Retinoic acid-induced caudal regression syndrome in the mouse fetus

Sirenomelia or mermaid syndrome with a cleft lip in a Tanzanian newborn: a case report

BACKGROUND

Sirenomelia or sirenomelia sequence, also known as mermaid syndrome, is a rare congenital anomaly involving the caudal region of the body. The syndrome is characterized by partial or complete fusion of lower extremities, renal agenesis, absent urinary tract, ambiguous external genitalia, imperforate anus, and single umbilical artery. Sirenomelia is often associated with several visceral congenital malformations, rendering it invariably incompatible with extrauterine life.

CASE PRESENTATION

We present the case of 22-year-old Black African woman who delivered a term newborn by caesarean section at a gestation age of 37 weeks due to obstructed labor with fetal distress. The newborn was a fresh stillbirth weighing 2100 g and had fusion of the lower extremities, a single upper limb, ambiguous genitalia, imperforate anus, and a cleft lip. The mother had made only two prenatal visits, at which she was found to be normotensive and normoglycemic. She was not screened for routine fetomaternal infections and missed supplementation for folic acid during the critical first trimester. She did not undergo any obstetric ultrasonography. The parents of the newborn were not close relatives and there was no family history of consanguinity. Further genetic testing was not performed due to lack of laboratory capacity, and post mortem examination was not permitted due to cultural taboo and restrictions relating to handling of deceased newborns.

CONCLUSION

Sirenomelia is a rare congenital malformation with very poor prognosis. Specific interventions during pre-conception and early prenatal care are critical in the prevention of specific congenital anomalies. Early obstetric ultrasonography is invaluable for diagnosis of sirenomelia as well as counseling for possible termination of pregnancy.

Prenatal sirenomelia diagnosis in the first trimester: A case report and literature review

Key Clinical Message

Sirenomelia is a lethal condition in the perinatal period. The sonographic examination in the first trimester can effectively detect sirenomelia with a high degree of accuracy. Furthermore, vascular examinations using color flow imaging and augmented imaging techniques such as 3D sonography can improve diagnostic accuracy. Parents should get advice from a multidisciplinary team concerning sirenomelia care and prognosis as soon as possible.

Abstract

Sirenomelia is a rare condition with an uncommon congenital malformation; its most remarkable feature is lower extremity fusion with multiple visceral anomalies. Accordingly, the appearance resembles a mermaid's tail, hence mermaid syndrome. Sirenomelia has an incidence rate of 1.5-4.2 per 100,000 births, a male-to-female 2.7:1 ratio, and shows no differences across races. The condition is generally associated with renal agenesis, exterior genitalia defects, a single umbilical artery, and an imperforate anus. Here, we describe the first sirenomelia case in our hospital; a 13-week-old fetus with conjoined lower limbs was identified by ultrasound in the first trimester. We discuss this rare case with reference to the literature and provide insights on diagnosing this condition by ultrasound.

Self-organizing models of human trunk organogenesis recapitulate spinal cord and spine co-morphogenesis

Integrated in vitro models of human organogenesis are needed to elucidate the multi-systemic events underlying development and disease. Here we report the generation of human trunk-like structures that model the co-morphogenesis, patterning and differentiation of the human spine and spinal cord. We identified differentiation conditions for human pluripotent stem cells favoring the formation of an embryo-like extending antero-posterior (AP) axis. Single-cell and spatial transcriptomics show that somitic and spinal cord differentiation trajectories organize along this axis and can self-assemble into a neural tube surrounded by somites upon extracellular matrix addition. Morphogenesis is coupled with AP patterning mechanisms, which results, at later stages of organogenesis, in in vivo-like arrays of neural subtypes along a neural tube surrounded by spine and muscle progenitors contacted by neuronal projections. This integrated system of trunk development indicates that in vivo-like multi-tissue co-morphogenesis and topographic organization of terminal cell types can be achieved in human organoids, opening windows for the development of more complex models of organogenesis.

Genetics of Atavism

Abstract

Atavisms have attracted people’s attention for a long time. First, atavisms excited their imagination and created fertile ground for myths and superstitions. With the development of science, atavisms became the subject of investigation, which soon provided evidence to support evolutionary theory. However, at the molecular level, the formation of atavisms remained insufficiently understood. Recent progress in comparative genomics and molecular developmental biology has helped in understanding the processes underlying the formation of one of the human atavisms: the vestigial tail.

Caudal duplication syndrome: a literature review and reappraisal of its pathoembryogenesis

INTRODUCTION

The term caudal duplication syndrome (CDS) was first introduced for complex anomalies of the distal caudal end of the trunk. The pathoembryogenesis of CDS is yet unknown, although a few theories have been proposed. We reviewed the previously proposed pathoembryogenetic theories and suggested a new perspective through the common clinical characteristics shown in CDS cases reported in the literature.

METHODS

We conducted a systematic literature search of the online database PubMed from October 1993 to October 2020, using the search term "caudal duplication syndrome", according to the first mention of this entity. A total of 17 articles with 23 patients were reviewed.

RESULTS

The most common manifestations were the duplication of the distal colon, genitourinary organs, and lower spine. Specifically, the duplicated bladders or uteri contacted their counterpart through a septum, and the duplicated bowels ran parallel. More caudal structures, such as the urethra or anus, were formed separately. The duplication seems to be a result of division by an intervening septum or structure in each part. In addition, duplication was not limited to the structures formed from the caudal cell mass (CCM), such as the distal spine and spinal cord, but also included hindgut structures. Moreover, anomalies involving caudal mesenchymal defects were also present. Considering clinical manifestations that are related to all three germ layers and seemingly the overseptation of these germ layers in CDS patients, with supporting data from animal experiments, events such as late-stage errors involving Hensen's node/the primitive streak and the duplication of the CCM with the hyperplasia of the abnormally located central caudal mesenchyme are probable pathoembryogenetic mechanisms for CDS. The "leakage" of the normal growth power of the caudal mesenchyme into the intervening midline space between the two CCMs and consequent weak lateral and caudal pushes of the caudal mesenchyme may explain the association of caudal agenesis or its related anomalies with CDS.

CONCLUSION

We propose a theory that by a molecular interaction, an insult causes late gastrulation phase problems, resulting in ectopic primitive streak formation, and therefore, a duplication of the CCM is induced. Subsequently, the overactivity of abnormally positioned midline mesenchyme between the two CCMs may divide the hindgut derivatives by a central septum. Underactive lateral and caudal pushes of the caudal mesenchyme may lead to an association of features shown in caudal agenesis.

Understanding axial progenitor biology in vivo and in vitro

The generation of the components that make up the embryonic body axis, such as the spinal cord and vertebral column, takes place in an anterior-to-posterior (head-to-tail) direction. This process is driven by the coordinated production of various cell types from a pool of posteriorly-located axial progenitors. Here, we review the key features of this process and the biology of axial progenitors, including neuromesodermal progenitors, the common precursors of the spinal cord and trunk musculature. We discuss recent developments in the in vitro production of axial progenitors and their potential implications in disease modelling and regenerative medicine.

Gastrulation : Current Concepts and Implications for Spinal Malformations

It has been recognised for over a century that the events of gastrulation are fundamental in determining, not only the development of the neuraxis but the organisation of the entire primitive embryo. Until recently our understanding of gastrulation was based on detailed histological analysis in animal models and relatively rare human tissue preparations from aborted fetuses. Such studies resulted in a model of gastrulation that neurosurgeons have subsequently used as a means of trying to explain some of the congenital anomalies of caudal spinal cord and vertebral development that present in paediatric neurosurgical practice. Recent advances in developmental biology, in particular cellular biology and molecular genetics have offered new insights into very early development. Understanding the processes that underlie cellular interactions, gene expression and activation/inhibition of signalling pathways has changed the way embryologists view gastrulation and this has led to a shift in emphasis from the ‘descriptive and morphological’ to the ‘mechanistic and functional’. Unfortunately, thus far it has proved difficult to translate this improved knowledge of normal development, typically derived from non-human models, into an understanding of the mechanisms underlying human malformations such as the spinal dysraphisms and anomalies of caudal development. A paediatric neurosurgeons perspective of current concepts in gastrulation is presented along with a critical review of the current hypotheses of human malformations that have been attributed to disorders of this stage of embryogenesis.

Mutation of amphioxus Pdx and Cdx demonstrates conserved roles for ParaHox genes in gut, anus and tail patterning

BACKGROUND

The homeobox genes Pdx and Cdx are widespread across the animal kingdom and part of the small ParaHox gene cluster. Gene expression patterns suggest ancient roles for Pdx and Cdx in patterning the through-gut of bilaterian animals although functional data are available for few lineages. To examine evolutionary conservation of Pdx and Cdx gene functions, we focus on amphioxus, small marine animals that occupy a pivotal position in chordate evolution and in which ParaHox gene clustering was first reported.

RESULTS

Using transcription activator-like effector nucleases (TALENs), we engineer frameshift mutations in the Pdx and Cdx genes of the amphioxus Branchiostoma floridae and establish mutant lines. Homozygous Pdx mutants have a defect in amphioxus endoderm, manifest as loss of a midgut region expressing endogenous GFP. The anus fails to open in homozygous Cdx mutants, which also have defects in posterior body extension and epidermal tail fin development. Treatment with an inverse agonist of retinoic acid (RA) signalling partially rescues the axial and tail fin phenotypes indicating they are caused by increased RA signalling. Gene expression analyses and luciferase assays suggest that posterior RA levels are kept low in wild type animals by a likely direct transcriptional regulation of a Cyp26 gene by Cdx. Transcriptome analysis reveals extensive gene expression changes in mutants, with a disproportionate effect of Pdx and Cdx on gut-enriched genes and a colinear-like effect of Cdx on Hox genes.

CONCLUSIONS

These data reveal that amphioxus Pdx and Cdx have roles in specifying middle and posterior cell fates in the endoderm of the gut, roles that likely date to the origin of Bilateria. This conclusion is consistent with these two ParaHox genes playing a role in the origin of the bilaterian through-gut with a distinct anus, morphological innovations that contributed to ecological change in the Cambrian. In addition, we find that amphioxus Cdx promotes body axis extension through a molecular mechanism conserved with vertebrates. The axial extension role for Cdx dates back at least to the origin of Chordata and may have facilitated the evolution of the post-anal tail and active locomotion in chordates.

Sirenomelia, case report and review of the literature

We present a case of sirenomelia diagnosed in the first trimester of pregnancy. The ultrasound examination showed fused lower extremities and an anechoic structure in the lower abdomen that is clue in the early diagnosis. The postmortem study showed the existence of a single umbilical artery (vitelline artery), with an origin in the abdominal aorta. This finding not only explained the presence of a vascular steal with subsequent underdeveloped of pelvic organs, but also differentiated this condition from caudal regression syndrome.

Caudal Regression Syndrome-A Review Focusing on Genetic Associations

Caudal regression syndrome (CRS) represents a spectrum of clinical phenotypes with varying degrees of malformation of the lower body with involvement of structures deriving from all 3 layers of the trilaminar embryo. We review areas of active investigation in the diagnosis, etiology, epidemiology, and treatment of the disease with a focus on underlying genetics. CRS pathobiology is complex and multifactorial with a significant contribution from environmental factors as evidenced in twin studies. Contemporary genomic and genetic investigations in both human primary tissue and murine in vitro and in vivo models implicate various genes associated with caudal differentiation and neural cell migration in embryogenesis. A large number of identified targets center around the metabolic regulation of retinoic acid and its derivatives. Dysregulation of retinoic acid homeostasis has been associated with abnormal embryonic cell migration, differentiation, and organogenesis with resulting malformations and agenesis in both a laboratory and a clinical setting. There appears to be a significant overlap in potential genetic targets with CRS and other developmental syndromes with similar presentations, such as VACTERL (vertebral defects, anal atresia, cardiac defects, tracheo-esophageal fistula, renal anomalies, and limb abnormalities) association. CRS represents a spectrum of caudal developmental abnormalities with treatment options limited to mild and moderate expressions of disease. Continued research is necessary to further clarify mechanisms of disease pathobiology and complex polygenetic and environmental interaction. Despite this, progress has been made in identifying genetic targets and downstream effectors contributing to preclinical and clinical progression.

Descriptive and risk factor analysis of nonsyndromic sacral agenesis: National Birth Defects Prevention Study, 1997-2011

Sacral agenesis is a rare birth defect characterized by partial or complete absence of the sacrum. We sought to (a) describe case characteristics, (b) estimate birth prevalence, and (c) identify risk factors for nonsyndromic sacral agenesis using data from the National Birth Defects Prevention Study (NBDPS). The NBDPS was a population-based, case-control study involving pregnancies with estimated dates of delivery from October 1997 through December 2011. We estimated birth prevalence using all NBDPS eligible cases. Using self-reported maternal exposure information, we conducted multivariable logistic regression analysis to identify potential risk factors overall and among women without diabetes. The birth prevalence of sacral agenesis was 2.6/100,000 live births. In the multivariable analysis, multifetal pregnancy, pre-existing Type 1 diabetes, and pre-existing Type 2 diabetes were positively and significantly associated with sacral agenesis, albeit estimates were imprecise. Preexisting Type 1 diabetes was the strongest risk factor (adjusted odds ratio = 96.6, 95% confidence interval = 43.5-214.7). Among women without diabetes, periconceptional smoking was positively and significantly associated with sacral agenesis. Our findings underscore the importance of smoking cessation programs among women planning pregnancy and the importance of better understanding the role of glycemic control before and during pregnancy when designing interventions for primary prevention of sacral agenesis.

Sonic Hedgehog Signaling Is Required for Cyp26 Expression during Embryonic Development

Deciphering how signaling pathways interact during development is necessary for understanding the etiopathogenesis of congenital malformations and disease. In several embryonic structures, components of the Hedgehog and retinoic acid pathways, two potent players in development and disease are expressed and operate in the same or adjacent tissues and cells. Yet whether and, if so, how these pathways interact during organogenesis is, to a large extent, unclear. Using genetic and experimental approaches in the mouse, we show that during development of ontogenetically different organs, including the tail, genital tubercle, and secondary palate, Sonic hedgehog (SHH) loss-of-function causes anomalies phenocopying those induced by enhanced retinoic acid signaling and that SHH is required to prevent supraphysiological activation of retinoic signaling through maintenance and reinforcement of expression of the Cyp26 genes. Furthermore, in other tissues and organs, disruptions of the Hedgehog or the retinoic acid pathways during development generate similar phenotypes. These findings reveal that rigidly calibrated Hedgehog and retinoic acid activities are required for normal organogenesis and tissue patterning.

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.

Bones in human CYP26B1 deficiency and rats with hypervitaminosis A phenocopy Vegfa overexpression

Angulated femurs are present prenatally both in CYP26B1 deficient humans with a reduced capacity to degrade retinoic acid (RA, the active metabolite of vitamin A), and mice overexpressing vascular endothelial growth factor a (Vegfa). Since excessive ingestion of vitamin A is known to induce spontaneous fractures and as the Vegfa-induced femur angulation in mice appears to be caused by intrauterine fractures, we analyzed bones from a CYP26B1 deficient human and rats with hypervitaminosis A to further explore Vegfa as a mechanistic link for the effect of vitamin A on bone. We show that bone from a human with CYP26B1 mutations displayed periosteal osteoclasts in piles within deep resorption pits, a pathognomonic sign of hypervitaminosis A. Analysis of the human angulated fetal femur revealed excessive bone formation in the marrow cavity and abundant blood vessels. Normal human endothelial cells showed disturbed cell-cell junctions and increased CYP26B1 and VEGFA expression upon RA exposure. Studies in rats showed increased plasma and tissue Vegfa concentrations and signs of bone marrow microhemorrhage on the first day of excess dietary vitamin A intake. Subsequently hypervitaminosis A rats displayed excess bone formation, fibrosis and an increased number of megakaryocytes in the bone marrow, which are known characteristics of Vegfa overexpression. This study supports the notion that the skeletal phenotype in CYP26B1 deficient human bone is caused by excess RA. Our findings suggest that an initial part of the vitamin A mechanism causing bone alterations is mediated by excess Vegfa and disturbed bone marrow microvessel integrity.

•

Human CYP26B1 deficit and rat hypervitaminosis A phenocopy Vegf bone overexpression

•

Hypervitaminosis A cause rapid microhemorrhage in rat bone marrow.

•

Retinoic acid treatment disrupt cell-cell junction integrity between endothelial cells.

•

Hypervitaminosis A have a persistent negative effect on rat bone marrow perfusion.

•

Hypervitaminosis A rat bones resemble bones of patients with myelofibrotic disorders.

Medulloblastoma in association with sacral agenesis; a case report

Medulloblastoma is a congenital brain tumor which can be associated with different congenital anomalies. However, coincidence of cerebellar medulloblastoma with sacral agenesis has not been reported so far. A variety of genetic and/or environmental predisposing factors have been proposed for both diseases. Herein, an unprecedented coincidence of these two conditions is presented. A neonate was born with lumbosacral agenesis, paraplegia, and atrophic legs, and he developed medulloblastoma with three ventricular hydrocephalus 3 years later. Different aspects regarding the embryology and etiology of both ailments are discussed, assuming the possibility that the same genetic and/or environmental risk factors may have played a part in both conditions.

Mermaid Syndrome: A Case Report in Mauritius

Sirenomelia is a rare congenital malformation that results in the fusion of the lower limbs together with multiple visceral anomalies. We report a case of sirenomelia observed in Mauritius and the different findings seen in the baby. The baby had fused lower extremities and bony structures for each leg were present. The umbilical cord consisted of a single artery and one vein. The external genitalia was absent and an imperforate anus was also seen. An x-ray revealed poorly expanded lungs and two distinct sets of femur and tibia were seen on imaging. However, a fused fibula and a fused talus were also noted. Multiple theories have been suggested for the pathogenesis of this condition, and despite recent progress in pathology, this condition remains debated.

Perturbation of Retinoid Homeostasis Increases Malformation Risk in Embryos Exposed to Pregestational Diabetes

Pregestational diabetes is highly associated with an increased risk of birth defects. However, factors that can increase or reduce the expressivity and penetrance of malformations in pregnancies in women with diabetes remain poorly identified. All-trans retinoic acid (RA) plays crucial roles in embryogenesis. Here, we find that Cyp26a1, which encodes a key enzyme for catabolic inactivation of RA required for tight control of local RA concentrations, is significantly downregulated in embryos of diabetic mice. Embryonic tissues expressing Cyp26a1 show reduced efficiency of RA clearance. Embryos exposed to diabetes are thus sensitized to RA and more vulnerable to the deleterious effects of increased RA signaling. Susceptibility to RA teratogenesis is further potentiated in embryos with a preexisting genetic defect of RA metabolism. Increasing RA clearance efficiency using a preconditioning approach can counteract the increased susceptibility to RA teratogenesis in embryos of diabetic mice. Our findings provide new insight into gene-environment interactions that influence individual risk in the manifestation of diabetes-related birth defects and shed light on environmental risk factors and genetic variants for a stratified medicine approach to screening women with diabetes who are of childbearing age and assessing the risk of birth defects during pregnancy.

Sacral agenesis: a pilot whole exome sequencing and copy number study

Background

Caudal regression syndrome (CRS) or sacral agenesis is a rare congenital disorder characterized by a constellation of congenital caudal anomalies affecting the caudal spine and spinal cord, the hindgut, the urogenital system, and the lower limbs. CRS is a complex condition, attributed to an abnormal development of the caudal mesoderm, likely caused by the effect of interacting genetic and environmental factors. A well-known risk factor is maternal type 1 diabetes.

Method

Whole exome sequencing and copy number variation (CNV) analyses were conducted on 4 Caucasian trios to identify de novo and inherited rare mutations.

Results

In this pilot study, exome sequencing and copy number variation (CNV) analyses implicate a number of candidate genes, including SPTBN5, MORN1, ZNF330, CLTCL1 and PDZD2. De novo mutations were found in SPTBN5, MORN1 and ZNF330 and inherited predicted damaging mutations in PDZD2 (homozygous) and CLTCL1 (compound heterozygous). Importantly, predicted damaging mutations in PTEN (heterozygous), in its direct regulator GLTSCR2 (compound heterozygous) and in VANGL1 (heterozygous) were identified. These genes had previously been linked with the CRS phenotype.

Two CNV deletions, one de novo (chr3q13.13) and one homozygous (chr8p23.2), were detected in one of our CRS patients. These deletions overlapped with CNVs previously reported in patients with similar phenotype.

Conclusion

Despite the genetic diversity and the complexity of the phenotype, this pilot study identified genetic features common across CRS patients.

Electronic supplementary material

The online version of this article (doi:10.1186/s12881-016-0359-2) contains supplementary material, which is available to authorized users.

Magnetic Resonance Imaging Analysis of Caudal Regression Syndrome and Concomitant Anomalies in Pediatric Patients

OBJECTIVE

The aim of this study was to evaluate the magnetic resonance imaging (MRI) findings of caudal regression syndrome (CRS) and concomitant anomalies in pediatric patients.

MATERIALS AND METHODS

A hospital-based cross-sectional retrospective study was conducted. The study group comprised 21 pediatric patients presenting to the Departments of Radiodiagnosis and Pediatric Surgery in a tertiary care hospital from May 2011 to April 2016. All patients were initially evaluated clinically followed by MRI.

RESULTS

In our study, 21 pediatric patients were diagnosed with sacral agenesis/dysgenesis related to CRS. According to the Pang's classification, 2 (9.5%) patients were Type I, 5 (23.8%) patients were Type III, 7 (33.3%) patients were Type IV, and 7 (33.3%) patients were of Type V CRS. Clinically, 17 (81%) patients presented with urinary incontinence, 6 (28.6%) with fecal incontinence, 9 patients (42.9%) had poor gluteal musculatures and shallow intergluteal cleft, 7 (33.3%) patients had associated subcutaneous mass over spine, and 6 (28.6%) patients presented with distal leg muscle atrophy. MRI showed wedge-shaped conus termination in 5 (23.8%) patients and bulbous conus termination in 3 (14.3%) patients above the L1 vertebral level falling into Group 1 CRS while 7 (33.3%) patients had tethered cord and 6 (28.6%) patients had stretched conus falling into Group 2 CRS.

CONCLUSION

MRI is the ideal modality for detailed evaluation of the status of the vertebra, spinal cord, intra- and extra-dural lesions and helps in early diagnosis, detailed preoperative MRI evaluation and assessing concomitant anomalies and guiding further management with early institution of treatment to maximize recovery.

Systematic stereoscopic analyses for cloacal development: The origin of anorectal malformations

The division of the embryonic cloaca is the most essential event for the formation of digestive and urinary tracts. The defective development of the cloaca results in anorectal malformations (ARMs; 2–5 per 10,000 live births). However, the developmental and pathogenic mechanisms of ARMs are unclear. In the current study, we visualized the epithelia in the developing cloaca and nephric ducts (NDs). Systemic stereoscopic analyses revealed that the ND-cloaca connection sites shifted from the lateral-middle to dorsal-anterior part of the cloaca during cloacal division from E10.5 to E11.5 in mouse embryos. Genetic cell labeling analyses revealed that the cells in the ventral cloacal epithelium in the early stages rarely contributed to the dorsal part. Moreover, we revealed the possible morphogenetic movement of endodermal cells within the anterior part of the urogenital sinus and hindgut. These results provide the basis for understanding both cloacal development and the ARM pathogenesis.

MBTPS1/SKI-1/S1P proprotein convertase is required for ECM signaling and axial elongation during somitogenesis and vertebral development†

Caudal regression syndrome (sacral agenesis), which impairs development of the caudal region of the body, occurs with a frequency of about 2 live births per 100 000 newborns although this incidence rises to 1 in 350 infants born to mothers with gestational diabetes. The lower back and limbs can be affected as well as the genitourinary and gastrointestinal tracts. The axial skeleton is formed during embryogenesis through the process of somitogenesis in which the paraxial mesoderm periodically segments into bilateral tissue blocks, called somites. Somites are the precursors of vertebrae and associated muscle, tendons and dorsal dermis. Vertebral anomalies in caudal regression syndrome may arise through perturbation of somitogenesis or, alternatively, could result from defective bone formation and patterning. We discovered that MBTPS1/SKI-1/S1P, which proteolytically activates a class of transmembrane transcription factors, plays a critical role in somitogenesis and the pathogenesis of lumbar/sacral vertebral anomalies. Conditional deletion of Mbtps1 yields a viable mouse with misshapen, fused and reduced number of lumbar and sacral vertebrae, under-developed hind limb bones and a kinky, shortened tail. We show that Mbtps1 is required to (i) maintain the Fgf8 'wavefront' in the presomitic mesoderm that underpins axial elongation, (ii) sustain the Lfng oscillatory 'clock' activity that governs the periodicity of somite formation and (iii) preserve the composition and character of the somitic extracellular matrix containing fibronectin, fibrillin2 and laminin. Based on this spinal phenotype and known functions of MBTPS1, we reason that loss-of-function mutations in Mbtps1 may cause the etiology of caudal regression syndrome.

Disruption of the temporally regulated cloaca endodermal β-catenin signaling causes anorectal malformations

The cloaca is temporally formed and eventually divided by the urorectal septum (URS) during urogenital and anorectal organ development. Although congenital malformations, such as anorectal malformations (ARMs), are frequently observed during this process, the underlying pathogenic mechanisms remain unclear. β-Catenin is a critical component of canonical Wnt signaling and is essential for the regulation of cell differentiation and morphogenesis during embryogenesis. The expression of β-catenin is observed in endodermal epithelia, including URS epithelia. We modulated the β-catenin gene conditionally in endodermal epithelia by utilizing tamoxifen-inducible Cre driver line (Shh^CreERT2). Both β-catenin loss- and gain-of-function (LOF and GOF) mutants displayed abnormal clefts in the perineal region and hypoplastic elongation of the URS. The mutants also displayed reduced cell proliferation in the URS mesenchyme. In addition, the β-catenin GOF mutants displayed reduced apoptosis and subsequently increased apoptosis in the URS epithelium. This instability possibly resulted in reduced expression levels of differentiation markers, such as keratin 1 and filaggrin, in the perineal epithelia. The expression of bone morphogenetic protein (Bmp) genes, such as Bmp4 and Bmp7, was also ectopically induced in the epithelia of the URS in the β-catenin GOF mutants. The expression of the Msx2 gene and phosphorylated-Smad1/5/8, possible readouts of Bmp signaling, was also increased in the mutants. Moreover, we introduced an additional mutation for a Bmp receptor gene: BmprIA. The Shh^CreERT2/+; β-catenin^flox(ex3)/+; BmprIA^flox/− mutants displayed partial restoration of URS elongation compared with the β-catenin GOF mutants. These results indicate that some ARM phenotypes in the β-catenin GOF mutants were caused by abnormal Bmp signaling. The current analysis revealed the close relation of endodermal β-catenin signaling to the ARM phenotypes. These results are considered to shed light on the pathogenic mechanisms of human ARMs.

Proceedings of the 2013 National Toxicology Program Satellite Symposium

The 2013 annual National Toxicology Program (NTP) Satellite Symposium, entitled "Pathology Potpourri," was held in Portland, Oregon, in advance of the Society of Toxicologic Pathology's 32nd annual meeting. The goal of the NTP Symposium is to present current diagnostic pathology or nomenclature issues to the toxicologic pathology community. This article presents summaries of the speakers' presentations, including diagnostic or nomenclature issues that were presented, along with select images that were used for audience voting and discussion. Some lesions and topics covered during the symposium included a caudal tail vertebra duplication in mice; nephroblastematosis in rats; ectopic C cell tumor in a hamster; granular cell aggregates/tumor in the uterus of a hamster; Pneumocystis carinii in the lung of a rat; iatrogenic chronic inflammation in the lungs of control rats; hepatoblastoma arising within an adenoma in a mouse; humoral hypercalcemia of benignancy in a transgenic mouse; acetaminophen-induced hepatotoxicity in rats; electron microscopy images of iatrogenic intraerythrocytic inclusions in transgenic mice; questionable hepatocellular degeneration/cell death/artifact in rats; atypical endometrial hyperplasia in rats; malignant mixed Müllerian tumors/carcinosarcomas in rats; differential diagnoses of proliferative lesions of the intestine of rodents; and finally obstructive nephropathy caused by melamine poisoning in a rat.

Associations of anorectal malformations and related syndromes

Anorectal malformations (ARMs) represent a complex group of congenital anomalies resulting from abnormal development of the hindgut, allantois and Mullerian duct resulting in complete or partial urorectal septal malformations. There is a wide variety of phenotypic expression, ranging from mild anorectal to very complex severe ARM with >75 % having other associated malformations. 50 % of cases are syndromic although many may have other associated anomalies. This suggests a genetic link but the genetics of ARM are highly complex with a number of candidate genes being identified. Many can be classified as "field defects" as a result of a complex set of genetic interactions. Patients with associated malformations can be classified into those with multiple congenital anomalies (non-syndromic), those with chromosomal abnormalities and those with non-chromosomal syndromic associations, also, those with non-chromosomal syndromes and the influence of environmental factors (e.g. drugs in pregnancy). Although much is not known about the aetiology of ARM, the weight of evidence points to genetic factors as major causes for the condition. In this review, we look at the chromosomal and genetic associations and their underlying signalling pathways, to obtain a better understanding of the pathogenetic mechanisms involved in developing ARM. The spectrum of ARM phenotypic expression probably results from involvement and crosstalk between a number of critical signalling systems involved in development of this region. As a result, it may be expressed as a "field developmental defect" with many associated abnormalities. The role of environmental factors in the development of ARM is probably less.

Sirenomelia: a review on embryogenic enviromental theories, novel three-dimensional ultrasound imaging and first trimester diagnosis in a case of mosaic 69,XXX/46,XX fetus

PURPOSE

Sirenomelia is caused by atrophy of the lower extremities that is commonly associated with gastrointestinal and urogenital malformations.

METHODS

Embryogenic environmental theories and systematic review of the literature are reported.

RESULTS

Genetic basis of the condition has been demonstrated in the animal model. In humans, association with de novo balanced translocation has only recently been documented.

CONCLUSIONS

A case of triploidy mosaic fetus with sirenomelia and posterior fossa anomaly diagnosed at first trimester using novel three-dimensional ultrasound imaging techniques is presented.

A fetus with hemifacial microsomia and sirenomelia. The same mesodermal defect spectrum?

Sirenomelia is the most severe malformation complex affecting the human caudal pole, although its etiology is unclear, a primary defect of blastogenesis has been proposed. Studies consider sirenomelia as the most severe form of caudal dysgenesis, VACTERL association, or axial mesodermal dysplasia, although others still support the idea of a different pathologic entity. We report the prenatal, clinical, and pathologic features of a fetus with cleft lip and palate, microtia, cardiac, renal and intestinal malformations, radial aplasia, and sirenomelia. Karyotype, chromosomal breakage studies, and SHH sequence analysis were normal. The occurrence of cephalic, midline-paramedial, and caudal malformations in the same patient imply the diagnosis of hemifacial microsomia and sirenomelia. These entities are part of the same mesodermal malformation spectrum and the clinical presentation depends on environmental and genetic interactions in embrionic development. Future clinical and genome wide studies will help to better delineate this spectrum.

Tail regression induced by elevated retinoic acid signaling in amphioxus larvae occurs by tissue remodeling, not cell death

The vitamin A derived morphogen retinoic acid (RA) is known to function in the regulation of tissue proliferation and differentiation. Here, we show that exogenous RA applied to late larvae of the invertebrate chordate amphioxus can reverse some differentiated states. Although treatment with the RA antagonist BMS009 has no obvious effect on late larvae of amphioxus, administration of excess RA alters the morphology of the posterior end of the body. The anus closes over, and gut contents accumulate in the hindgut. In addition, the larval tail fin regresses, although little apoptosis takes place. This fin normally consists of columnar epidermal cells, each characterized by a ciliary rootlet running all the way from an apical centriole to the base of the cell and likely contributing substantial cytoskeletal support. After a few days of RA treatment, the rootlet becomes disrupted, and the cell shape changes from columnar to cuboidal. Transmission electron microscopy (TEM) shows fragments of the rootlet in the basal cytoplasm of the cuboidal cell. A major component of the ciliary rootlet in amphioxus is the protein Rootletin, which is encoded by a single AmphiRootletin gene. This gene is highly expressed in the tail epithelial cells of control larvae, but becomes downregulated after about a day of RA treatment, and the breakup of the ciliary rootlet soon follows. The effect of excess RA on these epidermal cells of the larval tail in amphioxus is unlike posterior regression in developing zebrafish, where elevated RA signaling alters connective tissues of mesodermal origin. In contrast, however, the RA-induced closure of the amphioxus anus has parallels in the RA-induced caudal regression syndrome of mammals.

Birth defects associated with perturbations in preimplantation, gastrulation, and axis extension: from conjoined twinning to caudal dysgenesis

Congenital malformations represent approximately 3 in 100 live births within the human population. Understanding their pathogenesis and ultimately formulating effective treatments are underpinned by knowledge of the events and factors that regulate normal embryonic development. Studies in model organisms, primarily in the mouse, the most prominent genetically tractable mammalian model, have equipped us with a rudimentary understanding of mammalian development from early lineage commitment to morphogenetic processes. In this way, information provided by studies in the mouse can, in some cases, be used to draw parallels with other mammals, including human. Here, we provide an overview of our current understanding of the general sequence of developmental events from early cell cleavages to gastrulation and axis extension occurring in human embryos. We will also review some of the rare birth defects occurring at these stages, in particular those resulting in conjoined twinning or caudal dysgenesis.

In vivo knockdown of Brachyury results in skeletal defects and urorectal malformations resembling caudal regression syndrome

The T-box transcription factor BRACHYURY (T) is a key regulator of mesoderm formation during early development. Complete loss of T has been shown to lead to embryonic lethality around E10.0. Here we characterize an inducible miRNA-based in vivo knockdown mouse model of T, termed KD3-T, which exhibits a hypomorphic phenotype. KD3-T embryos display axial skeletal defects caused by apoptosis of paraxial mesoderm, which is accompanied by urorectal malformations resembling the murine uro-recto-caudal syndrome and human caudal regression syndrome phenotypes. We show that there is a reduction of T in the notochord of KD3-T embryos which results in impaired notochord differentiation and its subsequent loss, whereas levels of T in the tailbud are sufficient for axis extension and patterning. Furthermore, the notochord in KD3-T embryos adopts a neural character and loses its ability to act as a signaling center. Since KD3-T animals survive until birth, they are useful for examining later roles for T in the development of urorectal tissues.

Sirenomelia phenotype in bmp7;shh compound mutants: a novel experimental model for studies of caudal body malformations

Sirenomelia is a severe congenital malformation of the lower body characterized by the fusion of the legs into a single lower limb. This striking external phenotype consistently associates severe visceral abnormalities, most commonly of the kidneys, intestine, and genitalia that generally make the condition lethal. Although the causes of sirenomelia remain unknown, clinical studies have yielded two major hypotheses: i) a primary defect in the generation of caudal mesoderm, ii) a primary vascular defect that leaves the caudal part of the embryo hypoperfused. Interestingly, Sirenomelia has been shown to have a genetic basis in mice, and although it has been considered a sporadic condition in humans, recently some possible familial cases have been reported. Here, we report that the removal of one or both functional alleles of Shh from the Bmp7-null background leads to a sirenomelia phenotype that faithfully replicates the constellation of external and internal malformations, typical of the human condition. These mutants represent an invaluable model in which we have analyzed the pathogenesis of sirenomelia. We show that the signaling defect predominantly impacts the morphogenesis of the hindgut and the development of the caudal end of the dorsal aortas. The deficient formation of ventral midline structures, including the interlimb mesoderm caudal to the umbilicus, leads to the approximation and merging of the hindlimb fields. Our study provides new insights for the understanding of the mechanisms resulting in caudal body malformations, including sirenomelia.

Sirenomelia: case reports and current concepts of pathogenesis

We present 2 cases of sirenomelia and highlight the recent theories about its pathogenesis. Both cases had a large aberrant abdominal umbilical artery (AAUA) arising from the aorta, suggesting vascular steal as the pathophysiology. However, the bilateral upper limb defects noted in 1 case, the reported 10% association of holoprosencephaly and anencephaly, and the reports of sirenomelia with normal umbilical arteries point to the alternative caudal dysgenesis (CD) theory. This proposes that an insult at the early blastogenic stage interferes with the formation of the notochord, resulting in abnormal development of caudal structures, an AAUA, and occasional neural tube defects. We have also analyzed the implications of the similarities between sirenomelia/CD and the VATER association; the increased risk of CD but not sirenomelia in infants of diabetic mothers; the fact that sirenomelia, holoprosencephaly, and the VATER association are all more common in monozygotic twins; the experimental production of sirenomelia in mice; and the possible genetic implications of the co-occurrence of sirenomelia and CD.

Sirenomelia with a de novo balanced translocation 46,X,t(X;16)(p11.23;p12.3)

We report a female fetus with sirenomelia with 46,X,t(X;16)(p11.23;p12.3) de novo. Fluorescence in situ hybridization (FISH) with bacterial artificial chromosomes were employed for narrowing down the breakpoint regions. On chromosome 16, the breakpoint was mapped in the region of RP11-453F10 (19 920 640-20 118 153 bp from 16pter). On chromosome X, the breakpoint was mapped in the region of RP11-794A15 (47 333 744-47 524 066 bp from Xpter). This is the first case report of sirenomelia associated with translocations. The abnormal phenotype, associated with a balanced translocation, was caused by deletion or breakage of dosage-sensitive genes of the breakpoint, disruption of an imprinted gene, or uniparental disomy. Although the parental origin of normal 16 and der(16) remained undetermined, this case will provide insight into the pathogenetic mechanism of sirenomelia.

Retinoic acid synthesis and functions in early embryonic development

Retinoic acid (RA) is a morphogen derived from retinol (vitamin A) that plays important roles in cell growth, differentiation, and organogenesis. The production of RA from retinol requires two consecutive enzymatic reactions catalyzed by different sets of dehydrogenases. The retinol is first oxidized into retinal, which is then oxidized into RA. The RA interacts with retinoic acid receptor (RAR) and retinoic acid X receptor (RXR) which then regulate the target gene expression. In this review, we have discussed the metabolism of RA and the important components of RA signaling pathway, and highlighted current understanding of the functions of RA during early embryonic development.

Sirenomelia: an epidemiologic study in a large dataset from the International Clearinghouse of Birth Defects Surveillance and Research, and literature review

Sirenomelia is a very rare limb anomaly in which the normally paired lower limbs are replaced by a single midline limb. This study describes the prevalence, associated malformations, and maternal characteristics among cases with sirenomelia. Data originated from 19 birth defect surveillance system members of the International Clearinghouse for Birth Defects Surveillance and Research, and were reported according to a single pre-established protocol. Cases were clinically evaluated locally and reviewed centrally. A total of 249 cases with sirenomelia were identified among 25,290,172 births, for a prevalence of 0.98 per 100,000, with higher prevalence in the Mexican registry. An increase of sirenomelia prevalence with maternal age less than 20 years was statistically significant. The proportion of twinning was 9%, higher than the 1% expected. Sex was ambiguous in 47% of cases, and no different from expectation in the rest. The proportion of cases born alive, premature, and weighting less than 2,500 g were 47%, 71.2%, and 88.2%, respectively. Half of the cases with sirenomelia also presented with genital, large bowel, and urinary defects. About 10-15% of the cases had lower spinal column defects, single or anomalous umbilical artery, upper limb, cardiac, and central nervous system defects. There was a greater than expected association of sirenomelia with other very rare defects such as bladder exstrophy, cyclopia/holoprosencephaly, and acardia-acephalus. The application of the new biological network analysis approach, including molecular results, to these associated very rare diseases is suggested for future studies.

Concerted involvement of Cdx/Hox genes and Wnt signaling in morphogenesis of the caudal neural tube and cloacal derivatives from the posterior growth zone

Decrease in Cdx dosage in an allelic series of mouse Cdx mutants leads to progressively more severe posterior vertebral defects. These defects are corrected by posterior gain of function of the Wnt effector Lef1. Precocious expression of Hox paralogous 13 genes also induces vertebral axis truncation by antagonizing Cdx function. We report here that the phenotypic similarity also applies to patterning of the caudal neural tube and uro-rectal tracts in Cdx and Wnt3a mutants, and in embryos precociously expressing Hox13 genes. Cdx2 inactivation after placentation leads to posterior defects, including incomplete uro-rectal septation. Compound mutants carrying one active Cdx2 allele in the Cdx4-null background (Cdx2/4), transgenic embryos precociously expressing Hox13 genes and a novel Wnt3a hypomorph mutant all manifest a comparable phenotype with similar uro-rectal defects. Phenotype and transcriptome analysis in early Cdx mutants, genetic rescue experiments and gene expression studies lead us to propose that Cdx transcription factors act via Wnt signaling during the laying down of uro-rectal mesoderm, and that they are operative in an early phase of these events, at the site of tissue progenitors in the posterior growth zone of the embryo. Cdx and Wnt mutations and premature Hox13 expression also cause similar neural dysmorphology, including ectopic neural structures that sometimes lead to neural tube splitting at caudal axial levels. These findings involve the Cdx genes, canonical Wnt signaling and the temporal control of posterior Hox gene expression in posterior morphogenesis in the different embryonic germ layers. They shed a new light on the etiology of the caudal dysplasia or caudal regression range of human congenital defects.

A clinical and experimental overview of sirenomelia: insight into the mechanisms of congenital limb malformations

Sirenomelia, also known as sirenomelia sequence, is a severe malformation of the lower body characterized by fusion of the legs and a variable combination of visceral abnormalities. The causes of this malformation remain unknown, although the discovery that it can have a genetic basis in mice represents an important step towards the understanding of its pathogenesis. Sirenomelia occurs in mice lacking Cyp26a1, an enzyme that degrades retinoic acid (RA), and in mice that develop with reduced bone morphogenetic protein (Bmp) signaling in the caudal embryonic region. The phenotypes of these mutant mice suggest that sirenomelia in humans is associated with an excess of RA signaling and a deficit in Bmp signaling in the caudal body. Clinical studies of sirenomelia have given rise to two main pathogenic hypotheses. The first hypothesis, based on the aberrant abdominal and umbilical vascular pattern of affected individuals, postulates a primary vascular defect that leaves the caudal part of the embryo hypoperfused. The second hypothesis, based on the overall malformation of the caudal body, postulates a primary defect in the generation of the mesoderm. This review gathers experimental and clinical information on sirenomelia together with the necessary background to understand how deviations from normal development of the caudal part of the embryo might lead to this multisystemic malformation.

Perosomus elumbis, cerebral aplasia, and spina bifida in an aborted thoroughbred foal

Perosomus elumbis represents a rare congenital anomaly characterized by aplasia of the lumbosacral spinal cord and vertebrae. This anomaly is often associated with arthrogryposis and malformations of the urogenital and intestinal tract. This report describes the first case of perosomus elumbis in an aborted Thoroughbred foal associated with cerebral aplasia with meningocele, cranioschisis, spina bifida, a fused urogenital and intestinal tracts lined by a cutaneous mucosa without uterine glands, atresia ani, and arthrogryposis of the hind legs. Immunohistochemistry detected no abnormalities in the GFAP, S-100, vimentin, NeuN, doublecortin, and neurofilament expression pattern in the remnants of the brainstem. Pathogenic bacteria or equine herpesviruses one and four were not found using a microbiological investigation or polymerase chain reaction, respectively. The observed malformations may be induced by a severe developmental disturbance during the formation of the neural tube with secondary distortion of the surrounding mesoderm. Although the aetiology of this dysorganogenesis remains undetermined, inherited chromosomal mutations, an intake of different xenobiotics, and/or a disturbed metabolism of the mare may have caused the defect.

Glyphosate-based herbicides produce teratogenic effects on vertebrates by impairing retinoic acid signaling

The broad spectrum herbicide glyphosate is widely used in agriculture worldwide. There has been ongoing controversy regarding the possible adverse effects of glyphosate on the environment and on human health. Reports of neural defects and craniofacial malformations from regions where glyphosate-based herbicides (GBH) are used led us to undertake an embryological approach to explore the effects of low doses of glyphosate in development. Xenopus laevis embryos were incubated with 1/5000 dilutions of a commercial GBH. The treated embryos were highly abnormal with marked alterations in cephalic and neural crest development and shortening of the anterior-posterior (A-P) axis. Alterations on neural crest markers were later correlated with deformities in the cranial cartilages at tadpole stages. Embryos injected with pure glyphosate showed very similar phenotypes. Moreover, GBH produced similar effects in chicken embryos, showing a gradual loss of rhombomere domains, reduction of the optic vesicles, and microcephaly. This suggests that glyphosate itself was responsible for the phenotypes observed, rather than a surfactant or other component of the commercial formulation. A reporter gene assay revealed that GBH treatment increased endogenous retinoic acid (RA) activity in Xenopus embryos and cotreatment with a RA antagonist rescued the teratogenic effects of the GBH. Therefore, we conclude that the phenotypes produced by GBH are mainly a consequence of the increase of endogenous retinoid activity. This is consistent with the decrease of Sonic hedgehog (Shh) signaling from the embryonic dorsal midline, with the inhibition of otx2 expression and with the disruption of cephalic neural crest development. The direct effect of glyphosate on early mechanisms of morphogenesis in vertebrate embryos opens concerns about the clinical findings from human offspring in populations exposed to GBH in agricultural fields.

The sirenomelia sequence: a case history

We report a case of sirenomelia sequence observed in an incident of preterm labor during the 29th gestational week. According to some authors, this syndrome should be classified separately from caudal regression syndrome and is likely to be the result of an abnormality taking place during the fourth gestational week, causing developmental abnormalities in the lower extremities, pelvis, genitalia, urinary tract and digestive organs. Despite recent progress in pathology, the etiopathogenesis of sirenomelia is still debated.

Anorectal and urinary anomalies and aberrant retinoic acid metabolism in cytochrome P450 oxidoreductase deficiency

CONTEXT

Cytochrome P450 oxidoreductase (POR) is an electron donor for all microsomal P450 enzymes including CYP26 involved in inactivation of all-trans retinoic acid (atRA). Although previous studies in Por knockout mice suggest that atRA accumulation is relevant to various posterior organ abnormalities, a systematic analysis has not been performed for anorectal and urinary anomalies in patients with POR deficiency (PORD).

OBJECTIVE

To report the frequencies of anorectal and urinary anomalies and plasma atRA values in PORD patients.

PATIENTS

We studied 37 Japanese patients with PORD, consisting of 15 homozygotes for R457H (group A), 15 compound heterozygotes for R457H and one apparently null mutation (group B), and seven patients with other combinations of mutations (group C). Since R457H is a severe hypomorphic mutation, the residual POR function is predicted to be higher in group A than in group B.

RESULTS

Imperforate anus was observed in four patients (10.8%) and vesicoureteral reflux was found in three patients (8.1%), with no significant difference in the frequencies of such anomalies between groups A and B. In addition, a complex urogenital malformation including penile agenesis was identified in one patient. Plasma atRA values were above the reference range in nine of 12 patients examined, and were similar between groups A and B and between patients with and without anomalies.

CONCLUSIONS

The results imply that aberrant atRA metabolism due to CYP26 deficiency underlies various anorectal and urinary anomalies in patients with PORD. Clinical phenotypes may be primarily determined by maternal oral retinol intake during pregnancy, and plasma atRA values may be largely influenced by the amount of postnatal oral retinol intake in such patients.

The chick somitogenesis oscillator is arrested before all paraxial mesoderm is segmented into somites

BACKGROUND

Somitogenesis is the earliest sign of segmentation in the developing vertebrate embryo. This process starts very early, soon after gastrulation has initiated and proceeds in an anterior-to-posterior direction during body axis elongation. It is widely accepted that somitogenesis is controlled by a molecular oscillator with the same periodicity as somite formation. This periodic mechanism is repeated a specific number of times until the embryo acquires a defined specie-specific final number of somites at the end of the process of axis elongation. This final number of somites varies widely between vertebrate species. How termination of the process of somitogenesis is determined is still unknown.

RESULTS

Here we show that during development there is an imbalance between the speed of somite formation and growth of the presomitic mesoderm (PSM)/tail bud. This decrease in the PSM size of the chick embryo is not due to an acceleration of the speed of somite formation because it remains constant until the last stages of somitogenesis, when it slows down. When the chick embryo reaches its final number of somites at stage HH 24-25 there is still some remaining unsegmented PSM in which expression of components of the somitogenesis oscillator is no longer dynamic. Finally, we identify a change in expression of retinoic acid regulating factors in the tail bud at late stages of somitogenesis, such that in the chick embryo there is a pronounced onset of Raldh2 expression while in the mouse embryo the expression of the RA inhibitor Cyp26A1 is downregulated.

CONCLUSIONS

Our results show that the chick somitogenesis oscillator is arrested before all paraxial mesoderm is segmented into somites. In addition, endogenous retinoic acid is probably also involved in the termination of the process of segmentation, and in tail growth in general.

The effects of all-trans retinoic acid on leukocytes in rat's embryo

This study was planned to determine the effects of All-Trans Retinoic Acid (ATRA) on the progenitors of White Blood Cells (WBC) and survey their outcomes in rat's embryo during both late-yolk sac and fetal liver stages of hematopoiesis. Single oral dose (100 mg kg(-1)) of ATRA was administered to rat on Gestation Day (GD) 10 and fetuses were observed on GD 18. The fetus's blood (from experimental group and control each, n = 24) were obtained directly from heart, as placental and mother circulation was continued and subsequently processed for Giemsa staining and followed by WBC counting and measuring. Statistical analysis was made by student t-test. A p-value <0.05 was considered significant. In the experimental embryos on GD 18, the mean number of WBC (29.2%), neutrophil, lymphocyte and monocyte were increased. There was a significant difference in WBC (p<0.0001) and neutrophil (p<0.001) between the groups in this regard. The mean diameter of neutrophil, lymphocyte and monocyte were compared in two groups. The results showed no significant change on experimental and control groups. We concluded that ATRA may have positive effects on proliferation, differentiation and maturation of neutrophil without having any significant effects on the diameter of cells throughout normal granulocyte differentiation in embryo during both late-yolk sac and fetal liver stages of hematopoiesis.

Teratogens inducing congenital abdominal wall defects in animal models

Congenital abdominal wall defects are common anomalies which include gastroschisis, omphalocele and umbilical cord hernia. Recent reports indicate that gastroschisis is increasing in prevalence, whereas omphalocele has remained steady, suggesting that environmental factors may play a part in their pathogenesis. The aim of this study is to review animal teratogen studies resulting in abdominal wall defects to investigate their possible causes. Each report was examined not only for the teratogens causing the defects, but also to carefully identify the defect occurring and its correlation with the known clinical anomalies. We found many discrepancies between the nomenclature used by animal teratology investigators and that used by clinicians. We were able to confirm the induction of gastroschisis by 22 teratogens, omphalocele by 9 teratogens and umbilical cord hernia by 8. There is no doubt that environmental factors may be responsible, at least in part, for all three of the clinical abdominal wall defects. Future studies should take care to appreciate the differences between these anomalies and describe them in detail, so that accurate and meaningful conclusions can be obtained.