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 role of the testicular factor INSL3 in establishing the gonadal position

INSL3, also designated Leydig insulin-like (Ley I-L) or relaxin-like factor (RLF), belongs to the insulin-like hormone superfamily. It is expressed in pre- and postnatal Leydig cells of the testis and in postnatal theca cells of the ovary. This sexual dimorphic pattern of INSL3 expression during development led us to suggest that the INSL3 factor could play an essential role in sexual differentiation, gonadal function and germ cell development. Key insights into the role of INSL3 came from analyses of INSL3 knockout mice. These mice showed impaired development of the gubernaculum ligament, a structure that is believed to mediate transabdominal descent of the testis during male embryogenesis. In double mutant XY-mice lacking INSL3 and a functional androgen receptor, it was demonstrated that both are essential for establishment of the sexual dimorphic position of the gonads through regulation of gubernaculum development and regression of the cranial suspensory ligament (CSL) during fetal life. Defects in this developmental process can cause cryptorchidism in the male, which is a most common disorder of sexual differentiation in human.     

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 μm) 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 μm) 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.     

Fluorescent anterograde labelling of the genitofemoral nerve shows that it supplies the scrotal region before migration of the gubernaculum

The genitofemoral nerve (GFN) contains a sexually dimorphic neuropeptide transmitter, calcitonin generelated peptide (CGRP). It has been proposed that release of CGRP from the nerve may mediate testicular descent. The aim of this study was to determine the course of the GFN in order to see if CGRP-containing fibres reached the future scrotum before gubernacular migration occurs, since this arrangement would be expected if the nerve controls gubernacular migration by CGRP release. Fluorescent anterograde labelling of the cut GFN in young rats using diamidinophenyl indole (DAPI) or Fast Blue was performed to determine the distal course of the nerve. On frozen serial sections, the nerve was found running posterolateral to the developing spermatic cord in the inguinal canal, then distally on the surface of the cremaster muscle. It then turned cranially to enter the gubernaculum from its distal attachment while some branches continued past the gubernaculum to end in the skin of the future scrotum. Immunoperoxidase staining for CGRP showed labelling in all GFN fibre bundles, including those reaching the scrotum. The course of the nerve with its sexually dimorphic neurotransmitter, CGRP, suggests that the nerve may influence the direction of gubernacular migration from the groin into the scrotum. Offprint requests to: J. M. Hutson     

Testicular descent

Sixty male rats were treated twice weekly from birth with either oestradiol benzoate (E₂B) or sesame oil (controls) to see if oestrogen caused cryptorchidism by inhibition of gubernacular migration between 3 and 10 days after birth. Dissection was carried out at 10, 15, or 28 days, the processus vaginalis measured using a probe, and the weight of the testis recorded. The length of the processus was used as a measure of the length of migration of the hollow gubernaculum. Although E₂B affected testicular weight significantly at 15 days, the length of the processus vaginalis was not significantly different from controls during the normal time of gubernacular migration (3–10 days of age). Histology confirmed this observation, with the gubernaculum within but slightly higher in the scrotum in E₂B-treated males than in 15-day controls. Similar findings were found in a second group of 10 rats subjected to orchidectomy at birth to remove the androgen source. By 28 days all E₂B-treated animals were found to be cryptorchid, although the gubernaculum had completed its migration to mid-scrotal level. It is proposed that pubertal testicular descent in the rodent is inhibited by E₂B by mechanisms other than inhibition of gubernacular migration between birth and 10 days of age, which may be preprogrammed before birth.     

Calcitonin gene-related peptide injected ectopically alters gubernacular migration in the flutamide-treated rat with cryptorchidism

Calcitonin gene-related peptide (CGRP) is proposed to be a neurotransmitter that is released from the genitofemoral nerve (GFN) to organize the migration of the gubernaculum. The hypothesis predicts that exogenous CGRP might alter gubernacular migration. In a pilot study prenatal administration of an antiandrogen, flutamide, produced rats with undescended testes that were treated from birth with injections of CGRP into the future scrotum. The exogenous CGRP rapidly extravasated throughout the groin, retarding growth of the vaginal processus (VP). In flutamide-treated rats, the definitive study aimed to show that gubernacular migration could be redirected upward by injecting CGRP into a position cranial to the developing gubernaculum. Injection of exogenous CGRP into the suprapubic region significantly altered caudal growth of the VP, with 77.3% of testes undescended compared with 30% undescended in control rats (P < 0.01). It was concluded that exogenous CGRP injected cranial to the developing gubernaculum prevented its caudal growth in the animals treated with flutamide. CGRP injection directly into the scrotum of a larger animal than the neonatal rat could determine whether CGRP can stimulate gubernacular migration and cure cryptochidism.     

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.     

Does chemical sympathectomy alter the ontogeny of gubernacular migration in vivo?

Background

Testicular descent occurs in several stages, but the exact mechanism remains obscure. Sympathetic nerves have been proposed to have a role by a possible action on developing cremaster muscle, following observations of sympathetic dysfunction in cremaster from boys with cryptorchidism. This study aimed to see if chemical sympathectomy affected testicular descent in rats.

Methods

Sprague-Dawley dams were injected with 6-hydroxydopamine (days 15-19; 75 μg/kg) or control vehicle alone, and male pups examined at 0 to 10, 20, 30, and 60 days of age. The length of the processus vaginalis was measured and sections taken for histology.

Results

No difference in processus vaginalis growth was found between experimental and control groups, both macro- and microscopically. Chemical sympathectomy was confirmed by loss of adrenergic fibres in the adrenal cortex of experimental, but not control, animals.

Conclusions

These studies suggest that sympathetic innervation is not an important part of testicular descent in the rat but does not exclude a pathologic role in undescended testes or effects in humans only.     

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.     

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.