Growth of the rat gubernaculum in vitro and localisation of its growth centre

Testicular descent revisited: a micro-computed tomography study in fetal rats

PURPOSE

There is a long history of research dealing with the embryology of the testicular descent. However, important aspects like the role of the gubernaculum and the development of the processus vaginalis peritonei are not understood. Micro-computed tomography (µCT) is an established tool for anatomical studies in rodents. Our study applied µCT imaging to visualize the testicular descent in rats and focused on the role of the gubernacular bulb and the development of the processus vaginalis peritonei.

METHODS

Rats from embryonic day 15 (ED15) to ED21 and newborns (N0) were fixed and dried using the "critical point" technique. We ran a SkyScan^® µCT system and scans were analyzed for gender-specific differentiation of the genital ridge and used for 3D visualization of relevant anatomic structures.

RESULTS

µCT imaging confirmed the intraperitoneal location of the testicles from ED15 to N0. The components of the inner genital moved closer together while the intestinal volume expanded. The gubernacular bulb seemed to be involved in the formation of the processus vaginalis peritonei.

CONCLUSION

Here, we utilized µCT imaging to visualize the testicular descent in the rat. Imaging provides new morphologic aspects on the development of the processus vaginalis peritonei.

Testicular descent: A review of a complex, multistaged process to identify potential hidden causes of UDT

BACKGROUND/PURPOSE

What causes normal descent of the testis in a fetus, and what goes wrong with this complex process to cause undescended testes (UDT), or cryptorchidism? Over the last 2 decades, most authors searching for the cause(s) of UDT have looked at the 2 main hormones involved, insulin-like hormone 3 (Insl3) and testosterone (T)/ dihydrotestosterone (DHT), and their known upstream (hypothalamic-pituitary axis) and intracellular 'downstream' pathways. Despite these detailed searches, the genetic causes of UDT remain elusive, which suggest the aetiology is multifactorial, and/or we are looking in the wrong place.

METHODS

In this review we highlight the intricate morphological steps involved in testicular descent, which we propose may contain the currently 'idiopathic' causes of UDT. By integrating decades of research, we have underlined many areas that have been overlooked in the search for causes of UDT.

RESULTS

It is quite likely that the common causes of UDT are still hidden in these areas, and we suggest examining these processes is worthwhile in the hope of finding the common genetic anomalies that lead to cryptorchidism. Given the fact that a fibrous barrier preventing descent is often described at orchidopexy, examination of the extracellular matrix enzymes needed to allow gubernacular migration may be a fruitful place to start.

CONCLUSION

This review of the complex anatomical steps and hormonal regulation of testicular descent highlights many areas of morphology and signalling pathways that have been overlooked in the search for causes of UDT.

Evaluation and management of the infant with cryptorchidism

PURPOSE OF REVIEW

Normal testicular descent is now recognized to occur in two steps with the first, transabdominal stage controlled by insulin-like hormone 3. The second, inguinoscrotal stage is controlled by androgens, mostly indirectly via the genitofemoral nerve, which appears to direct the migration of the gubernaculum to the scrotum. Undescended testis (UDT) is multifactorial, with only some of the genes identified. This review highlights recent developments that are leading to changes in practice.

RECENT FINDINGS

There is an emerging consensus among pediatric surgeons and urologists about the management of UDT with recommendations that the diagnosis of congenital UDT should be confirmed at 3-6 months of age and orchidopexy done at 6-12 months of age. With the recommendations for early surgery, recent studies focus on the complications of orchidopexy, to determine whether this is higher in infants than older children. In addition, there is general acceptance of the existence of 'acquired' UDT, which develops after about 2 years of age, but treatment for this group remains controversial.

SUMMARY

Evaluation of children with UDT now needs to be separated into the assessment of possible congenital UDT in infants at 0-6 months, for orchidopexy before 12 months, and preschool boys, who may be developing acquired UDT.

Regulation of testicular descent

Testicular descent occurs in two morphologically distinct phases, each under different hormonal control from the testis itself. The first phase occurs between 8 and 15 weeks when insulin-like hormone 3 (Insl3) from the Leydig cells stimulates the gubernaculum to swell, thereby anchoring the testis near the future inguinal canal as the foetus grows. Testosterone causes regression of the cranial suspensory ligament to augment the transabdominal phase. The second, or inguinoscrotal phase, occurs between 25 and 35 weeks, when the gubernaculum bulges out of the external ring and migrates to the scrotum, all under control of testosterone. However, androgen acts mostly indirectly via the genitofemoral nerve (GFN), which produces calcitonin gene-related peptide (CGRP) to control the direction of migration. In animal models the androgen receptors are in the inguinoscrotal fat pad, which probably produces a neurotrophin to masculinise the GFN sensory fibres that regulate gubernacular migration. There is little direct evidence that this same process occurs in humans, but CGRP can regulate closure of the processus vaginalis in inguinal hernia, confirming that the GFN probably mediates human testicular descent by a similar mechanism as seen in rodent models. Despite increased understanding about normal testicular descent, the common causes of cryptorchidism remain elusive.

Hoxa-11 maintains cell proliferation in the mouse gubernaculum to facilitate testicular descent

INTRODUCTION

The gubernaculum is a structure vital for guiding testicular descent. The Homeobox gene, Hoxa-11, is involved in patterning embryonic structures and is necessary for gubernacular development, as Hoxa-11 knock-out mice exhibit abnormal gubernacula and undescended testes. We aimed to elucidate how testicular descent fails by examining cell proliferation and androgen receptor (AR) expression in Hoxa-11 KO mice gubernacula.

METHODS

Postnatal day 2 wild type (n=6) and Hoxa-11 KO mice (n=6), were prepared for immunohistochemistry and confocal microscopy using antibodies against androgen receptor, slow skeletal myosin (My32), and Ki67, a marker of cell proliferation.

RESULTS

The gubernacula of Hoxa-11 KO mice were hypocellular compared with WT. AR was present in the gubernaculum and abutting inguinal fat pad in both WT and Hoxa-11 KO with no difference in expression. Slow skeletal myosin was present in a clear 'swirl' in the growth centre of WT animals which was absent in the Hoxa-11 KO mice. Ki67, expressed in the growth centre and cremaster muscle in WT, was greatly decreased in Hoxa-11 KO.

CONCLUSION

Hoxa-11 may regulate fibroblast proliferation in the gubernaculum, as it does in human uterosacral ligaments, allowing formation of the 'growth centre' within the bulb and facilitating myogenesis and elongation to the scrotum. Polymorphisms in Hoxa-11 may contribute to the aetiology of human 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.

Mathematical modelling of gubernaculum during inguino-scrotal migration shows limb bud characteristics

PURPOSE

The gubernaculum is postulated to grow like an embryonic limb bud during inguinoscrotal descent in rodents. Recently, modelling of limb bud growth suggests the undifferentiated, distal "progress zone" provides molecular morphogenic signals, rather than cell division, as previously thought. We aimed to develop a mathematical gubernacular growth model, hypothesising that it would mimic limb buds through evolutionary conservation.

METHODS

Histology was done on Sprague-Dawley rats (day 2, 8; n=7/group) to determine gubernacular length, width, cell density in distal growth centre, middle and proximal cremaster muscle. Analysis of measurements enabled gubernacular growth modelling under variable growth centre sizes/densities, assuming no apoptosis.

RESULTS

Modelling found that gubernacular growth occurred mostly within cremaster muscle, rather than primarily in the undifferentiated mesenchymal tip, despite its higher mitotic rate. The growth centre accounted for ≤ 10% of total gubernacular enlargement/elongation.

CONCLUSIONS

These results suggest the gubernaculum elongates by proliferation throughout cremaster muscle like a limb bud. The distal undifferentiated tip may provide signalling molecules for growth, which could be a fruitful source for causes of failed migration/elongation in cryptorchidism.

Journal of Pediatric Surgery-Sponsored Fred McLoed Lecture. Undescended testis: the underlying mechanisms and the effects on germ cells that cause infertility and cancer

Testicular descent is a complex morphological process that occurs in at least 2 stages, with different hormonal control. Insl3 controls the first step of gubernacular enlargement, although the abnormality long gubernacular cord in persistent Műllerian duct syndrome remains unexplained. Androgens control inguinoscrotal migration, which may be triggered by local signalling from the mammary line, and which requires the genitofemoral nerve. However, there is still much to learn about this phase, which when abnormal frequently leads to cryptorchidism. Orchidopexy is being recommended in the first year of age, because increasing research suggests that the stem cells for spermatogenesis form between 3 and 9 months, with surgery aiming to permit this normally, although this is not yet proven. Acquired cryptorchidism is now becoming accepted and is likely to be caused by inadequate elongation of the postnatal spermatic cord. It is not yet known whether orchidopexy is always needed, as this remains controversial.

The effect of flutamide on expression of androgen and estrogen receptors in the gubernaculum and surrounding structures during testicular descent

BACKGROUND/PURPOSE

Inguinoscrotal testicular descent is controlled by androgens between embryonic days E16-19, but androgen receptor (AR) and estrogen receptor (ER) locations are unknown. We aimed to find AR, ERα, and ERβ in the gubernaculum and inguinal fat pad (IFP) in normal rats and after flutamide treatment.

METHODS

Sprague-Dawley timed-mated rats were injected with flutamide (75 mg/kg body weight/5% ethanol + oil) on E16-19 or vehicle alone. Male fetuses or pups (5-10/group) were collected at E16; E19; and postnatal (P) days 0, 2, 4, 8. Sections were prepared for hematoxylin and eosin or immunohistochemistry for AR, ERα, and ERβ. Receptor labeling was quantitated as distinct nuclear labeling/100 μm(2) in gubernaculum and IFP.

RESULTS

There was minimal gubernacular AR-labeling until E19, dramatically increasing postnatally. By contrast, at E16-E19 there was significant IFP AR immunoreactivity suppressed by flutamide (P < .05). No ERα expression was observed, but ERβ was expressed in both gubernaculum and IFP, maximally at E16, but unchanged by flutamide.

CONCLUSIONS

During the androgen sensitivity window (E16-19), the gubernaculum contains ERβ but minimal ERα or AR, while the IFP, which is supplied by the genitofemoral nerve, contains abundant AR that are flutamide-sensitive. These results suggest that the IFP could be the site of androgenic action controlling gubernacular development.

The role of cremaster muscle in testicular descent in humans and animal models

Testicular descent is a complex developmental process involving anatomical and hormonal regulation. The gubernaculum undergoes a "swelling reaction" during the transabdominal phase and is mainly under the control of Insulin-Like Peptide 3 (INSL-3) and Mullerian Inhibitory Substance/Anti-Mullerian Hormone (MIS/AMH). The second phase of testicular descent is regulated by androgens and calcitonin gene-related peptide (CGRP) release from the sensory nucleus of the genitofemoral nerve (GFN). In rodents, the active proliferation of the gubernacular tip and cremaster muscle, its rhythmic contraction, as well as the chemotactic gradient provided by the CGRP result in eventual migration of the testis into the scrotum. This review illustrates the structural aspects and hormonal control of cremaster muscle development to better understand the mechanism of testicular descent in normal rodents and humans, compared to diseased rodent models. The analysis showed the cremaster muscle is formed from mesenchymal differentiation of the gubernacular tip and is not a direct passive extension of internal oblique muscle. Cremaster muscle matures slower than other body muscles, and the persistence of immature myogenic proteins seen in cardiac muscle allows rhythmic contraction to guide the testis into the scrotum. Finally, remodelling of the cremaster muscle enables gubernacular eversion. Further understanding of the molecular regulators governing the structural and hormonal changes in the cremaster muscle may lead to new advances in the treatment of undescended testes.

The role of the gubernaculum in the descent and undescent of the testis

Testicular descent to the scrotum involves complex anatomical rearrangements and hormonal regulation. The gubernaculum remains the key structure, undergoing the 'swelling reaction' in the transabdominal phase, and actively migrating out of the abdominal wall to the scrotum in the inguinoscrotal phase. Insulin-like hormone 3 (Insl3) is the primary regulator of the first phase, possibly augmented by Müllerian inhibiting substance/anitmüllerian hormone (MIS/AMH), and regression of the cranial suspensory ligament by testosterone. The inguinoscrotal phase is controlled by androgens acting both directly on the gubernaculum and indirectly via the genitofemoral nerve, and release of calcitonin gene-related peptide from its sensory fibres. Outgrowth of the gubernaculum and elongation to the scrotum has many similarities to an embryonic limb bud.Cryptorchidism occurs because of both failure of migration congenitally, and failure of elongation of the spermatic cord postnatally. Germ cell development postnatally is disturbed in congenital cryptorchidism, but our current understanding of germ cell biology suggests that early orchidopexy, around 6 months of age, should provide a significant improvement in prognosis compared with a previous generation. Hormone treatment is not currently recommended. Acquired cryptorchid testes may need orchidopexy once they no longer reach the scrotum, although this remains controversial.

Gubernacular development in the mouse is similar to the rat and suggests that the processus vaginalis is derived from the urogenital ridge and is different from the parietal peritoneum

BACKGROUND

Gubernacular development and testicular descent have been studied most extensively in rat models, but new transgenic mouse models require a deep understanding of normal mouse development so that results can be extrapolated to the human. We aimed to compare gubernacular anatomy during development in the mouse with that of the rat.

METHODS

Time-mated mice (C57BL/6) and Sprague-Dawley rats were used to collect male fetuses at embryonic (E) days E13, E14, E15, E17, E18, and E19 and neonates at postnatal (P) days P0 and P2. Fetuses and newborn were processed for serial sections (sagittal, transverse, and coronal) and stained with hematoxylin and eosin, muscle markers (embryonic myosin, desmin), a neuronal marker (Tuj1), a mitotic marker (Ki67), and keratin marker to label epithelium.

RESULTS

Early development of cremaster in the mouse was related to transversus abdominis muscle, but not internal oblique muscle (as in rats), and forms a monolaminar cremaster layer. There is close association between the regressing inguinal mammary bud and the gubernaculum in the mouse at E13. The peritoneal surface of the processus vaginalis (PV) covering the gubernaculum and epididymis was morphologically distinct from the remaining parietal peritoneum throughout development.

CONCLUSIONS

Gubernacular development in mouse is similar to that in the rat except for certain structures, such as cremaster muscle. The PV seems to be derived from the surface of the urogenital ridge, separate from the remaining parietal peritoneum. This study suggests that the PV has evolved to aid testicular descent in this species, rather than a nondescript diverticulum of parietal peritoneum.

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.