increased in patients with Klinefelter syndrome, however, the majority of sperm appear to be normal (56 - 59). Preliminary data suggest that in about 20 - 50 percent of patients with Klinefelter syndrome it may be possible to retrieve sperm by TESE (46, 60, 61). Several pregnancies have been achieved with testicular sperm used for ISCI (62). The embryos show normal or aneuploid karyotypes which can be identified by preimplantation or prenatal diagnosis (62). Interestingly, the birth of normal children conceived by assisted reproductive techniques seems to be rule (62), suggesting that the few sperm which can be found in about 50% of patients with Klinefelter syndrome possibly derive from the clonal expansion of spermatogonia with normal karyotype.

When testosterone serum levels are reduced, substitution with testosterone is necessary. To avoid symptoms of androgen deficiency hormone replacement therapy should be initiated as early as needed. In particular, Nielsen et al. (63) showed that early testosterone replacement not only relieves biological symptoms such as anemia, osteoporosis, muscular weakness and impotence, but also leads to better social adjustment and integration. Testosterone replacement must be considered a lifelong therapy in Klinefelter patients to assure quality of life. Usually gynecomastia is not influenced by hormone therapy. If it disturbs the patient, a plastic surgeon experienced in cosmetic breast surgery could perform a mastectomy.

XX-Male Syndrome

The XX-Male Syndrome is characterized by the combination of male external genitalia, testicular differentiation of the gonads and a 46,XX karyotype by conventional cytogenetic analysis. This disorder shows a prevalence of 1:9,000 to 1:20,000.

Applying molecular methods it has been demonstrated that about 3 of 4 XX-males have Y chromosomal material translocated onto the tip of one X chromosome (64). Translocation of a DNA-segment which contains the testis-determining gene (SRY = Sex Determining Region Y) from the Y to the X chromosome takes place during paternal meiosis (65). The presence of the gene is sufficient to cause the initially indifferent gonad to develop into a testis.

Most SRY-positive patients are very similar to patients with Klinefelter syndrome. In general, however, 46,XX males are significantly shorter than Klinefelter patients or healthy men, resembling female controls in height and weight. The incidence of maldescended testes is significantly higher than that in Klinefelter patients and controls (66). The testes are small (1 - 4 ml) and firm, and endocrine changes of primary testicular failure with decreased serum testosterone and elevated estrogen and gonadotropin levels. About every second patient develops gynecomastia. XX-males seem to have normal intelligence, however, exact data are lacking. Ejaculate analysis reveals azoospermia. The testicular histology of postpubertal SRY-positive XX males shows atrophy and hyalinization of the seminiferous tubules devoid of germ cells.

For the SRY-negative XX-males (1 of 4 XX-males) the mechanism underlying the sex reversal remains unclear. These patients are less virilized than SRY-positive men and show malformations of the genital organs such as maldescended testes, bifid scrotum or hypospadias (67).

Today, there is no therapy for infertility of men with XX-male syndrome. Patients with reduced testosterone production have to receive appropriate testosterone replacement therapy.

XYY-Syndrome

Most 47,XYY males have no health problems distinct from those of 46,XY males. The diagnosis relies entirely on the cytogenetic demonstration of two Y chromosomes with an otherwise normal karyotype. The non-mosaic chromosomal aneuploidy is caused by non-dysjunction in paternal meiosis. Usually the finding is incidental, occurring when karyotyping has been undertaken for unrelated issues. The prevalence among unselected newborns appears to be 1:1000.

Men with 47,XYY-syndrome have serum levels of testosterone and gonadotropins, as well as testicular volumes, comparable to those of normal healthy men. Most men with 47,XYY-syndrome have normal fertility. Onset of puberty seems to be delayed by 6 months, adult height is 7 cm in excess of the male population mean. The intelligence quotient lies within the normal range, but men score an average of ten points less than age-matched peers. Behavioural problems are more common in 47,XYY males, however, a history of violent behaviour is exceptional (68, 69).

Most 47,XYY-men do not need any specific therapy. Men who achieve fatherhood can expect chromosomally normal offspring probably with the same likelihood as normal men. Nevertheless, to be safe, prenatal diagnosis should be offered.

Numerical Chromosome Abnormalities Limited to Spermatozoa

It should be noted that meiotic disorders, i.e. numerical chromosome abnormalities limited to spermatogenic cells, are not detectable through the study of the somatic karyotype (70 - 72). In a recent study assessing more than 600,000 sperm from 30 men with normal somatic karyotype and different degrees of oligozoospermia a significant inverse correlation between the frequency of numerical sperm chromosome abnormalities (XY, XX, and YY disomy, and diploidy) and sperm concentration was demonstrated (73). In six men with obstructive azoospermia and normal somatic karyotype, analysis of approximately 60,000 sperm revealed an increased frequency of chromosomal disomy and diploidy of spermatozoa compared to 18 normal controls, but only YY disomy reached statistical significance (74).

STRUCTURAL CHROMOSOME ABNORMALITIES

Structural chromosome abnormalities encompass pathological alterations of chromosome structure that are detectable through light-microscopic examination of banded metaphase preparations. Smaller lesions that are only detectable with molecular genetics are not termed structural chromosomal abnormalities. By convention, structural rearrangements such as Robertsonian translocations, that also imply a change in chromosome number, are also regarded as structural abnormalities.

Structural anomalies of the autosomes are distinguished from anomalies of the sex chromosomes (gonosomes). Reciprocal and Robertsonian translocations, inversions, marker chromosomes, X and Y isochromosomes, and Y chromosomal deletions are of practical importance for andrology.

When evaluating a structural chromosomal anomaly for clinical purposes, the distinction between balanced and unbalanced structural aberrations is pivotal. The former are characterized by a deviation from normal chromosome structure but without a net loss or gain of genetic material. If no important gene is disrupted at the breakpoints, balanced structural aberrations exert no negative effect on general health. However, the balanced abnormalities could adversely affect fertility and could give rise to unbalanced karyotypes in the offspring (70, 75, 76).

In unbalanced structural chromosomal abnormalities, genetic material is either missing or there is an overall net excess of material in the cell. Almost all unbalanced karyotypes are associated with severe disturbances of general health, if they are at all compatible with survival. Exceptions are deletions of the Y chromosome that may limit reproductive functions selectively, and are therefore of importance in reproductive medicine.

The majority of male individuals carrying structural aberrations is probably fertile and need no specific therapy. Conversely, men with impaired spermatogenesis show an increased prevalence of structural chromosomal abnormalities (70). Infertile patients with structural chromosomal aberrations may conceive naturally or more severe cases may require 'symptomatic' treatment modalities such as intracytoplasmic sperm injection, however, success rates may be lower than in couples with normal karyotypes (77) It should also be considered that unbalanced karyotypes of the embryo may result from balanced parental chromosomal anomalies (78). For any carrier of a structural chromosome abnormality who considers fatherhood by any means, genetic counselling is strongly recommended, and it should be obligatory prior to any infertility treatment (79, 80). It should be mentioned that in many countries karyotyping of men with idiopathic infertility and decreased sperm concentration is recommended prior to ICSI therapy (e.g., in Germany for men with a sperm concentration < 5 million/ml). The risk of spontaneous pregnancy loss, possible developmental delay often associated with congenital anomalies as a result of an unbalanced karyotype in the offspring, options of prenatal and preimplantation genetic diagnosis, and - for certain aberrations - the possibility that other family members are also affected should be discussed with the patient.

Structural Abnormalities of the Autosomes

Balanced autosomal anomalies may interfere with the meiotic pairing of the chromosomes and thus adversely affect spermatogenesis. These abnormalities often do not display a typical clinical phenotype. The presence and extent of disturbed fertility cannot be foreseen in individual cases. The same balanced autosomal aberration can have a severe effect on spermatogenesis in one patient and none at all in another patient. Even brothers with the same pathological karyotype can have widely differing sperm densities. So far no clinical or laboratory parameter in an infertile male is known which reliably indicates the presence of an autosomal structural anomaly. Therefore, in cases of unclear azoospermia or severe oligozoospermia karyotyping is generally advised (79).

Translocations and inversions usually represent cases of familial chromosomal aberrations. In these patients a family study should be encouraged, as the presence of a translocation or inversion is often associated with a higher rate of abortion and, in some cases, the risk for the birth of a severely handicapped child.

Structural Abnormalities of Sex Chromosomes

An intact Y chromosome is essential for the male reproductive system. The male-specific region of the Y chromosome (MSY) differentiates the sexes and comprises 95% of the chromosome length (81). The SRY gene is localized on the short arm of the Y chromosome and it influences differentiation of the embryonic gonad in the testicular pathway. The long arm of the Y chromosome contains areas responsible for the regular spermatogenesis.

When speaking of deletions of the Y chromosome, those of the short and the long arm must be distinguished (82). Short arm deletions of the Y chromosome that encompass the sex determining SRY gene result in sex reversal. Clinically, affected men appear as phenotypically female individuals with somatic signs of Turner's syndrome. If the deletion affects the long arm, the phenotype will be male. Loss of the heterochromatic part of the Y chromosome's long arm (Yq12) leaves general and reproductive health unaffected. Deletions of the euchromatic part of the Y chromosome's long arm (Yq11) may affect spermatogenesis, because Yq11 harbours loci essential for spermatogenesis (81).

In addition to deletions, a series of further structural anomalies of the Y chromosome are known. Pericentric inversions are without consequence. An isodicentric Y chromosome is a more complex aberration nearly always occurring as a mosaic with a 45,X-cell line. The phenotype may be male, female or intermediate. Patients with a male phenotype are usually infertile. These patients have an increased risk developing testicular tumors (see Endotext.com, Endocrinology of Male Reproduction, Chapter 13: Testicular cancer pathogenesis, diagnosis and endocrine aspects). Reciprocal translocations between the Y chromosome and one of the autosomes are rare. In most cases spermatogenesis is severely disturbed, however, several men with this karyotype are fertile. Translocations between the X and Y chromosomes occur in several variations; often the karyotype is unbalanced. The correlation between karyotype and clinical presentation is complex. The phenotype may be male or female, fertility may be normal or disturbed.

The X chromosome contains numerous genes essential to survival. Every major deletion of this chromosome has a lethal effect in the male sex. Translocations between the X chromosome and an autosome usually result in disturbed spermatogenesis, whereas inversions of the X chromosome do not substantially affect male fertility.

Y CHROMOSOME MICRODELETIONS

The human Y chromosome is not only the dominant sex determinator, but plays an essential role in the genetic regulation of spermatogenesis (83). The long arm of the Y chromosome contains four partially overlapping but discrete regions that are essential for normal spermatogenesis (81, 84). The loss of one of these regions, designated as AZF(azoospermia factor)a, P5/proximal P1 (AZFb), P5/distal P1 (AZFb) and AZFc (or b2/b4),can lead to infertility (81). The deleted regions are usually of submicroscopic dimensions and are known as Y chromosome microdeletions. Their prevalence in azoospermic men lies between 5 - 10% and between 2 - 5% in cases of severe oligozoospermia (85). Clearly, the frequency of Y microdeletions is related to the criteria by which men have been selected (86, 87), whereas ethnic differences might exists as well (88). Deletions of the AZFc region represent about 80% of all AZF deletions (Simoni et al., 2007) [Figure 6]. The type and mechanism of deletions have been recently clarified and result from homologous recombination between retroviral or palindromic sequences (89). The AZFc region includes 12 genes and transcription units, each present in a variable number of copies making a total of 32 copies (90). The classical complete deletion of AZFc, removes 3.5 Mb, corresponding to 21 copies of genes and transcription units (91). Even more gene copies are removed by more extensive deletions (7.7 Mb and 42 copies removed in P5/distal P1 deletions; 7.0 Mb and 38 copies removed P4/distal P1 deletions) (92).It remains unclear if any of the genes of the respective regions is indeed pathologically relevant.

Clinically the patients present with severely disturbed spermatogenesis; endocrine testicular function may or may not be affected by the microdeletion as in other cases of spermatogenetic failure. Testicular histopathology varies from complete or focal Sertoli-cell-only pattern to spermatogenic arrest or hypospermatogenesis with qualitatively intact but quantitatively severely reduced spermatogenesis (93). In azoospermic men, the presence of a complete deletion of AZFa or AZFb seems to be associated with uniform germ cell aplasia and negative prognostic value for testicular sperm retrieval (93, 94). No clinical parameter can help distinguishing patients with microdeletions of the Y chromosome from infertile men without microdeletion (88).. It should be noted that Y chromosome microdeletions have also been described in proven fertile men (95).

A positive result of the analysis, which should be carried out according to the standard recommended by the current guidelines (96), provides a causal explanation for the patient's disturbed spermatogenesis. Beyond this, the test also has prognostic value, as TESE is possible in about 50% of men with AZFc deletion and every son of such a patient will carry the paternal Y chromosome microdeletion and will probably inherit disturbed fertility (97). Genetic counseling is indicated for all carriers of Y chromosome microdeletions (79, 93).

Smaller deletions removing only part of the AZFc region have been identified as a polymorphism significantly associated with infertility, especially oligozoospermia (90). These so-called gr/gr deletions arise by the same mechanism (homologous recombination) and have been extensively studied in large groups of men in different countries. They are found in about 6.8% of infertile men but also in 3.9% of the controls. Although they represent a a significant risk factor for male infertility, they should be regarded as a polymorphism and for the time being this type of diagnostics offers no advantage in male infertility workup as no clinical consequence can be derived from the results obtained (31).