Normal and abnormal embryonic development of the anorectum in human embryos

Developmental processes and ectodermal contribution to the anal canal in mice

The anorectal canal has two origins; the upper part is derived from endoderm and the lower part is derived from ectoderm. The process of ectodermal contribution to the canal remains unclear. To understand the development of this area, serial sagittal sections of mouse embryos were made every 12h from embryonic day 13.0 (E13.0) to E18.5. Three-dimensional (3-D) reconstructions were obtained from these sections. At the time of the disappearance of the cloacal membrane (E13.5), the endodermal lining reached the site of disintegrated membrane. Thus, the whole canal was of endodermal origin. The transitional zone between the dorsal end of the primary perineum and tail was thicker than other ectodermal epithelia. In this region, it changed from an acute to obtuse angle. After it straightened out and formed the canal, the secondary perineum appeared caudally. During these processes, the external sphincter appeared in the underlying mesenchyme of the thick ectoderm and functioned as a drawstring to form the ectodermal anal canal.

Urorectal septum malformation sequence: prenatal progression, clinical report, and embryology review

The urorectal septum malformation sequence (URSMS) is characterized by severe abnormalities of the urorectal septum (URS) and urogenital organs. The primary defect in this condition appears to be a deficiency in caudal mesoderm leading to the malformation of the URS and other structures in the pelvic region. Recent clinical reports discuss prental findings of URSMS [Lubusky et al. (2006); Prenatal Diagnosis 26: 345-349]. However, here we present a case of URSMS with prenatal findings not previously described, review the literature on URSMS, and summarize current embryological understanding of the pathology seen in hindgut development. The unique prenatal finding in the patient was an abdominally located cystic mass that was first seen at 18 weeks of gestation. Over the next 6 weeks, the mass decreased in size until it disappeared. Concurrent with reduction of the cyst, ascites developed. The patient displayed several traditional URSMS indicators including abnormal bladder and dysplastic kidneys. Our findings give additional insight into the embryology of urorectogenital development. Specifically, they suggest that the cystic mass may have been a persistent urachus prior to septation of the cloaca. Postnatal evaluation confirmed a URSMS diagnosis; the newborn had ambiguous genitalia, hypoplastic kidneys, absent uterus, imperforate anus, smooth perineum, and overall underdeveloped urogenital structures.

Epithelial and muscular regionalization of the human developing anorectum

In the past, interpretations of anorectal development were mainly based on analysis of serially sectioned embryos of various nonhuman species as well as some human specimens. A four-dimensional view of the developmental situation in the human has never been established nor connected to recent findings obtained from newer molecular techniques. We, therefore, investigated human embryonic and fetal pelves by means of immunohistochemistry and in situ hybridization to elucidate differentiation and interaction of epithelial and mesenchymal layers of the anorectum. To emphasize spatial as well as sequential morphological development, we produced three-dimensional reconstructions of the specimens at hand. Research conducted proved that the decisive steps of epithelial and muscular differentiation occur between the 7th and 9th week after conception. This study elucidates a biphasic epithelial "closure" in the anal canal and interactions between epithelium, smooth musculature, and skeletal musculature. Based on the results presented here, it is possible to describe the pathogenesis of two anorectal malformations: the imperforate anal membrane and the anal membrane stenosis. This study will now provide the basis for further research into developmental processes occurring before the ones examined.

Mutational analyses of UPIIIA, SHH, EFNB2 and HNF1beta in persistent cloaca and associated kidney malformations

OBJECTIVES

'Persistent cloaca' is a severe malformation affecting females in which the urinary, genital and alimentary tracts share a single conduit. Previously, a Uroplakin IIIA (UPIIIA) mutation was reported in one individual with persistent cloaca, and UPIIIA, Sonic Hedgehog (SHH), Ephrin B2 (EFNB2) and Hepatocyte Nuclear Factor 1beta (HNF1beta) are expressed during the normal development of organs that are affected in this condition. HNF1beta mutations have been associated with uterine malformations in humans, and mutations of genes homologous to human SHH or EFNB2 cause persistent cloaca in mice.

PATIENTS AND METHODS

We sought mutations of coding regions of UPIIIA, SHH, EFNB2 and HNF1beta genes by direct sequencing in a group of 20 patients with persistent cloaca. Most had associated malformations of the upper renal tract and over half had impaired renal excretory function. The majority of patients had congenital anomalies outside the renal/genital tracts and two had the VACTERL association.

RESULTS

Apart from a previously described index case, we failed to find UPIIIA mutations, and no patient had a SHH, EFNB2 or HNF1beta mutation.

CONCLUSION

Persistent cloaca is only rarely associated with UPIIIA mutation. Despite the fact that SHH and EFNB2 are appealing candidate genes, based on their expression patterns and mutant mice phenotypes, they were not mutated in these humans with persistent cloaca. Although HNF1beta mutations can perturb paramesonephric duct fusion in humans, HNF1beta was not mutated in persistent cloaca.

Patterns of severe abdominal wall defects: Insights into pathogenesis, delineation, and nomenclature

BACKGROUND

During the last decade, descriptions of malformation complexes involving an abdominal wall defect (AWD) have repeatedly appeared in the literature, and there has been frequent confusion regarding nomenclature, definitions, and delineations. The aims of this work were to evaluate possible embryological relationships among AWD cases, review the related nomenclature, identify patterns involving AWDs, and stress the importance of complete clinical descriptions. METHODS Cases diagnosed as AWD complexes were selected from live--and stillborn infants of the Hospital Materno Infantil Ramón Sardá, Buenos Aires, and from the Laboratory of Perinatal Pathology, Buenos Aires, Argentina. They were sorted by the location of the AWD, the umbilical cord length, and the presence or absence of a persistent cloaca. The findings in 26 cases were described, according to proposed definitions.

RESULTS

Three patterns could be identified: 1) the AWD involving the umbilical ring, a persistent or exstrophic cloaca, and a spinal cord anomaly; 2) the AWD extending laterally to the umbilical ring, severe unilateral limb defects, and same-sided agenesis of abdominal organs; and 3) the AWD not involving the umbilical ring, clefts, exencephaly, and amputations. Furthermore, overlapping among these patterns was observed, and possible involved mechanisms are discussed.

CONCLUSIONS

The observed overlapping among patterns suggested that malformation complexes involving AWDs might not be independent conditions but rather belong to a common and broader spectrum of anomalies. Complete clinical descriptions, the avoidance of synonyms and generalizations, and strictly defined inclusion criteria are proposed for a better understanding of pathogenetic pathways in, and relationships among, AWD complexes.

Anal canal duplication: a retrospective analysis of 12 cases from two European pediatric surgical departments

Anal canal duplication (ACD) represents an extremely rare intestinal congenital anomaly of unknown origin. Usually evidenced within 2 years of age, nearly 45% of reported cases present associated malformations such as presacral mass, anorectal malformation (ARM) and genitourinary anomalies. The confirmative diagnosis is histopathological, with evidence of an anal mucosal lining (squamous +/- transitional epithelium), surrounded from a smooth muscle coat and anal glands. We review a conjoined experience from two European pediatric surgical departments. From 1970 to 2005, 12 patients were observed, seven in Pescara, Italy (1997-2005), five in Barcelona, Spain (1970-2004) - mean age at diagnosis 17.8 months, range 0-60; M:F = 1:11. Clinical presentation, diagnostic-surgical approach, and complications were reviewed. According to clinical presentation, patients could be divided in three age groups: asymptomatic (mean age 4.8 months, six patients - one with an associated complex genitourinary malformation, one with a presacral mature teratoma, one with ACD evidenced hysthologically on a retroanal mass removed during the correction of an ARM), mildly symptomatic - constipation, mucous discharge (mean age 29.2 months, four patients - one with associated presacral ependymoma and intestinal neuronal dysplasia type B, one with presacral mass) and complicated - perineal abscess, recurrent fistula (mean age 34 months, two patients). In 11 cases a perianal orifice was evident (ten posteriorly located). The pelvic-MRI was the preferred diagnostic tool in Pescara (5/7, with presacral mass in two patients), fistulography in Barcelona (5/5), where one presacral mass was discovered intraoperatively. Eleven patients underwent surgical removal of the ACD (five perineal approach, five posterior sagittal approach, and one PSARP). Histopathological findings confirmed the diagnosis in operated cases (11). The parents of the male patient denied the consent to surgical treatment. The only major post-operative complication was a sphincteric insufficiency (one case), surgically treated. When facing a perianal orifice, attention should be paid to ACD, particularly in female patients with coexistent genitourinary or intestinal malformations. Pelvic US and MRI are the gold standard to evidence the not rarely associated presacral mass. Surgical early removal (mucosectomy or perineal/posterior sagittal approach, depending on length of ACD and associated presacral mass) is warranted, also in asymptomatic patients, because of the risk of inflammatory complications and cancer (the latter reported in literature in adults).

Molecular genetic cascades for external genitalia formation: an emerging organogenesis program

External genitalia are anatomical structures located at the posterior embryonic region as part of several urogenital/reproductive organs. The embryonic anlage of the external genitalia, the genital tubercle (GT) develops as a bud-shaped structure with an initial urethral plate and later urethra. Embryonic external genitalia are considered to be one of the appendages. Recent experiments suggest that essential regulatory genes possess similar functions for the outgrowth regulation of the GT and limb appendages. The transient embryonic epithelia located in the distal GT are called the distal urethral epithelium (DUE) regulating, at least in part, the (distal) GT development. This review covers the available data about early patterning of GT and discusses the molecular developmental similarities and points of divergence between the different appendages. Development of the male and female external genitalia is also reviewed.

Tissue fusion and cell sorting in embryonic development and disease: biomedical implications

Throughout embryonic development, segregated epithelial and/or mesenchymal cell populations make contact and fuse to shape new tissue units. This process, known as tissue fusion, is a key event in many essential morphogenetic mechanisms and its disruption can lead to congenital malformations. Another mechanism whereby complex tissues can arise involves a cell sorting process in which originally intermixed cells de-mix to generate distinct phases or layers. Different organisms use a combination of tissue fusion and cell sorting to acquire shape. Although the two processes appear to differ mechanistically, they are intricately linked inasmuch as they both involve the same molecular determinants and contribute to the same body plan. We aim to discuss the role of adhesion molecules and cell dynamics in tissue fusion and cell sorting, providing examples of their impact in embryonic development. Finally, we will advance the concept that malignant invasion may be viewed as cell sorting in reverse. Supplementary material for this article can be found on the BioEssays website (http://www.interscience.wiley.com/jpages/0265-9247/suppmat/index.html).

Cellular proliferation in the urorectal septation complex of the human embryo at Carnegie stages 13-18: a nuclear area-based morphometric analysis

In order to analyse the patterns of cellular proliferation both in the mesenchyme of the urorectal septum (URS) and in the adjacent territories (posterior urogenital mesenchyme, anterior intestinal mesenchyme and cloacal folds mesenchyme), as well as their contribution to the process of cloacal division, a computer-assisted method was used to obtain the nuclear area of 3874 mesenchymal cells from camera lucida drawings of nuclear contours of selected sections of human embryos [Carnegie stages (CSs) 13-18]. Based on changes in the size of the nucleus during the cellular cycle, we considered proliferating cells in each territory to be those with a nuclear area over the 75th percentile. The URS showed increasing cell proliferation, with proliferation patterns that coincided closely with cloacal folds mesenchyme, and with less overall proliferation than urogenital and intestinal mesenchymes. Furthermore, at CS 18, we observed the beginning of the rupture in the cloacal membrane; however, no fusion has been demonstrated either between the URS and the cloacal membrane or between the cloacal folds. The results suggest that cloacal division depends on a morphogenetic complex where the URS adjacent territories could determine septal displacement at the time that their mesenchymes could be partially incorporated within the proliferating URS.

Association of cloacal anomalies, caudal duplication, and twinning

Cloacal anomalies exhibit a wide variety of morphologic types and accompanying clinical severity. The association of malformations of the cloaca with partial, complete, or conjoined twinning has been appreciated for some time, but, with the advent of prenatal ultrasound technology, appears to occur with a greater frequency than once thought. This observation has important implications for pathogenesis. We present 2 representative cases, a 19-week-old female fetus with duplication of several caudal structures and a 21-week-old male fetus with cloacal exstrophy variant and demised co-twin with lower abdominal wall defect, extruded intestinal tract, absent external genitalia, and imperforate anus. These findings and previously published theories suggest that certain models of monozygotic twinning may apply to the pathogenesis of cloacal anomalies. Specifically, the partial or complete duplication of the organizing center within a single embryonic disc may increase the risk of mesodermal insufficiency and thus account for the failure of complete development of the cloacal membrane and consequent exstrophy or other aberration.

A proposed mechanism for intermediate atresia ani (AA), based on a porcine case of AA and hypospadias

BACKGROUND

Atresia Ani (AA) is one of the most common malformations of the lower gastrointestinal tract. It occurs in 1 in 1500 to 1 in 5000 human births. These patients suffer physically and psychologically from this disorder, and thus there is a research interest in the condition. There are 3 subcategories of AA: high, intermediate, and low. Each of these forms is likely related to separate etiological processes.

METHODS

An anatomical study was performed on a porcine case of AA with a urorectal fistula and malformed urethra.

RESULTS

This animal was found to have the intermediate form of AA.

CONCLUSIONS

A new mechanism is hypothesized, distinct from that described for the high and low forms of AA. This proposed mechanism involves the persistence of the cloacal membrane. Evidence to support this hypothesis includes: location of the urorectal fistula in the region of the embryonic cloacal duct, the lack of anomalies usually seen in conjunction with AA associated with mutations of the Sonic Hedgehog gene, and the presence of a malformed urethra, which is significantly correlated to intermediate AA. This form of AA could be related to a failure of the cloacal membrane to break down at the appropriate time during urorectal separation.

Spatiotemporal distribution of apoptosis during normal cloacal development in mice

To understand normal cloacal developmental processes, serial sagittal sections of mouse embryos were made every 6 hrs from embryonic day 11.5 (E11.5) to E13.5. During cloacal development to form the urogenital sinus and anorectal canal, fusion of the urorectal septum with the cloacal membrane was not observed, and the ventral and dorsal parts of the cloaca were continuously connected by the canal until disappearance of the cloacal membrane to open the vestibule formed by the urogenital sinus and anorectal canal to the outside at E13.5. Ventral shifting of the dorsal part of the cloaca was observed until E12.5. The dorsal part was transformed in accordance with ventral shifting. In addition, apoptosis was seen to occur around the dorsal part. However, from E12.25, apoptotic cells are observed in a linear arrangement in the urorectal septum just ventral to the peritoneal cavity. Interestingly, extension of this line reaches the area of the cloacal membrane disintegrated by apoptosis. The present findings suggest that in the early stages (until E12.0), distribution of apoptosis in mesenchyme around the dorsal part of the cloaca might be strongly related to the transformation and ventral shifting of this part. Conversely, the apoptosis pattern in urorectal septum mesenchyme in later stages (from E12.0) might be involved in transformation of the urorectal septum and disintegration of the cloacal membrane.

Clinical and molecular characterization of the bladder exstrophy-epispadias complex: analysis of 232 families

OBJECTIVE

To identify genetic and nongenetic factors contributing to the risk of bladder exstrophy-epispadias complex (BEEC).

PATIENTS AND METHODS

In all, 285 families with BEEC were invited to participate in the study, and 232 of them were recruited. Epidemiological information was obtained from 151 of the consenting families, with a detailed clinical genetic examination of 94 probands. In all, 440 DNA samples were collected from 163 families for molecular analysis.

RESULTS

Most of the cases were sporadic and had no family history of BEEC. Among patients, 95% were Caucasian, and males were more common in both the epispadias group (M/F, 2.2, 29 patients) and the classic bladder-exstrophy group (M/F 1.8, 164), but in the cloacal exstrophy group the sex ratio was close to unity (1.1, 15). There was a statistically significant association with advanced parental age (P < 0.001). Birth weight, gestational age and maternal reproductive history did not appear to be significantly different from those in the general population. Information on exposures to tobacco, alcohol and drugs was collected but none appeared to act as a risk factor. Karyotype analysis on 37 cases detected two chromosomal abnormalities, i.e. 46XY t(8;9)(p11.2; q13) and 47XYY. Molecular analysis of the HLXB9 gene, which causes Currarino syndrome, did not detect mutations in the blood or bladder DNA of 10 patients with bladder or cloacal exstrophy.

CONCLUSIONS

BEEC most commonly occurs as an isolated sporadic birth defect with a recurrence risk of << 1%. There was no evidence of a single-gene effect or common environmental factor in this study population. In addition to race and advanced parental age, birth order may be a risk factor for BEEC. We suggest somatic mutations in a gene(s) within the pathway regulating bladder development may be the cause of BEEC.

Bidirectional signaling mediated by ephrin-B2 and EphB2 controls urorectal development

Incomplete urethral tubularization (hypospadias) and anorectal abnormalities are two common and poorly understood birth defects that affect the extreme caudal midline of the human embryo. We now show that cell surface molecules essential for proper axon pathfinding in the developing nervous system, namely ephrin-B2 and the ephrin receptors EphB2 and EphB3, also play major roles in cell adhesion events that tubularize the urethra and partition the urinary and alimentary tracts. Mice carrying mutations which disrupt the bidirectional signals that these molecules transduce develop with variably penetrant severe hypospadias and incomplete midline fusion of the primitive cloaca. We further show that animals completely lacking ephrin-B2 reverse signaling present a fully penetrant failure in cloacal septation. This results in severe anorectal malformations characterized by an absence of the terminal-most hindgut (rectum) and formation of a fistula that aberrantly connects the intestines to the urethra at the base of the bladder. Consistent with an apparent requisite for both forward and reverse signaling in these caudal remodeling events, EphB2 and ephrin-B2 are coexpressed at the midline in the fusing urethral/cloacal endoderm and underlying lateral mesoderm of the urorectal septum that migrates toward the caudal midline as the cloaca septates. Our data thus indicate that B-subclass Eph and ephrin molecules play an important role in these clinically significant midline cell-cell adhesion and fusion events.

Developmental stages in the rabbit embryo: guidelines to choose an appropriate experimental model

Researchers involved in the field of congenital malformations are often forced to work on an animal model. Both accurate description of its normal development and comparative staging with human development will be mandatory. To complete the lacking medical literature, we herein provide such data for the rabbit model. Sampled rabbit embryos were staged using the Carnegie criteria, in order first to determine if they were consistent with the rabbit developmental pattern, and second to compare this pattern with the human one. Our results show a suitable comparison of rabbits and humans in early developmental stages, except for the neural growth.

Normal and abnormal embryonic development of the anorectum in rats

BACKGROUND

The embryologic and pathologic aspect of anorectal malformation (ARM) remains poorly understood. There is no universally accepted theory to explain anorectal embryology and the abnormal development that produces ARM. The aim of this study was to observe the developmental processes of anorectum in rats and to explore the abnormal embryonic development that leads to ARM.

METHODS

Rat embryos with ARM were obtained by treating pregnant rats with administration of ethylenethiourea (ETU). Normal rat embryos and embryos with ARM from gestational days 12.5 to 20 were sectioned serially and sagittally and stained with H & E. The relevant structure including cloaca and urorectal septum (URS) were examined in a temporospatial sequence.

RESULTS

Characteristics of anorectum development in ARM rat embryos treated by ETU were as follows: (1) URS never fused with cloaca membrane. (2) Dorsal cloacal membrane was maldeveloped. (3) Cloacal configuration was abnormal. (4) Tail groove never appeared. All type of ARM was the rectourethal fistula and common cloaca in ETU-treated rat embryos and was discernible on gestation day 16.

CONCLUSIONS

Absence of the tail groove and maldevelopment of the dorsal cloacal membrane, cloacal configuration, and urorectal septum are likely to be responsible for the formation of ARM. Failure of fusion of the URS with cloacal membrane is the immediate reason for rectourethral fistula or common cloaca in ETU-treated rats.

The absence of lateral fusion in cloacal partition

BACKGROUND/PURPOSE

The mechanism by which the cloaca becomes partitioned into a dorsal rectal part and a ventral genitourinary sinus has been the subject of speculation for more than a century. Despite repeated suggestions that partitioning of the cloaca by fusion of lateral folds does not occur, the concept continues to hold sway in many student and surgical texts. The authors reviewed the histologic and 3-dimensional appearance of the urorectal septum in human and rat embryos to see if there was any evidence of lateral fusion in its formation.

METHODS

Sprague-Dawley rat embryos (n = 143) were examined between 11 and 21 days' gestation and compared with human embryo sections (57 embryos) held in historical collections in Europe. Rat embryos were examined by microscopy, dissection, and serial histologic section. In addition, some specimens were sectioned in wax until the lumen of the cloaca was reached, after which they were dewaxed and the internal cavities imaged with scanning electronmicroscopy (n = 18 of 143).

RESULTS

Cloacal "partitioning" resulted from a combination of growth of the mesenchyme of the hindgut and genitourinary sinus, an alteration in the position of the cloaca in relation to surrounding structures secondary to growth in the ventral, infraumbilical abdominal wall and changes in the curvature of the developing spine, and apoptosis in the dorsal wall of the cloaca with shortening of the dorsal cloacal wall. There was no septum, as it is usually defined, between the developing bladder and hindgut. There was no evidence on either histologic section or scanning electronmicroscopy of any process of fusion occurring between the 2 lateral folds within the lumen of the cloaca.

CONCLUSIONS

Lateral fusion of the side walls of the cloaca does not play a role in cloacal "partition." Development of the bladder and hindgut occurs by a process that involves growth, differentiation, and remodeling.

Anal canal duplication in infants

BACKGROUND/PURPOSE

Anal canal duplication (ACD) is the most distal and the least frequent digestive duplication. A review of the English-language literature found 15 cases reported in the pediatric age group.

METHODS

A retrospective chart review was performed for our experience from 1999 to 2001 with 6 patients who presented with a midline postanal opening.

RESULTS

All of 6 patients were girls and included one set of twins. Patients were from 3 to 9 months of age at the time of diagnosis (mean, 4.5 months). The anal canal duplication was delineated clearly by contrast study of the tract. All were tubular structures, 10 to 12 mm in length. None of the patients had any other associated anomalies. Five of 6 patients underwent operation between the ages of 3 and 8 months (mean, 5.4 months). Excision of the ACD was accomplished through the posterior sagittal approach. The orifice of the ACD, measuring 1 to 2 mm in diameter located just behind anus, directed to the lumen of the anal canal by keeping in the midline and ended blindly 5 mm above the dentate line without luminal communication. Histology findings showed a squamous epithelium with smooth muscle bundles in 2 cases and pseudostratified columnar epithelium with focally squamous epithelial lining and adjacent smooth muscle bundles in 3 cases. The postoperative courses were uneventful with satisfactory anal function. One patient has not yet undergone operation and has been well on outpatient follow-up.

CONCLUSIONS

ACD is a congenital developmental lesion located in the midline posterior to the anus presenting as a tubular structure without communication with the anal canal, usually discovered in early infancy, and characteristically predominant in girls. The authors recommend that all ACDs, regardness of size and length, should be removed surgically to restore the normal anatomy and to avoid delayed presentation of infection such as perianal abscess or fistula formation.

Septation of the anorectal and genitourinary tracts in the human embryo: crucial role of the catenoidal shape of the urorectal sulcus

BACKGROUND

Previous studies of the tracheoesophageal sulcus and the sulci of the developing heart have suggested that the catenoidal or saddle-shaped configuration of the sulcus had mechanical properties that were important to developmental processes by causing regional growth limitation. We examined the development of the human perineal region to determine if a similar configuration exists in relation to the urorectal septum. We wished to re-examine the controversial issue of the role of the urorectal sulcus in the partitioning of the cloaca.

METHODS

Digitally scanned photomicrographs of serial histologic sections of embryos from Carnegie stages 13, 15, 18, and 22, obtained from the Carnegie Embryological Collection were used. Each image was digitally stacked, aligned, and isolated using image-editing software. Images were compiled using 3-D image-visualization software (T-Vox), into full 3-D voxel-based volume renderings. Similarly, digital models were made of the urogenital sinus, anorectum, cloaca, allantois, mesonephric ducts, ureters, and kidneys by isolating their associated epithelium in each histologic section and compiling the data in T-Vox. Methods were developed to create registration models for determining the exact position and orientation of outlined structures within the embryos.

RESULTS

Models were oriented and resectioned to determine the configuration of the urorectal sulcus. The results show that the urorectal sulcus maintains a catenoidal configuration during the developmental period studied and, thus, would be expected to limit caudal growth of the urorectal septum.

CONCLUSION

The observations support the concept that the urorectal septum is a passive structure that does not actively divide the cloaca into urogenital and anorectal components.

Clarification of the processes that lead to anorectal malformations in the ETU-induced rat model of imperforate anus

BACKGROUND

Ethylenethiourea (ETU) administered to timed-pregnant rats can induce anorectal malformations (ARMs) in about 80% of rat fetuses, thus providing an ideal animal model to study the embryogenesis of ARMs. The current study was undertaken to investigate the embryogenetic events that may be responsible for the development of ARMs in rats.

METHODS

Time-mated pregnant rats were divided randomly into control and experimental groups. The experimental rats received 1% ETU (125 mg/kg) by gavage on gestational day 10, and control rats received only the vehicle. Their embryos were harvested by cesarean section on gestational days 13, 14, 15, and 16. They were fixed and embedded in paraffin and serially sectioned in either the sagittal or transverse plane. The sections were stained with H&E, examined, and photographed. The comparative morphogenesis of the hindgut, cloaca, and tailgut of age-matched embryos was studied.

RESULTS

The key abnormalities in the experimental embryos were: (1) maldevelopment of cloaca and urorectal septum with no sign of the fusion between the urorectal septum and the cloacal membrane, (2) delay of tailgut regression, (3) abnormal and massive apoptotic cell death involving the posterior cloacal wall, and (4) underdevelopment of the dorsal aspect of the cloaca and its membrane. The type of ARM that was developing was discernible by gestational day 15 and 16.

CONCLUSIONS

ARMs induced by ETU in rat embryos seem to be caused by the cumulative effect of aberrations in the development of several components of the hindgut and cloaca. Variation in the extent of maldevelopment of these structures may result in a spectrum of ARMs.

Magnetic resonance imaging in children with anorectal malformations: embryologic implications

PURPOSE

The aim of this study was to evaluate the spectrum of developmental anomalies in the caudal region of patients with anorectal malformations (ARM) and relate them to the new insights in the embryogenesis of this region.

METHODS

Forty-nine patients with ARM were investigated with magnetic resonance imaging (MRI); 24 preoperatively (preop) and 25 postoperatively (postop). Of this group, 17 patients had a high (or intermediate) ARM, 28 a low ARM, and 4 a cloacal malformation (CM). The evaluation of the imaging studies included the level and type of ARM (preop); the developmental state of the sphincter muscle complex (SMC); and the associated anomalies of spinal cord, spine, and urogenital system. The anomalies detected were ordered and related to recent embryologic observations in the caudal region.

RESULTS

With MRI, the level of ARM was depicted effectively in 23 of 24 patients (96%) investigated preop, including 5 of 9 clinically proven fistulae in those with high ARM and CM (56%). Maldevelopment of the SMC was observed in 14 of 49 patients (29%). Associated anomalies of spinal cord and spine were found in 51% of patients, more frequently in those with high ARM, low ARM with fistulae, and cloacal malformations. Associated anomalies of the urogenital system were found in 37% of patients.

CONCLUSIONS

As illustrated by the current study, MRI has become indispensable for the visualization of the nature of congenital ARM and associated anomalies. Moreover, MRI aids in understanding the morphology and pathogenesis of these complex congenital malformations. Based on the MRI and recent embryologic observations, a new and simplified classification of ARM is introduced, including a more appropriate nomenclature.

Cell reorganisation during epithelial fusion and perforation: the case of ascidian branchial fissures

In this study, we have analysed ultrastructurally the mechanism of epithelial fusion and perforation during the development of branchial fissures in the larva and bud of the colonial urochordate Botryllus schlosseri. Perforation of membranes represents an important process during embryogenesis, occurring to create communication between two separate compartments. For example, all chordate embryos share the formation of pharyngeal plates, which are constituted of apposed endodermal and ectodermal epithelia, which have the capacity to fuse and perforate. Although the process of perforation is extremely common, its cellular mechanism remains little understood in detail, because of the complexity of the structures involved. In B. schlosseri, two epithelial monolayers, the peribranchial and the branchial ones, with no interposed mesenchymal cells, participate in pharyngeal perforation. Blood flows in the interspace between the two cellular leaflets. Apico-lateral zonulae occludentes seal the cells of each epithelium, so that the blood compartment is separated from the environment of the peribranchial and branchial chambers; here, sea water will flow when the zooid siphons open. Stigmata primordia appear as contiguous thickened discs of palisading cells of branchial and peribranchial epithelia. The peribranchial component invaginates to contact the branchial one. Here, the basal laminae intermingle, compact, and are degraded, while the intercellular space between the two epithelia is reduced to achieve the same width as that found between the lateral membranes of adjacent cells. Cells involved in this fusion rapidly change their polarity: they acquire a new epithelial axis, because part of the adhering basal membrane becomes a new lateral surface, whereas the original lateral membranes become new apical surfaces. Before disassembling the old tight junctions and establishing communication between branchial and peribranchial chambers, cells of the stigmata rudiments form new tight junctions organised as distinct entities, so that the structural continuum of the epithelial layers is maintained throughout the time of fusion and perforation.

Anorectal anomalies associated with or as part of other anomalies

Anorectal anomalies occurring with other anomalies or as part of syndromes were analyzed to determine how their epidemiological characteristics differed from those of isolated anal anomalies. Almost 15% of cases were chromosomal, monogenic or teratogenic syndromes, whereas the rest were of unknown cause including sequences (9.3%), VACTERL associations (15.4%) and multiple congenital anomalies (MCA) (60.2%). Almost half of babies with MCA had one or two VACTERL anomalies with distribution frequencies that did not differ significantly from those in babies with the full VACTERL association. There were considerable differences in the frequency of the VACTERL association among babies with different types of anorectal anomaly. Babies with anal anomalies occurring with sequences, VACTERL or MCA showed the same sex differences as babies with isolated anal anomalies, namely male predominance in anal atresia without fistula or cloaca, no sex difference in anal atresia with fistula, and female predominance in ectopic anus and congenital anal fistula. These anomalies, however, were associated with significantly lower mean gestational lengths and birth weights, and higher frequencies of fetal death and pregnancy termination than babies with isolated anal anomalies. Twins were more frequent in sequences, VACTERL and MCA than in isolated anomalies, monogenic syndromes or chromosome anomalies. Five cases were conjoined twins, representing 15% of all cases of twin pregnancies with an anal anomaly. Indeterminate sex was more frequent in babies with anal atresias without fistula than in those with fistula. Anal anomalies are defects of blastogenesis attributable to disorders in expression of pattern determining genes. The differential sex involvement in different types of anal anomaly may be manifestations of expression of the HY/SRY genes during blastogenesis or of X-linkage.

The cloacal plate: the missing link in anorectal and urogenital development

OBJECTIVE

To determine whether differences in the embryology of the anorectal and urogenital area, previously examined in sheep, pigs, rats, rabbits and guinea pigs, producing varying conclusions, may be secondary to differences in species development.

MATERIALS AND METHODS

Rat and human embryos were studied at the time of genital tubercle development and cloacal partition by standard serial-section histology, and by immunohistochemistry, dissection and scanning electron microscopy. The images obtained were compared with those previously reported for pig and sheep embryos.

RESULTS

The cloacal plate, a vertically orientated midline plate of epithelial cells in the caudal half of the genital tubercle, was the key structure that varied between the different species. In rats the plate maintained a vertical height along its length, while in humans and pigs it reverted to a two-layer membrane dorsally, shortly before it degenerated to expose both the anorectal and urogenital tracts. In sheep the plate was taller ventrally than in the other species but also reverted to a short membrane dorsally that exposed the hindgut when it degenerated. The anterior part of the cloacal plate persisted in all embryos as the urethral plate, which then participated in the formation of the urethra in the male and the vestibule in the female. The animal that most closely resembled humans in anogenital development was the pig.

CONCLUSIONS

The cloacal plate is the key to understanding early anorectal and urogenital development and yet it is barely recognized in published reports. An appreciation of its relevance helps to explain the variations in the described embryology of the region, and is mandatory when interpreting embryological findings in animal models of human anomalies. Given the similarities between porcine and human development, the pig may be the most legitimate animal model for the study of anorectal and urogenital anomalies in humans.

Partial urorectal septum malformation sequence: a report of 25 cases

We have identified 25 cases with what we are calling the partial urorectal septum malformation (URSM) sequence, which were seen in our institution over the past 27 years. The partial URSM sequence is defined as a single perineal/anal opening that drains a common cloaca in combination with an absent (imperforate) anus. In the 25 patients reported here, the partial URSM sequence was more common in females, with a female to male ratio of 18 to 7. Ambiguous genitalia were common in both sexes. Internal pelvic structures typically showed a cloaca with the bladder and rectum (and vagina in females) coalescing into a common canal that connected to the external surface in the perineal or anal area. Abnormalities of the internal genitalia were also common, with 12 females having a bifid or septate vagina and 11 having a bicornuate uterus. Renal anomalies were frequent in both sexes, with 10 of 25 patients having unilateral cystic renal dysplasia and 7 of 25 patients having unilateral renal agenesis. Twenty-one of 25 patients survived long term. By definition, the partial URSM sequence is a milder expression of the full URSM sequence, which is defined as having no perineal or anal openings and is typically associated with an internal cloaca. The URSM spectrum, which encompasses the partial and full URSM sequences, is believed to be caused by abnormalities of septation of the primitive cloaca. The URSM spectrum is distinct from the VATER association and conditions caused by sex hormone abnormalities, such as congenital adrenal hyperplasia.

Anorectal malformations caused by defects in sonic hedgehog signaling

Anorectal malformations are a common clinical problem affecting the development of the distal hindgut in infants. The spectrum of anorectal malformations ranges from the mildly stenotic anus to imperforate anus with a fistula between the urinary and intestinal tracts to the most severe form, persistent cloaca. The etiology, embryology, and pathogenesis of anorectal malformations are poorly understood and controversial. Sonic hedgehog (Shh) is an endoderm-derived signaling molecule that induces mesodermal gene expression in the chick hindgut. However, the role of Shh signaling in mammalian hindgut development is unknown. Here, we show that mutant mice with various defects in the Shh signaling pathway exhibit a spectrum of distal hindgut defects mimicking human anorectal malformations. Shh null-mutant mice display persistent cloaca. Mutant mice lacking Gli2 or Gli3, two zinc finger transcription factors involved in Shh signaling, respectively, exhibit imperforate anus with recto-urethral fistula and anal stenosis. Furthermore, persistent cloaca is also observed in Gli2(-/-);Gli3(+/-), Gli2(+/-);Gli3(-/-), and Gli2(-/-);Gli3(-/-) mice demonstrating a gene dose-dependent effect. Therefore, Shh signaling is essential for normal development of the distal hindgut in mice and mutations affecting Shh signaling produce a spectrum of anorectal malformations that may reveal new insights into their human disease equivalents.

Perineo-pelvic diaphragmatic split: anorectal exstrophy

Anorectal malformations are among the most common congenital anomalies that require neonatal surgery. Although the headlines of anorectal malformations present with a spectrum of defects of this region, generally, imperforate anus and its fistulous or nonfistulous connections are examined. Here the authors present a boy with anorectal malformation, with sacrococcygeal teratoma and meningocele. Interestingly, the patient had neither imperforate anus nor anorectal stenosis, but had widely open anorectum. This kind of anorectal malformation has never been described previously in the English-language literature.

Clarification of the process of separation of the cloaca into rectum and urogenital sinus in the rat embryo

BACKGROUND/PURPOSE

The normal process of division of the cloaca into a rectum and urogenital tract is still not fully understood. The main controversies relate to how the urorectal septum (URS) divides the cloaca and whether the URS fuses with the cloacal membrane. This study used a 3-dimensional reconstruction technique, combined with histologic correlation, to observe the developmental and septational processes of the cloaca of the normal rat embryo from gestational days 11 to 16.

METHODS

Normal rat embryos from gestational days 11 to 16 were sectioned serially both transversely and sagittally and stained with H&E. 3-dimensional reconstructions were performed on embryos younger than day 13.5. The relevant structures were examined in a temporo-spatial sequence.

RESULTS

The tailgut started to regress by apoptosis on day 12 in a cranio-caudal direction. The URS, first evident in day-12.5 embryos, extended and fused with the cloacal membrane on day 15 of gestation, completing the separation of the cloaca into rectum and bladder. Regression of the tailgut and ventral protrusion of the urogenital sinus markedly changed the configuration of the cloaca. The cloacal membrane did not break down until after it had fused with the URS.

CONCLUSIONS

These findings clarify the relative contributions made by active septation of the cloaca by the URS and configurational changes of the cloaca to produce a rectum and bladder. The URS fuses with the cloacal membrane before the anal and urogenital membranes break down.

Does the urorectal septum fuse with the cloacal membrane?

PURPOSE

Traditional theories of cloacal embryogenesis assume that the urorectal septum fuses with the cloacal membrane before the anal membrane disintegrates. However, recent observations in humans and other species raise doubt about this assumption. We determined whether urorectal septum fusion occurs in rats.

MATERIALS AND METHODS

Rat embryos were harvested at specific times between days 11 and 16 of gestation. We evaluated the morphology, growth and relationship of the urorectal septum to the cloacal membrane on serial histological sections.

RESULTS

The urorectal septum consistently fused with the cloacal membrane on day 15 of gestation before the cloacal membrane began to disintegrate.

CONCLUSIONS

In rats the urorectal septum fuses with the cloacal membrane, after which the urogenital membrane and anal membrane disintegrate by a process of apoptosis.

Urorectal septum malformation sequence: prenatal sonographic diagnosis in two sets of discordant twins

Urorectal septum malformation sequence (URSMS) is a rare congenital malformation, which includes ambiguous genitalia, a phallus-like structure, imperforate anus, bladder, vaginal and rectal fistulas and Müllerian duct defects. We report two cases of prenatally diagnosed URSMS, both occurring in two sets of discordant twins. To the best of our knowledge, this is the first antenatal description of such an anomaly. The first fetus, one of a set of monochorionic, monoamniotic twins was detected sonographically at 21 weeks of gestation due to an enlarged phallus-like formation. The second fetus, one of dichorionic, diamniotic twins, was suspected of having an abnormally enlarged rectum at 13 weeks of gestation. The diagnosis of URSMS was established at 29 weeks of gestation by showing abnormal female external genitalia, with a dilated bowel that contained echogenic foci due to enterolithiasis. The diagnosis of both cases was confirmed postnatally. Sonographic findings and differential diagnosis are presented.

Apoptosis during regression of the tailgut and septation of the cloaca

PURPOSE

Apoptosis is involved in the embryonic morphogenesis of many organs. The current study was undertaken to ascertain the role of apoptosis during cloacal development in the rat.

METHODS

One hundred five rat embryos, ranging from gestational days 11 to 16, were sectioned serially in the transverse or sagittal planes and stained with H&E. The cloaca, urorectal septum, rectum, urogenital sinus, Wolffian ducts, and tailgut (TG) were examined consecutively in temporospatial sequence.

RESULTS

The tailgut immediately distal to the hindgut starts to regress by apoptosis on day 12 of gestation in a craniocaudal direction and has regressed completely by day 13.5. A large number of apoptotic cells and debris can be identified in the urorectal septum during cloacal septation. Vacuoles are formed by coalescence of apoptotic cells at the tip of urogenital sinus from day 15 to 16, and, at the same time, sporadic apoptotic bodies in the anal membrane contribute to its thinning.

CONCLUSION

Results of the current study confirm that apoptosis occurs in a specific temporo-spatial sequence in the hindgut and cloaca and appears to be an important mechanism in TG regression, uro-rectal separation, urethral opening, and rupture of the anal membrane.

Normal development of the male anterior urethra

A histological study was performed on serially sectioned human and mouse embryos to study the influences of programmed cell death (PCD) during morphogenesis for clarifying the existing controversies on the morphology and basic processes involved in the embryonic development of the male anterior urethra. The following new insights into the development of the anterior urethra could be established. The formation of the urethra starts with the early adhesion of the arms of the genital tubercle. In this way an epithelial plate is formed, located in the ventral midline, that is in continuity with the cloacal membrane. Male sex differentiation takes place following rupture of this cloacal membrane through programmed cell death. Fusion of the urogenital swellings with primary luminization gives rise to the penile urethra, whereas the glandular part of the urethra is formed through secondary luminization of the epithelial cord that is formed during fusion of the arms of the genital tubercle, i.e., the glans. In both fusion processes, apoptosis plays a key role. The consequence of fusion of the urogenital swellings is that their mesodermal cores unite on the ventral aspect of the penile urethra, where they differentiate into the integumental structures. The prepuce starts to develop as a fold of ectoderm with a mesodermal core after complete fusion of the entire urethra. Finally, the scrotum was found to develop through merging of the labioscrotal swellings and not by fusion.

New mouse models of congenital anorectal malformations

BACKGROUND/PURPOSE

The genetic, embryological, and pathogenetic aspects of hindgut development remain poorly understood. Recently, the morphogenetic pathway involving the Sonic hedgehog (Shh) gene has been shown essential to the normal development of many midaxial organs, including the foregut. This study reports genetically based murine models of congenital anorectal malformations (CAM) involving the Shh-responsive transcription factors, Gli2 and Gli3. Its purpose is to show the necessity of these 2 factors to normal hindgut development.

METHODS

Gli2-/- mutants were generated by a targeted deletion. Gli3-/- mutants are spontaneous mutants involving the Gli3 gene. Gli2-/- Gli3+/- mutants were generated by intercrossing double heterozygotes. Whole-mount midsagittal sections of the embryos were analyzed on embryonic days (E) 11.5 and E13.5.

RESULTS

Gli3-/- mutants had anal stenosis and ectopic anus, and Gli2-/- mutants showed imperforate anus and rectourethral fistula. Gli2-/- Gli3+/- mutants had a cloacal abnormality.

CONCLUSIONS

The phenotypic abnormalities observed in these mutant mice are identical to the spectrum of human CAM. The severity of the phenotype appears to reflect the gene dose. Gli2 and Gli3 play an important role in the normal development of murine hindgut. The results of this study provide, for the first time, a molecular basis for CAM.

Septation and differentiation of the embryonic human cloaca

BACKGROUND/PURPOSE

Limitations in methodologies have fostered controversy regarding the septation of the human embryonic cloaca. The aim of this study was to evaluate the septation of the human embryonic cloaca.

METHODS

Using the Carnegie Embryological Collection and specimens at Johns Hopkins, Baltimore, MD, the authors studied 12 embryos and five fetuses. Embryo photomicrographs were reconstructed using three-dimensional modeling.

RESULTS

In Carnegie stage 13 the authors observed a cloaca, distinct primitive urogenital sinus, and anorectum separated by the urorectal septum. The primitive urogenital sinus and anorectum enter the cloaca separated from the amniotic space by the cloacal membrane. As the embryo becomes a fetus it lengthens, grows, expands and rotates through a process called transformation. Transformation gives rise to a loss of caudal curvature and a decrease in distance between the septum and membrane, but these structures do not fuse. Disintegration of the cloacal membrane produces openings for the urogenital sinus and anorectum.

CONCLUSIONS

The observations suggest that the urogenital sinus and anorectum form early and are separated by the urorectal septum as a passive structure. There does not appear to be septation or differentiation of the cloaca itself.