Embryology of the distal urethra and external genitals

Cloaca-Like Anomalies in the Male: A Report on Two Cases

“Cloaca” is a term used to describe an anomaly in the female where a single orifice is located in the perineum draining both urogenital and gastrointestinal tracts. Few reports used the same term “cloaca” to describe the counterpart anomaly in the male. We present two “male” cases of anorectal anomalies associated with significant penile deformity (caudally displaced penis) that were managed during the period between January 2010 and September 2021. Characteristically, both cases had a single “central” perineal orifice. The latter was located anterior to the predestined site of the normal anus and just beneath a caudally positioned hypospadiac phallus. The caudal displacement of the penis was strikingly obvious by the presence of severe form of penoscrotal transposition. Both cases were associated with a perineal swelling (hamartoma) just beside the central perineal orifice. The urethra was very short (like that in the female), besides the single perineal orifice, which makes the presentation very similar to cloacal anomalies.

Etiology of Hypospadias: A Comparative Review of Genetic Factors and Developmental Processes Between Human and Animal Models

Hypospadias is a congenital anomaly of the penis with an occurrence of approximately 1 in 200 boys, but the etiology of the majority of hypospadias has remained unknown. Numerous genes have been reported as having variants in hypospadias patients, and many studies on genetic deletion of key genes in mouse genital development have also been published. Until now, no comparative analysis in the genes related literature has been reported. The basic knowledge of penile development and hypospadias is mainly obtained from animal model studies. Understanding of the differences and similarities between human and animal models is crucial for studies of hypospadias. In this review, mutations and polymorphisms of hypospadias-related genes have been compared between humans and mice, and differential genotype–phenotype relationships of certain genes between humans and mice have been discussed using the data available in PubMed and MGI online databases, and our analysis only revealed mutations in seven out of 43 human hypospadias related genes which have been reported to show similar phenotypes in mutant mice. The differences and similarities in the processes of penile development and hypospadias malformation among human and commonly used animal models suggest that the guinea pig may be a good model to study the mechanism of human penile development and etiology of hypospadias.

Increased hand digit length ratio (2D:4D) is associated with increased severity of hypospadias in pre-pubertal boys

INTRODUCTION

Hypospadias is a common congenital male disorder, with much research focusing on prenatal androgen exposure as a causative factor. Whilst digit length ratios were apparent in sexual dimorphism since the nineteenth century, their role in hypospadias remains unknown. The objective of our study was to determine the correlation between digit length (2D:4D) ratio, hypospadias severity, and anogenital distance.

METHODS

Pre-pubertal boys (<3 years old) seen intra/postoperatively following hypospadias repair (June 2018-January 2019 inc.) were included. These were age-matched to non-hypospadias controls. Anthropomorphic measurements of digit lengths, penile/glans width, and anogenital distance were measured using digital calipers.

RESULTS

Data measurements were collected for 105 boys with hypospadias (60 distal; 45 proximal) and 55 controls. There were significant differences in 2D:4D ratios in each hand (p < 0.001), as well as individual digits (p < 0.001), and a reduced anogenital distance (p < 0.001), when comparing the proximal group with distal or control groups. There were no significant differences in glans width, or between term- and preterm births.

CONCLUSIONS

This study is the first to demonstrate increased 2D:4D ratios with proximal hypospadias, which also correlate with a shortened anogenital distance. This may provide a non-invasive, potentially antenatal, anthropomorphic measurement, as an indirect indicator of aberrant urogenital development.

[Di (2-ethylhexyl) phthalate-induced hypospadias in SD rats is related with Mafb expression: a transcriptome profiling-based study]

OBJECTIVE

To investigate the transcriptome profile of genital tubercles (GTs) in male SD rats and explore the mechanism of hypospadias induced by Di (2-ethylhexyl) phthalate (DEHP).

METHODS

Forty time-pregnant SD rats were randomly divided into 4 equal groups, namely GD16 group and GD19 group (in which the male GTs were collected on gestation day[GD]16 and GD19 for RNA-seq, respectively), control group and DEHP exposure group (with administration of oil and 750 mg/kg DEHP by gavage from GD12 to GD19, respectively).In the control and DEHP exposure groups, the GTs were collected from the male fetuses on GD19.5, and scanning electron microscopy and HE staining were used to observe the morphological changes.The differentially expressed genes (DEGs) in the GTs were screened using lllumina HiSeq 2000 followed by GO and KEGG enrichment analyses to characterize the transcriptome profile.Immunofluorescence assay was performed to verify the DEGs (Mafb) identified by RNA-seq results.Immunofluorescence assay and Western blotting were used to examine the expression levels of Mafb in the penile tissue.

RESULTS

A total of 1360 DEGs were detected in the GTs between GD16 group and GD19 group by RNA-seq.Among these genes, 797 were up-regulated and 563 were down-regulated.These DEGs were mainly enriched in the cell adhesion plaque signaling pathway, axon guidance signaling pathway, and extracellular matrix receptor signaling pathway.Compared with that in GD16 group, Mafb was significantly up-regulated in GD19 group, which was consistent with the sequencing results.Mafb and β-catenin were significantly down-regulated in DEHP-exposed group compared with the control group (P &lt; 0.01).

CONCLUSIONS

Mafb expression increases progressively with the development of GTs in male SD rats.DEHP exposure causes significant down-regulation of Mafb and β-catenin, suggesting that β-catenin signaling pathway that affects Mafb is related to DEHP-induced hypospadias in SD rats.

The development of the cloaca in the human embryo

Subdivision of cloaca into urogenital and anorectal passages has remained controversial because of disagreements about the identity and role of the septum developing between both passages. This study aimed to clarify the development of the cloaca using a quantitative 3D morphological approach in human embryos of 4–10 post‐fertilisation weeks. Embryos were visualised with Amira 3D‐reconstruction and Cinema 4D‐remodelling software. Distances between landmarks were computed with Amira3D software. Our main finding was a pronounced difference in growth between rapidly expanding central and ventral parts, and slowly or non‐growing cranial and dorsal parts. The entrance of the Wolffian duct into the cloaca proved a stable landmark that remained linked to the position of vertebra S3. Suppressed growth in the cranial cloaca resulted in an apparent craniodorsal migration of the entrance of the Wolffian duct, while suppressed growth in the dorsal cloaca changed the entrance of the hindgut from cranial to dorsal on the cloaca. Transformation of this ‘end‐to‐end’ into an ‘end‐to‐side’ junction produced temporary ‘lateral (Rathke's) folds’. The persistent difference in dorsoventral growth straightened the embryonic caudal body axis and concomitantly extended the frontally oriented ‘urorectal (Tourneux's) septum’ caudally between the ventral urogenital and dorsal anorectal parts of the cloaca. The dorsoventral growth difference also divided the cloacal membrane into a well‐developed ventral urethral plate and a thin dorsal cloacal membrane proper, which ruptured at 6.5 weeks. The expansion of the pericloacal mesenchyme followed the dorsoventral growth difference and produced the genital tubercle. Dysregulation of dorsal cloacal development is probably an important cause of anorectal malformations: too little regressive development may result in anorectal agenesis, and too much regression in stenosis or atresia of the remaining part of the dorsal cloaca.

Contrasting mechanisms of penile urethral formation in mouse and human

This paper addresses the developmental mechanisms of formation of the mouse and human penile urethra and the possibility that two disparate mechanisms are at play. It has been suggested that the entire penile urethra of the mouse forms via direct canalization of the endodermal urethral plate. While this mechanism surely accounts for development of the proximal portion of the mouse penile urethra, we suggest that the distal portion of the mouse penile urethra forms via a series of epithelial fusion events. Through review of the recent literature in combination with new data, it is unlikely that the entire mouse urethra is formed from the endodermal urethral plate due in part to the fact that from E14 onward the urethral plate is not present in the distal aspect of the genital tubercle. Formation of the distal portion of the mouse urethra receives substantial contribution from the preputial swellings that form the preputial-urethral groove and subsequently the preputial-urethral canal, the later of which is subdivided by a fusion event to form the distal portion of the mouse penile urethra. Examination of human penile development also reveals comparable dual morphogenetic mechanisms. However, in the case of human, direct canalization of the urethral plate occurs in the glans, while fusion events are involved in formation of the urethra within the penile shaft, a pattern exactly opposite to that of the mouse. The highest incidence of hypospadias in humans occurs at the junction of these two different developmental mechanisms. The relevance of the mouse as a model of human hypospadias is discussed.

X-ray micro-computed-tomography in pediatric surgery: a new tool for studying embryos

PURPOSE

The embryology of common congenital malformations is discussed controversially. Studies are hampered by a shortage of study material and techniques which require partial or complete preparation and therewith destruction of embryos. X-ray micro-computed-tomography (µCT) is a technical opportunity keeping the embryos intact. Thus, the aim of this study was to assess the applicability of µCT in embryonic research compared to the anatomical information obtained by scanning electron microscopy (SEM).

METHODS

Chicken, rat, mouse and sheep embryos, processed either for SEM studies or as whole embryos, were imaged in three-dimensional (3D) using µCT. The obtained two-dimensional (2D) digital datasets were volume rendered by tomographic reconstruction software and studied using analysis software.

RESULTS

All embryos were µCT scanned without technical problems. The quality of the µCT images (image contrast, anatomical details) was excellent, but varied depending on age and species studied. µCT imaging allowed a more comprehensive anatomical/morphological analysis but showed less surface details compared to SEM.

CONCLUSION

µCT is a technique suitable and innovative for pediatric surgical research, which allows detailed evaluation of entire embryos without time- and specimen-consuming micro-dissection. Samples prepared for SEM can be used for µCT and vice versa.

Epithelial-mesenchymal transformation and apoptosis in rat urethra development

BackgroundTo examine the mechanism of urethral seam formation during embryonal development of rat urethra.MethodsTime-mated Sprague-Dawley rats were killed and the genital tubercles of male pups harvested on embryonic day (ED) 15, 16, 18, and 19. External morphology was observed under scanning electron microscope. Serial transverse sections were prepared to examine dynamic changes in the urethral seam morphology with hematoxylin-eosin staining, immunohistochemistry, transmission electron microscopy, and double immunofluorescence.ResultsBilateral outgrowth of urethral swelling followed by urethral plate fusion in the midline to form urethral seam was observed from ED 16 onwards. Coexpression of epithelial and mesenchymal markers was observed in several cells at the urethral seam; a few cells with coexpression of epithelial and apoptotic markers were also observed. Mesenchymal transformation of epithelial cells and apoptotic epithelial cells was observed under transmission electron microscope.ConclusionUrethral formation occurs by tubulogenesis, which initiates proximally and progresses distally. This is the first study to demonstrate epithelial-mesenchymal transformation and epithelial cell apoptosis in the urethral seam cells of fetal rats. These findings provide new insights into the mechanisms involved in embryonal development of the urethra.

Clarification of mammalian cloacal morphogenesis using high-resolution episcopic microscopy

The developmental process through which the cloaca transforms from one hollow structure to two separated urinary and digestive outlets remains controversial and speculative. Here, we use high-resolution episcopic microscopy to examine a comprehensive series of normal and mutant mouse cloaca in which the detailed 3-dimensional (3-D) morphological features are illuminated throughout the development. We provide evidence that the dorsal peri-cloacal mesenchyme (dPCM) remains stationary while other surrounding tissues grow towards it. This causes dramatic changes of spatial relationship among caudal structures and morphological transformation of the cloaca. The 3-D characterizations of Dkk1 mutants reveal a hyperplastic defect of dPCM, which leads to a significant anterior shift of the caudal boundary of the cloaca, premature occlusion of the cloaca and, imperforate anus phenotype. Conversely, Shh knockout causes a severe hypoplastic defect of cloaca mesenchyme including dPCM and persistent cloaca. Collectively, these findings suggest that formation of the dPCM is critical for cloacal morphogenesis and furthermore, growth and movement of the mesenchymal tissues towards the dPCM lead to the cloaca occlusion and separation of the urinary and digestive outlets.

Development of the human male urethra: a histochemical study on human embryos

PURPOSE

Controversy persists regarding the formation of human penile urethra. The classic fusion theory for the development of the spongy urethra and ectodermal ingrowth or endodermal transformation theories for the development of the glanular urethra do not explain the wide spectrum of anomalies seen in patients with hypospadias. This histological study was made to clarify the mechanism of urethral development.

MATERIALS & METHODS

15 human male embryos ranging from 6 to 14 weeks were studied. The phalluses were examined microscopically and photographed. Tissues were prepared as serial histological sections and stained with haematoxylin and eosin and with special immuno-histochemical stains.

RESULTS

1) The penile urethra: At 6 weeks of gestation, the urethral plate which is solid distally and partially grooved proximally becomes grooved distally and has fused proximally by 8 weeks. At 14 weeks of gestation; the urethral opening migrates only to the middle of the shaft. 2) The glanular urethra: At the 6th week of gestation, a solid epithelial plate reached the tip of the genital tubercle, and a glans cannot be identified. At the 7th week, a central vacuolation appears and the penile urethral groove does not reach the tip of the phallus. At the 8th week; coronal sulcus starts to appear, and a well defined blind central canal was evident in the 13th week. During the 14th week, the floor of the glanular canal degenerated to form a glanular groove.

CONCLUSIONS

Our observations suggest that the spongy urethra passes through 3 stages of development: a solid epithelial plate, deep urethral groove, and fused urethra. The glanular urethra passes through 4 developmental stages: a solid epithelial plate, a blind central canal, a deep glanular groove, and the floor from the preputial lamella. There was no evidence of ectodermal ingrowth. These observations raise serious questions to the current theories for human urethra development. Further studies on fresh human embryos are needed.

A tortuous proximal urethra in urorectal septum malformation sequence?

We observed a newborn boy with urorectal septum malformation sequence. Anomalies of the genitalia and rectum were present. He expired on the first day of life, due to severe lung hypoplasia. Autopsy showed a colon that ended in a blind sac, an enlarged bladder with no grossly visible urethra, and dysplastic kidneys. A cone-shaped tissue at the usual site of the bladder outlet contained tortuous and slit-like lumina, suggesting an undeveloped proximal urethra. The urethral structure was lined by transitional epithelium with squamous metaplasia. Many small buds-lined with columnar epithelium-branched from the urethral structure. These ductal buds lined with columnar epithelium stained for prostatic acid phosphatase. Basal cells surrounding the ductal buds stained for p63 and high molecular weight cytokeratin-supporting an interpretation that the buds were early prostatic ducts with normal histology. To our knowledge, these are the first histological images of an undeveloped, obstructed urethra associated with the urorectal septum malformation sequence.