Does the gubernaculum migrate during inguinoscrotal testicular descent in the rat?

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.

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.

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

PURPOSE

Recent studies suggest that testicular descent is accomplished by outgrowth of the gubernaculum from the abdominal wall. The tip of the gubernaculum has been proposed as the primary site of growth, similar to an embryonic limb bud. We aimed to determine the maximum site of growth in organ culture.

METHODS

Gubernacula from 1-day-old Sprague-Dawley rats (n = 40) were collected and divided into 4 groups as follows: whole gubernaculum (control), truncated gubernaculum (tip excised), gubernacular tip alone, and grafted gubernaculum with an extra tip on its side. Tissues were cultured with or without calcitonin gene-related peptide (CGRP) (714nmol/L) in medium for 24 hours. The area of each gubernaculum was determined by "Image J" analysis of digital photos collected via a Leica Wild M28 microscope (Leica Microsystems, Wetzler GmbH Germany) taken before and after culture.

RESULTS

In organ culture, the neonatal rat gubernaculum normally shrank 10% to 15%, but this was prevented by the presence of exogenous CGRP (0.8% vs 11.8%; P < .003). By contrast, gubernacula with their tips excised were not affected by CGRP (3.4% vs 4.7%; not significant). Gubernacular tips alone did respond to CGRP (2.7% vs 13.5%; P < .03). Transplantation of the tip to another gubernaculum caused it to develop 2 tips.

CONCLUSIONS

These results suggest that the rat gubernaculum contains a growth centre in its distal tip that can respond to CGRP. This is consistent with a limb bud model of gubernacular growth during the inguinoscrotal descent of the testis.

The migrating gubernaculum grows like a "limb bud"

BACKGROUND

The gubernaculum is crucial for testicular descent, and in the second, or inguinoscrotal, phase of descent it has no caudal attachments. Cranially, it is attached to the testis, but its caudal free tip migrates to the scrotum controlled by the genitofemoral nerve. Recent studies show active proliferation in the tip. We hypothesized that the gubernacular tip may grow like a limb bud.

METHODS

We performed whole-mount in situ hybridization studies on male and female fetal mice (ages, E14.5-E18.5; n = 162) looking for limb bud regulatory factors.

RESULTS

Our results showed that a member of the fibroblast growth factor (Fgf) family, Fgf10, and Hoxa10 were both expressed in the male gubernaculum at E14.5, and Hoxa10 was also expressed in the E16.5 mice. Weak staining was seen in the female gubernaculum for Hoxa10 on days E14.5 and E16.5, whereas no staining for Fgf10 was seen in the female gubernaculums.

CONCLUSIONS

These studies, although preliminary, suggest limb bud regulators are essential for gubernacular growth. Hox genes and Fgfs may be fruitful areas of research to unravel the molecular control of gubernacular migration during testicular descent.

The anatomy of the cremaster muscle during inguinoscrotal testicular descent in the rat

BACKGROUND

Extrapolation of rat testicular descent studies to humans has been criticized because of anatomical differences of the cremaster muscle. Human cremaster is described as a thin strip rather than a large, complete sac as in rats, which is proposed to be more important in propelling the testis during descent. This study investigated cremaster muscle anatomy and ontogeny in both normal and cryptorchid rat models.

METHODS

Gubernacula from 4 groups of neonatal rats were sectioned longitudinally and transversely: normal Sprague-Dawley, capsaicin pretreated, flutamide pretreated, and congenital cryptorchid rats. Gubernacula were stained with hematoxylin-eosin, Masson trichrome, and desmin immunohistochemistry to study muscle development.

RESULTS

Myoblasts are more numerous at the gubernacular tip, whereas the most differentiated muscle is proximal. Rat cremaster develops as an elongated strip rather than a complete sac derived from abdominal wall muscles. Flutamide and capsaicin pretreatment disrupts development.

CONCLUSION

Rat cremaster muscle develops as a strip, bearing close resemblance to human cremaster muscle, permitting extrapolation of cremaster function to human testicular descent. The cremaster muscle appears to differentiate from the gubernacular tip during elongation to the scrotum, and requires intact sensory innervation and androgen.

Cell membrane and mitotic markers show that the neonatal rat gubernaculum grows in a similar way to an embryonic limb bud

AIM/BACKGROUND

How the gubernaculum guides the testis into the scrotum remains controversial, with various proposals from passive inversion to active growth. We aimed to determine if the gubernaculum contains an area of active proliferation, such as a "progress zone" in a growing embryonic limb bud, using a fluorescent cell membrane marker, 1,1'-didodecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate [DiIC12(3)], to trace cell migration, and 5-bromodeoxyuridine (BUDR) (a thymidine analogue) as a mitotic marker.

METHODS

Gubernacula were collected from neonatal male rats (n = 42, day 1-2, Sprague-Dawley) and cultured with calcitonin gene-related peptide (CGRP; 714 nmol/L). 1,1'-didodecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate-coated glass beads (diameter, 150-212 microm) were placed next to the bulb for the first 3 hours. Gubernacula were cultured for 3, 18, and 24 hours, then frozen sections cut and examined by confocal microscopy (wavelength, 549 nm). In a second experiment, pups not exposed to exogenous CGRP (n = 53, day 0, Sprague-Dawley) were injected intraperitoneally with BUDR (50 mg/kg of body weight); gubernacula were collected at 2, 48, 72, and 96 hours postinjection (PI), sectioned, and stained using immunohistochemistry to count the number of BUDR-positive cells per 100 cells (labeling index) in the bulb, cremaster, cord, and epididymis.

RESULTS

After 24 hours' culture with CGRP, the bulb showed an oval region (diameter, 300 microm) of high fluorescence, and the cremaster region showed elongated cells migrating out of the bulb. When cultured without CGRP, the same oval region contained no fluorescence. In vivo BUDR labeling index increased in all areas until 48 hours postinjection and then decreased most rapidly in the bulb (P < .05), in the presence of endogenous CGRP from the genitofemoral nerve.

CONCLUSIONS

The rat gubernaculum contains a putative progress zone, such as in a growing limb bud, in the presence of CGRP. Cells migrate out of this zone to form cremaster muscle. We hypothesize that proliferation in the bulb elongates the gubernaculum, whereas proliferation of cremaster cells would increase gubernacular diameter. This brings to "life" the gubernaculum as an actively growing organ in contrast to the inert ligament connecting the testis to the scrotum portrayed in most anatomy textbooks.

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.

A new role for androgen in testicular descent: permitting gubernacular cell proliferation in response to the neuropeptide, calcitonin gene-related peptide

BACKGROUND/PURPOSE

Cell proliferation at the gubernacular tip increases in response to exogenous calcitonin gene-related peptide (CGRP) during migration into the scrotum. Calcitonin gene-related peptide is contained in the masculinized sensory branches of the genitofemoral nerve. We tested the independent effects of chemical sensory nerve disruption and prenatal androgen blockade on the in vitro gubernacular proliferative response to CGRP.

METHODS

Neonatal Sprague-Dawley rats were injected with capsaicin, a sensory nerve toxin, and gubernacula dissected 2 days later (D2). Sprague-Dawley dams were injected with flutamide, an androgen receptor antagonist, between days 15 and 19 of gestation. Flutamide pretreated males, and normal neonatal rats, were dissected at D0 and D2. Gubernacula were cultured for 24 hours +/- CGRP, pulse-labelled for the last 4 hours of culture with bromodeoxyuridine, a thymidine analogue marker for DNA replication, sectioned, and stained using immunohistochemistry. The percentage of positively staining cells in the gubernacular tip was calculated from three separate counts by a blinded observer and compared using analysis of variance.

RESULTS

Normal D0 gubernacular tips showed a significant response of cell proliferation to exogenous CGRP (34% vs 9% in controls, P < .001), which resolved by day 2 (16% vs 12%, P > .05). Calcitonin gene-related peptide markedly increased cell proliferation in D2 capsaicin pretreated gubernacula compared with controls (25% vs 14%, P < .01) and normal D2 gubernacula cultured with CGRP (P < .01). D0 flutamide pretreated cultured with CGRP showed no increase in cell proliferation compared with controls (16% vs 11%), but a small response was seen by D2 (19% vs 9%, P < .05). There was no significant difference between proliferation rates in the control groups.

CONCLUSIONS

Sensory innervation interruption sensitises the gubernaculum to exogenous CGRP, suggesting upregulation of CGRP receptors. In contrast, androgen blockade abolishes the increased rate of cell proliferation within the gubernacular tip. We conclude that androgens are necessary to "preprogramme" the proliferative response of the gubernaculum to CGRP.

Testicular descent, cryptorchidism and inguinal hernia: the Melbourne perspective

Cryptorchidism is the commonest congenital genitourinary anomaly in males and results when the testis does not descend into its normal intrascrotal position during development. In full-term infants, the incidence is approximately 3% at birth. Cryptorchidism results in several abnormalities, including attenuated spermatogenesis, infertility and a greater risk of malignancy. The normal mechanism of testicular descent appears to be multi-staged, with various anatomical factors and hormonal influences, but the exact process is still unclear. In this article we review the current theories of normal testicular descent, with a focus on the hormones and anatomical factors, and current treatments for undescended testis.

Testicular descent and cryptorchidism: the state of the art in 2004

The understanding of testicular descent has changed much in the 20 years since the authors' laboratory began studying the mechanism. The process is now known to occur in 2 steps with different anatomy and hormonal regulation but with many still unresolved controversies. Recent advances include the recognition of acquired cryptorchidism of critical early postnatal germ cell development and the recommendation for surgery at 6 months of age. The authors still await long-term outcome studies.

Gubernacular fibroblasts express the androgen receptor during testis descent in cryptorchid rats treated with human chorionic gonadotrophin

Cryptorchidism was provoked in 3 day old rats treated with 17-beta-estradiol over 30 days to identify the cells that express the androgen receptor (AR) during experimental testis descent in the gubernaculum. In one group of animals, testis descent was induced with human chorionic gonadotrophin (hCG) applied daily for 5 or 10 days. A correlative study using a testosterone radioimmunoassay with electron microscopy and immunocytochemical detection of AR was performed in gubernacula of hCG treated and untreated control animals. The gubernaculum of rats undergoing testes descent showed a dramatic increase in the number of AR-positive cells. These were located in the connective tissue among smooth muscle cells in the gubernacular cord and between striated muscle fibers in the bulb. In both regions, the AR-positive cells were identified as fibroblasts. Several clusters of amorphous material appeared in the extracellular matrix of the connective tissue in hCG treated rats. Our results suggest that testosterone induces the expression of AR in gubernacular fibroblasts which seem to degrade the extracellular matrix during gubernacular involution.

Measurement of the linear dynamics of the descent of the bovine fetal testis

Measurements were made on 86 male bovine fetuses collected from abattoirs in the vicinity of Sydney, Australia. The fetal body length was used to calculate the approximate day of gestational age (DGA); most fetuses were between 60 and 150 DGA. The distances from the caudal pole of the kidney (metanephros) to, respectively, the tip of the scrotum, the distal end of the testis and the internal ring of the inguinal canal were measured, as well as the dimensions of the testis and gubernaculum testis. Distances of (1) testis to inguinal canal, (2) inguinal canal to scrotum, (3) testis to scrotum and (4) gubernaculum to scrotum were calculated from these measurements, which were made on both left and right sides. The total length of the gubernaculum testis increased during transabdominal passage and during transinguinal passage of the testis. Furthermore, the gubernaculum appeared to maintain the testis at a relatively fixed distance from the scrotum during transabdominal passage so that the inguinal canal appeared to move towards the testis. The greatest distance between the testis and the tip of the scrotum was found during the transinguinal passage of the testis and was 2.8 cm for the left testis and 2.3 cm for the right. When located within the scrotum, each testis was still 1.6-1.7 cm from the tip of the scrotum, so the distance to be traversed was only 0.6-1.2 cm. Following passage of the testis through the inguinal canal, the gubernaculum became shorter and its distal tip was displaced toward the distal end of the scrotum. Traction by the gubernaculum could account for the final transposition of the testis from the external inguinal ring to the scrotum. Other factors involved in displacement of the testis include differential growth patterns as well as increases in the dimensions of the testis itself.

The development of the gubernaculum and inguinal closure in the marsupial Macropus eugenii

This study reports the developmental anatomy of testicular descent and inguinal closure of the tammar wallaby (Macropus eugenii) from birth to maturity. In females the ovary migrated caudally between days 10 and 20 after birth. The gubernaculum differentiates into the round ligament in the abdomen and extra-abdominally as the ilio-marsupialis muscle of the mammary glands. In males the testes migrated to the internal inguinal ring by day 20 post partum (pp), coinciding with the enlargement of the gubernaculum, and from the internal inguinal ring to the scrotum between days 20 and 65 pp. During descent there was an increase in the hyaluronic acid concentration in cells of the gubernaculum and scrotum. Development of the cremaster muscle began by day 10 pp on the periphery of the gubernaculum and its basic structure was completed by day 60 pp. After descent the inguinal canal closed between days 50 and 60 pp, but a small irregular lumen persisted, somewhat similar to that seen in the congenital scrotal hydrocoele of humans. Tammars have a hopping mode of locomotion and, like humans, are essentially bipedal. We suggest that inguinal closure evolved in these two species because their upright posture may otherwise lead to a high incidence of inguinal hernias.

Role of the gubernacular bulb in cremaster muscle development of the rat

The role of the gubernaculum during the inguino-scrotal phase of testicular descent remains controversial. Some authors propose involution and eversion while others suggest active migration, although the site of growth is unknown. We aimed to determine whether the gubernacular bulb is actively proliferating or regressing during inguino-scrotal testicular descent in the rat. Gubernacula were removed from Sprague-Dawley rats and congenitally-cryptorchid TS mutant rats. Animals (0, 3, 7, 10, and 11 days of age) were treated with bromodeoxyuridine (BUdR) 2 hr before they were killed. BUdR incorporation into newly synthesized DNA served as a marker of cell division. The gubernacula were histologically processed for hematoxylin-eosin (H&E) and immunoperoxidase staining. Four different areas within the gubernaculum were examined for BUdR-positive cells: area 1: plica gubernaculi (cord); area 2: pars infravaginalis gubernaculi (bulb); area 3: distal part of the cremaster muscle; and area 4: proximal part of the cremaster muscle. The rate of cell division for each of these areas was determined by counting the number of BUdR-positive cells per 100 cells. The highest rate of BUdR labeling in both types of rats was in area 2, which is the tip of the gubernacular bulb, and this was significantly greater (P < 0.0001) than in the gubernacular cord or developing cremaster muscle. The mitotic activity was also noted to be significantly greater (P < 0.0001) at the distal end of the cremaster muscle than at the proximal end. The amount of mitosis decreased significantly (P < 0.01) in areas 2 and 4 of the gubernaculum in Sprague-Dawley rats across the period studied. This trend was not observed in TS rats. Our results suggest that the bulb actively proliferates after birth, with possible differentiation into new cremaster muscle cells. We propose that the bulb is the growing end of the elongating gubernaculum, analogous to the growth of a limb bud.

Localization of calcitonin gene-related peptide within the genitofemoral nerve in immature rats

BACKGROUND/PURPOSE

Calcitonin gene-related peptide (CGRP) has been proposed to influence migration and testicular descent by release from the genitofemoral nerve. The site of CGRP within the nerve has been controversial, with conflicting views on whether CGRP is synthesised and released from the motor nerves.

METHODS

The genitofemoral nerve (GFN) was retrogradely labelled by fluorescent dye (DAPI) in 25 Sprague-Dawley rats (days 5, 16, and 31, n = 8 in each group; day 35, n = 1). Spinal cords and dorsal root ganglia (DRG) were removed two to three days later and sectioned for immunofluorescence. Substance P and CGRP-containing cells were labelled with fluorescein-linked antibodies. Specimens were examined by double fluorescence to identify cells with both markers.

RESULTS

The motor nucleus of the GFN contained 119 cells on day 7 and 284 cells by days 19 through 34. A prominent band of CGRP-containing fibers, arising from the dorsal horn, synapsed with the GFN motor nucleus itself. CGRP-labelled GFN cells were found in the DRG by double labelling.

CONCLUSIONS

CGRP from the GFN may affect gubernacular migration by release from the sensory nerves, rather than motor nerves as previously thought. The GFN motor nucleus receives CGRP-containing innervation from the dorsal horn, which may form part of the cremasteric reflex.