The gubernaculum in adult female, adult male and TFM male mice

The curious case of testicular descent: factors controlling testicular descent with a note on cryptorchidism

Background

The testicular descent is a uniquely complex process depending upon multiple factors like growth and reorganisation of the specific gonadal ligaments, hormones, etc., which interplay with each other. Though an unambiguous event, it is still laced with incredulity since the data interpretation were intermingled between different species creating more ambiguity in certain aspects of this process. In order to understand the aetiopathology of cryptorchidism the extensive study of the factors controlling the descent is necessitous.

Main body

Though testes originate in the abdomen, they migrate to an extra abdominal site the scrotum, which makes it vulnerable to pathological conditions associated with the descent. The hormones that play vital role in the first phase of descent are insulin-like hormone 3 (INSL3), Anti-müllerian hormone as well as testosterone, whereas androgens, genitofemoral nerve and its neurotransmitter calcitonin gene-related peptide (CGRP) influence the second phase. Despite the vast research regarding the complex nexus of events involving the descent there are disparities among the cross species studies. However all these discrepancies make testicular descent yet again fascinating and perplexing. Our aim is to provide a comprehensive review including recent advances which provides thorough coverage of anatomical and hormonal factors in the descent as well as cryptorchidism.

Conclusion

Though our understanding on testicular descent has evolved over the decades there still has obscurity surrounding it and the studies on the factors responsible for descent are becoming more intense with the time. Our knowledge on many factors such as INSL3 and CGRP is more established now; however, on the other hand the role of androgens still remains speculative. As the knowledge and understanding of the biological process of testicular descent increases it will pave ways to new treatment plans to treat cryptorchidism more effectively.

Morphologic alterations of the genital mesentery implicated in testis non-descent in rats prenatally exposed to flutamide

BACKGROUND

There is an endless debate on whether androgens mediate testis descent through developmental changes in the gubernacular or the cranial suspensory ligament.

OBJECTIVE

To investigate the relation of any possible morphologic changes in the genital mesentery, that is, the system of genital peritoneal folds including the gubernacular and cranial suspensory ligaments, with the event of testis non-descent in rats prenatally exposed to the antiandrogen flutamide.

MATERIALS AND METHODS

Time-pregnant Sprague Dawley rats received flutamide (100 mg/kg/d) or vehicle subcutaneously on gestational days 16-17. Flutamide-treated male offspring (n = 67), and vehicle-treated male (n = 34) and female (n = 28) offspring were surgically explored under microscope on postnatal day 50. Testicular position was examined bilaterally. Dimensions of genital mesentery parts were also assessed bilaterally. Association of flutamide-induced morphologic changes with descended (n = 61) and undescended (n = 50; 33 cryptorchid and 17 ectopic) testes was investigated with logistic regression analysis.

RESULTS

The male genital mesentery comprised a cranial and a caudal fold converging on the vas deferens. Flutamide resulted in enlarged cranial and reduced caudal folds. Of all flutamide-induced alterations, the increased length of the posterior fixation of the cranial fold and the decreased length of the gubernacular ligament of the caudal fold were found to independently increase the odds of testis non-descent. Testicular ectopy, unlike cryptorchidism, was associated with a short gubernacular ligament only. The female genital mesentery consisted of a cranial fold only.

CONCLUSION

Our findings showed a combined contribution of both cranial and caudal folds of the genital mesentery to testis non-descent, through an abnormally long mesentery root and an abnormally short gubernacular ligament, respectively. Inhibition of male-specific development of the genital mesentery with flutamide did not result in a feminized architecture.

Regional Variation in Androgen Receptor Expression and Biomechanical Properties May Contribute to Cryptorchidism Susceptibility in the LE/orl Rat

Background: The process of testicular descent requires androgen and insulin-like 3, hormones secreted by fetal Leydig cells. Knowledge concerning distinct and common functions of these hormones in regulating development of the fetal gubernaculum remains limited and/or conflicting. The current studies were designed to better define characteristics of androgen receptor (AR) expression, function and regulation, as well as the biomechanical properties of normal and cryptorchid gubernaculum during fetal development. Methods: We studied fetal gubernacula from Long Evans outbred (LE/wt) rats and an inbred (LE/orl) strain with an inherited form of cryptorchidism associated with an AR signaling defect. Gubernacular cells or whole organs obtained from LE/wt and LE/orl fetal gubernacula underwent AR immunostaining and quantitative image analysis. The effects of dihydrotestosterone (DHT) on AR expression, muscle fiber morphology, hyaluronan (HA) levels and glycosaminoglycan (GAG) content were measured in LE/wt gubernacula. Finally, the spatial mechanics of freshly harvested LE/wt and LE/orl fetal gubernacula were compared using micropipette aspiration. Results: AR is expressed in the nucleus of mesenchymal core, tip and cord cells of the embryonic (E) day 17 and 21 fetal gubernaculum, and is enhanced by DHT in primary cultures of gubernacular mesenchymal cells. Enhanced AR expression at the tip was observed in LE/wt but not LE/orl gubernacula. In in vitro studies of whole mount fetal gubernaculum, DHT did not alter muscle fiber morphology, HA content or GAG production. Progressive swelling with reduced cellular density of the LE/wt gubernaculum at E19-21 was associated with increased central stiffness in LE/wt but not in LE/orl fetuses. Conclusions: These data confirm nuclear AR expression in gubernacular mesenchyme with distal enhancement at the tip/cord region in LE/wt but not LE/orl rat fetuses. DHT enhanced cellular AR expression but had no major effects on muscle morphology or matrix composition in the rat fetal gubernaculum in vitro. Regional increased stiffness and decreased cell density between E19 and E21 were observed in LE/wt but not LE/orl fetal gubernacula. Developmental differences in cell-specific AR expression in LE/orl fetal gubernacula may contribute to the dysmorphism and aberrant function that underlies cryptorchidism susceptibility in this strain.

Perspective: A Neuro-Hormonal Systems Approach to Understanding the Complexity of Cryptorchidism Susceptibility

Nonsyndromic cryptorchidism is a common multifactorial, condition with long-term risks of subfertility and testicular cancer. Revealing the causes of cryptorchidism will likely improve prediction and prevention of adverse outcomes. Herein we provide our current perspective of cryptorchidism complexity in a synthesis of cumulative clinical and translational data generated by ourselves and others. From our recent comparison of genome-wide association study (GWAS) data of cryptorchidism with or without testicular germ cell tumor, we identified RBFOX family genes as candidate susceptibility loci. Notably, RBFOX proteins regulate production of calcitonin gene-related peptide (CGRP), a sensory neuropeptide linked to testicular descent in animal models. We also re-analyzed existing fetal testis transcriptome data from a rat model of inherited cryptorchidism (the LE/orl strain) for enrichment of Leydig cell progenitor genes. The majority are coordinately downregulated, consistent with known reduced testicular testosterone levels in the LE/orl fetus, and similarly suppressed in the gubernaculum. Using qRT-PCR, we found dysregulation of dorsal root ganglia (DRG) sensory transcripts ipsilateral to undescended testes. These data suggest that LE/orl cryptorchidism is associated with altered signaling in possibly related cell types in the testis and gubernaculum as well as DRG. Complementary rat and human studies thus lead us to propose a multi-level, integrated neuro-hormonal model of testicular descent. Variants in genes encoding RBFOX family proteins and/or their transcriptional targets combined with environmental exposures may disrupt this complex pathway to enhance cryptorchidism susceptibility. We believe that a systems approach is necessary to provide further insight into the causes and consequences of cryptorchidism.

The regulation of testicular descent and the effects of cryptorchidism

The first half of this review examines the boundary between endocrinology and embryonic development, with the aim of highlighting the way hormones and signaling systems regulate the complex morphological changes to enable the intra-abdominal fetal testes to reach the scrotum. The genitoinguinal ligament, or gubernaculum, first enlarges to hold the testis near the groin, and then it develops limb-bud-like properties and migrates across the pubic region to reach the scrotum. Recent advances show key roles for insulin-like hormone 3 in the first step, with androgen and the genitofemoral nerve involved in the second step. The mammary line may also be involved in initiating the migration. The key events in early postnatal germ cell development are then reviewed because there is mounting evidence for this to be crucial in preventing infertility and malignancy later in life. We review the recent advances in what is known about the etiology of cryptorchidism and summarize the syndromes where a specific molecular cause has been found. Finally, we cover the recent literature on timing of surgery, the issues around acquired cryptorchidism, and the limited role of hormone therapy. We conclude with some observations about the differences between animal models and baby boys with cryptorchidism.

Androgen receptor (AR) physiological roles in male and female reproductive systems: lessons learned from AR-knockout mice lacking AR in selective cells

Androgens/androgen receptor (AR) signaling is involved primarily in the development of male-specific phenotypes during embryogenesis, spermatogenesis, sexual behavior, and fertility during adult life. However, this signaling has also been shown to play an important role in development of female reproductive organs and their functions, such as ovarian folliculogenesis, embryonic implantation, and uterine and breast development. The establishment of the testicular feminization (Tfm) mouse model exploiting the X-linked Tfm mutation in mice has been a good in vivo tool for studying the human complete androgen insensitivity syndrome, but this mouse may not be the perfect in vivo model. Mouse models with various cell-specific AR knockout (ARKO) might allow us to study AR roles in individual types of cells in these male and female reproductive systems, although discrepancies are found in results between labs, probably due to using various Cre mice and/or knocking out AR in different AR domains. Nevertheless, no doubt exists that the continuous development of these ARKO mouse models and careful studies will provide information useful for understanding AR roles in reproductive systems of humans and may help us to develop more effective and more specific therapeutic approaches for reproductive system-related diseases.

Cryptorchidism

Cryptorchidism is a very common anomaly of the male genitalia, affecting 2%-4% of male infants and is more common in premature infants. There are two separate stages of testicular descent. The first stage occurs at 8-15 weeks' gestation in the human fetus and is characterized by enlargement of the genito-inguinal ligament, or gubernaculum, and regression of the cranial suspensory ligament. The testis remains close to the future inguinal region as the fetal abdomen grows. Leydig cells in the testis produce insulin-like hormone 3, which stimulates the caudal gubernaculum to grow and become thicker. Mullerian inhibiting substance may have a role in the first phase of descent by stimulating the swelling reaction in the gubernaculum. The second phase of testicular descent requires migration of the gubernaculum and testis from the inguinal region to the scrotum, between 25 and 35 weeks' gestation. The genitofemoral nerve releases calcitonin gene-related peptide, a neurotransmitter that provides a chemotactic gradient to guide migration. The exact cause of cyrptorchidism remains elusive. Information is mainly derived from animal studies (especially in rodents), which may not extrapolate to the human setting. These findings, however, do have some similarities among mammalian species. The current recommended timing for orchidopexy is between 6 and 12 months of life in an effort to preserve the spermatogonia--the stem cells for subsequent spermatogenesis. Despite surgical treatment by orchidopexy, the long-term outcome still remains problematic and controversial. Impaired fertility (33% in unilateral cases and 66% in bilateral undescended testes) and a cancer risk 5-10 times greater than normal is observed over time. Further research into the cause and management of undescended testes is necessary.

Cryptorchidism in common eutherian mammals

Cryptorchidism is failure of one or both testes to descend into the scrotum. Primary fault lies in the testis. We provide a unifying cross-species interpretation of testis descent and urge the use of precise terminology. After differentiation, a testis is relocated to the scrotum in three sequential phases: abdominal translocation, holding a testis near the internal inguinal ring as the abdominal cavity expands away, along with slight downward migration; transinguinal migration, moving a cauda epididymidis and testis through the abdominal wall; and inguinoscrotal migration, moving a s.c. cauda epididymidis and testis to the bottom of the scrotum. The gubernaculum enlarges under stimulation of insulin-like peptide 3, to anchor the testis in place during gradual abdominal translocation. Concurrently, testosterone masculinizes the genitofemoral nerve. Cylindrical downward growth of the peritoneal lining into the gubernaculum forms the vaginal process, cremaster muscle(s) develop within the gubernaculum, and the cranial suspensory ligament regresses (testosterone not obligatory for latter). Transinguinal migration of a testis is rapid, apparently mediated by intra-abdominal pressure. Testosterone is not obligatory for correct inguinoscrotal migration of testes. However, normally testosterone stimulates growth of the vaginal process, secretion of calcitonin gene-related peptide by the genitofemoral nerve to provide directional guidance to the gubernaculum, and then regression of the gubernaculum and constriction of the inguinal canal. Cryptorchidism is more common in companion animals, pigs, or humans (2–12%) than in cattle or sheep (≤1%). Laboratory animals rarely are cryptorchid. In respect to non-scrotal locations, abdominal testes predominate in cats, dogs, and horses. Inguinal testes predominate in rabbits, are common in horses, and occasionally are found in cats and dogs. S.c. testes are found in cattle, cats and dogs, but are most common in humans.

Female mice carrying a ubiquitin promoter-Insl3 transgene have descended ovaries and inguinal hernias but normal fertility

Mouse knockout studies have indicated that Insl3 is involved in development of the gubernaculum in males, which is essential for normal testicular descent. To determine further the functions of Insl3 we have generated transgenic (TG) mice ubiquitously expressing Insl3. In these mice low levels of transgenic Insl3 mRNA are expressed in all tissues analyzed. In the TG females the ovaries descend to the base of the abdominal cavity during the fetal period, as a consequence of the formation of male-like gubernaculum structures. Furthermore, the gubernacular structures developed express androgen receptor, identically to the corresponding structures in males. At adult age the ligaments formed connect the uterine horns to the inguinal region of the abdomen. Ligaments are also formed between the lower and upper parts of the uterine horns, and these ligaments force the uterus to form a coiled structure. However, the TG females retain their reproductive functions, indicating that neither the location of the ovaries nor the macroscopic structure of the uterus is vital for reproduction. In addition, Insl3 expression causes inguinal hernia in females, suggesting that a combination of estrogen and Insl3 action disrupts proper development of the muscular and connective tissue structures of the abdomen. The lack of a phenotype in other tissues indicates that gubernaculum formation is the most sensitive biological response as regards Insl3.

Inguinal canal "lipoma"

Groin dissection was performed in adult male post-mortem subjects to establish the prevalence of inguinal canal "lipoma." Thirty-six body halves (age range 24-92 years) were studied. Of these, 27 (75%) contained a discrete mass of fat within the inguinal canal. This mass was always continuous with the preperitoneal fat through the deep inguinal ring. Nineteen of these 27 masses (70%), displayed a characteristic pedunculated form with a bulbous distal end. Eighteen of the 36 dissections (50%), revealed a mass more than 4 cm in length. Six dissections showed extension of the mass beyond the superficial inguinal ring and three of these six (8% of the 36 groins studied) showed distortion of the proximal spermatic cord with a mass at the superficial inguinal ring. The masses submitted for histology comprised mature adipose tissue and all but two of these were reported as having an adherent capsule. No significant correlation was found between mass length and either subject age or body mass index (BMI) but a statistically significant correlation between the length of the fat mass on the left and right sides was shown. This study demonstrates that the inguinal canal "lipoma" is a common feature in an adult male population and may be of sufficient size to cause clinical misdiagnosis. The high prevalence, characteristic location and appearance of the "lipoma" suggest a developmental etiology.

Cryptorchidism in mice mutant for Insl3

Impaired testicular descent (cryptorchidism) is one of the most frequent congenital abnormalities in humans, involving 2% of male births. Cryptorchidism can result in infertility and increases risk for development of germ-cell tumours. Testicular descent from abdomen to scrotum occurs in two distinct phases: the trans-abdominal phase and the inguino-scrotal phase. Currently, little is known about the factors that regulate the trans-abdominal phase of testicular descent. Leydig insulin-like hormone (Insl3) is a member of the insulin hormone superfamily expressed in the developing testis. We show here that mice mutant for Insl3 are viable, but exhibit bilateral cryptorchidism due to developmental abnormalities of the gubernaculum, resulting in abnormal spermatogenesis and infertility. Female homozygotes have impaired fertility associated with deregulation of the oestrus cycle. These findings reveal roles for Insl3 in the development of the urogenital tract and in female fertility. Insl3 may act as a hormone to regulate the growth and differentiation of the gubernaculum, thereby mediating intra-abdominal testicular descent.