Rodent anogenital distance recommendations.

BACKGROUND: Measurement of rat anogenital distance (AGD) dates to at least 1912. Increased interest in endocrine disrupting chemicals and the use of AGD as a biomarker for fetal androgen effects have increased the number of studies with this endpoint in recent decades. A literature review revealed different landmarks, methods of measurement, and methods to adjust for body weight differences. AGD is often reported to hundredths of millimeters and as such, deserves precision in all these aspects. This paper presents recommendations for the measurement and analysis of rodent AGD. METHODS: Literature and regulatory guidance documents that mentioned or measured rodent AGD were reviewed. Four adjustment methods were evaluated using available online data from three rat studies each with two generations of offspring. RESULTS: Tabulation of studies reveals that species/stocks and time of data collection, but more importantly anatomical landmarks and methods of measurement have produced a variety of results which are difficult to compare. Not all studies have adjusted for test article effects on body weight (and thus size). The four adjustment methods were fairly comparable. CONCLUSION: Recommendations are as follows. A microscopic method should be used to measure AGD of late rodent fetuses and early postnatal pups. The caudal edge of the genital tubercle and the cranial edge of the anus are clear and identifiable landmarks. The simplest adjustment is to divide individual AGDs by the cube root of animals' body weight. These recommendations will help ensure data consistency and accuracy, and facilitate meaningful comparisons across laboratories and chemical classes.

Prenatal diagnosis of persistent left superior vena cava, polyhydramnios and a small gastric bubble in a fetus with VACTERL association.

OBJECTIVE: We reported a fetus that presenting with persistent left superior vena cava (PLSVC), polyhydramnios, and a small gastric bubble during prenatal examination and identified VACTERL association after birth. CASE REPORT: A 34-year-old woman underwent amniocentesis at 18 weeks of gestation because of advanced maternal age and the result was normal. Subsequently, an ultrasound revealed single umbilical artery (SUA) at 21 weeks of gestation. She received a detailed fetal anatomy survey that presented the same findings and PLSVC. A small visible gastric bubble was noted at that time, and the other organs were unremarkable. Polyhydramnios was identified at 30 weeks of gestation and amnioreduction was subsequently performed at 32 weeks of gestation. However, polyhydramnios was persisted despite amnioreduction and intrauterine growth restriction was also detected. A cesarean section was performed because of fetal distress at 36 + 2 weeks, and a 1832-g female baby was delivered. Pre-axial polydactyly at left thumb, SUA and esophageal atresia with distal tracheoesophageal fistula (TEF) were identified after birth. The neonate died at age of 4 days because of surgical complication following esophageal anastomosis. CONCLUSION: Prenatal diagnosis of PLSVC associated with polyhydramnios and a small gastric bubble may indicate esophageal atresia with TEF, and further examination for associated syndromes such as VACTERL association is warranted.

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.

Imaging of anorectal malformations in utero.

PURPOSE: To document the imaging findings suggestive of anorectal malformation (ARMs) on prenatal US and MRI. METHODS: Retrospective evaluation of the screening US and prenatal MRI exams of the rectum and ano-perineal region in normal fetuses and in patients with ARMs. RESULTS: Examples showing the normal rectal and anoperineal anatomy on prenatal US and MRI exams and the imaging findings observed in different types of confirmed ARMS. CONCLUSIONS: Prenatal diagnosis of ARMs requires both a systematic evaluation of the fetal pelvis and perineum and an appropriate knowledge of its suggestive imaging findings.

Developmental biology meets toxicology: contributing reproductive mechanisms to build adverse outcome pathways.

An adverse outcome pathway (AOP) is a simplified description of the sequence of mechanistic events that lead to a particular toxicological effect, from initial trigger to adverse outcome. Although designed to inform regulatory risk assessors, the AOP framework also provides a platform for innovative collaborations between experts from relevant research fields and the regulatory community. The underpinning for any AOP is basic knowledge about molecular and developmental processes; such knowledge can only be attained by solid bioscientific research. Starting with this fundamental knowledge, the objective is to devise novel testing strategies that focus on key events in a causative pathway. It is anticipated that such a knowledge-based approach will ultimately alleviate many of the burdens associated with classical chemical testing strategies that typically involve large-scale animal toxicity regimens. This hails from the notion that a solid understanding of the underlying mechanisms will allow the development and use of alternative test methods, including both in vitro and in silico approaches. This review is specifically targeted at professionals working in bioscientific fields, such as developmental and reproductive biology, and aims to (i) inform on the existence of the AOP framework and (ii) encourage new cross-disciplinary collaborations. It is hoped that fundamental biological knowledge can thus be better exploited for applied purposes: firstly, an improved understanding of how our perpetual exposure to environmental chemicals is causing human reproductive disease and, secondly, new approaches to screen for harmful chemicals more efficiently. This is not an instructional manual on how to create AOPs; rather, we discuss how to harness fundamental knowledge from the biosciences to assist regulatory toxicologists in their efforts to protect humans against chemicals that harm human reproductive development and function.

Fetal anogenital distance using ultrasound.

OBJECTIVE: This study measured anogenital distance (AGD) during late second/early third trimester of pregnancy to confirm previous findings that AGD can be measured noninvasively in the fetus using ultrasound and further showed differences in reference ranges between populations. METHOD: Two hundred ten singleton pregnancies were recruited at the Rosie Hospital, Cambridge, UK. A 2D ultrasound was performed between 26 and 30 weeks of pregnancy. AGD was measured from the centre of the anus to the base of the scrotum in males and to the posterior convergence of the fourchette in females. RESULTS: A significant difference in AGD between males and females (P < .0001) was found, replicating previous results with a significant correlation between estimated fetal weight (EFW) and AGD in males only (P = .006). A comparison of AGD using reference data from an Israeli sample (n = 118) and our UK sample (n = 208) showed a significant difference (P < .0001) in both males and females, after controlling for gestational age (GA). CONCLUSION: Our results confirm that AGD measurement in utero using ultrasound is feasible. In addition, there are strong sex differences, consistent with previous suggestions that AGD is influenced by prenatal androgen exposure. AGD lengths differ between the UK and Israel; therefore, population-specific normative values may be required for accurate clinical assessments.

Distinct Transcriptional Profiles of the Female, Male, and Finasteride-Induced Feminized Male Anogenital Region in Rat Fetuses.

A short anogenital distance (AGD) in males is a marker for incomplete masculinization and a predictor of adverse effects on male reproductive health. For this reason, AGD is used to assess the endocrine disrupting potential of chemicals for risk assessment purposes. The molecular mechanisms underpinning this chemically induced shortening of the AGD, however, remains unclear. Although it is clear that androgen receptor-mediated signaling is essential, evidence also suggest the involvement of other signaling pathways. This study presents the first global transcriptional profile of the anogenital tissue in male rat fetuses with chemically induced short AGD, also including comparison to normal male and female control animals. The antiandrogenic drug finasteride (10 mg/kg bw/day) was used to induce short AGD by exposing time-mated Sprague Dawley rats at gestation days 7-21. The AGD was 37% shorter in exposed male fetuses compared with control males at gestation day 21. Transcriptomics analysis on anogenital tissues revealed a sexually dimorphic transcriptional profile. More than 350 genes were found to be differentially expressed between the 3 groups. The expression pattern of 4 genes of particular interest (Esr1, Padi2, Wnt2, and Sfrp4) was also tested by RT-qPCR analyses, indicating that estrogen and Wnt2 signaling play a role in the sexually dimorphic development of the anogenital region. Our transcriptomics profiles provide a stepping-stone for future studies aimed at characterizing the molecular events governing development of the anogenital tissues, as well as describing the detailed Adverse Outcome Pathways for short AGD; an accepted biomarker of endocrine effects for chemical risk assessment.

Prenatal Evaluation for Detection of Anorectal Atresia: Value of Ultrasound.

OBJECTIVES: To investigate the applicability and value of ultrasound (US) in the diagnosis of anorectal atresia. METHODS: Between January 2008 and January 2016, we prospectively evaluated 63,101 fetuses (gestational age, 20-38 weeks), including low- and high-risk populations using 2-dimensional US scans. An abnormal imaging finding was defined as an anal canal diameter of less than the 95% confidence interval (small anal canal) of the normal range or the absence of an anal canal and rectum. Imaging findings were considered normal on detection of an anal canal with a normal width and the absence of abnormalities. Prenatal imaging findings were confirmed by a postnatal or postmortem examination. RESULTS: Among the investigated fetuses, 28 showed evidence of anorectal atresia on US scans, and 22 of those with anorectal atresia had additional anomalies. Six cases of isolated anorectal atresia were successfully detected during the preclusive prenatal US scans. Four cases of a low imperforate anus (including 2 covered anuses) yielded false-negative results, indicating a diagnostic rate of 87.5% (28 of 32). The normal appearance of the fetal rectum and anal canal ruled out anorectal atresia in 30 fetuses with a dilated colon. Additionally, there were 3 false-positive cases, in which a narrow anal canal was observed. CONCLUSIONS: Identifying the abnormal appearance or absence of the fetal anal canal and rectum on preclusive US anomaly scans is useful for prenatal diagnosis or exclusion of anorectal atresia, which may help improve the detection of isolated anorectal atresia. Furthermore, a combined evaluation of the longitudinal and axial appearances of the fetal anal canal and rectum can improve diagnostic accuracy.

Prenatal exposure to persistent organic compounds and their association with anogenital distance in infants.

RESEARCH QUESTION: What is the association between prenatal exposure to persistent organic pollutants, separately and combined, and anogenital distance (in-utero endocrine disruption marker). DESIGN: A cohort study conducted in Sonora, Mexico. Blood concentrations of polychlorobiphenyls (PCB) 28, 74, 118, 138/158, 153, 170, 180 and the isomers of dichlorodiphenyltrichloroethane (DDT) and its metabolites were determined in women in the third trimester of pregnancy; three variants of anogenital distance were measured on five occasions during the first year of life of their infants: 82 girls (402 observations) and 74 boys (356 observations). RESULTS: Boys had negative and significant associations between anogenital distance/height and the concentrations of PCB 28 (beta = - 0.005;P = 0.006), PCB 74 (beta = - 0.003;P = 0.013), and PCB 170 (beta = - 0.005;P = 0.001) when analysed individually. Negative and significant associations were also found using statistical models applied to mixtures of compounds. The latter associations were sometimes larger in magnitude and significance, suggesting a possible potentiation of the compounds. No associations were observed between anogenital distance and DDT in either sex or with PCB in girls. CONCLUSIONS: The decreased anogenital distance associated with prenatal exposure to the persistent organic pollutants, observed consistently in different analyses, suggests an under-masculinizing effect of these environmental pollutants in boys.

First trimester expression of anorectal malformation: Case report and review of the literature.

A 26-years-old woman, underwent an ultrasound examination at 13.4 weeks. A cystic structure was identified in the right lower abdomen. Gradually, the cystic mass was replaced by echogenic content and eventually attained the appearance of hyperechoic bowel. At 21.2 weeks, the anal sphincter could not be demonstrated which was consistent with the diagnosis of isolated anal agenesis. Amniocentesis revealed 46XY karyotype with normal comparative genomic hybridization. After termination of pregnancy at 23 weeks, an autopsy revealed an isolated high type anorectal malformation (ARM) without fistula. We reviewed all 14 cases reported in the literature of first trimester sonographic expression of ARM.

Transient Distention of Right Posterior Located Sigma, a New Sonographic Sign for the Prenatal Diagnosis of Anal Atresia.

We describe a new sonographic sign for the detection of anal atresia in the early midtrimester on transvaginal sonography. In six cases of fetal anal atresia, the finding of a transient, distended, and right-sided sigmoid colon was observed at 13-16 weeks' gestation. Three cases have undergone pregnancy termination due to multiple anomalies. In the other three, the colonic distension resolved by 19 weeks' gestation. In two of these, the finding was isolated, and no other anomalies were detected. In all six cases, anal atresia or cloaca was confirmed on postabortal autopsy or after delivery. © 2016 Wiley Periodicals, Inc. J Clin Ultrasound 45:160-162, 2017.

First-trimester determination of fetal gender by ultrasound: measurement of the ano-genital distance.

INTRODUCTION: Early ultrasound fetal sex determination is of obvious interest, particularly in the context of X-linked diseases. In the human, the anogenital distance, i.e., the distance between the caudal end and the base of the genital tubercule is sexually dimorphic. This difference is apparent from 11 weeks of gestation. The aim of this prospective study was to evaluate the accuracy of anogenital distance measurement during the first trimester ultrasound in the early determination of fetal gender. MATERIALS AND METHODS: Fetal gender was assessed by ultrasound in 310 singleton pregnancies at 11-14 weeks of gestation. The optimal cut-off was determined by the minimal p-value technic and validated using bootstrap simulation. RESULTS: 310 women were included. A cut-off of 4.8mm was determined to predict male (≥4.8mm) or female (<4.8mm) fetuses. Sex was correctly determined for 87% of the males and 89% of the females. The inter-observer variability was excellent. CONCLUSION: This study presents a new sonographic sign for early fetal sex determination that has not been previously explored. It appears to be an accurate tool but it requires further validation in larger series.

Anatomical basis of the coordination between smooth and striated urethral and anal sphincters: loops of regulation between inferior hypogastric plexus and pudendal nerve. Immuno-histological study with 3D reconstruction.

PURPOSE: (1) Describe both nervous pathways to the sphincters, and highlight the anatomical support of their coordination. (2) Obtain a 3D representation of this complex innervation system. METHODS: A computer-assisted anatomical dissection technique was used. Serial histological sections were cut in the pelvis of four female human foetuses (aged 19-32 weeks of gestation). The sections were treated with conventional staining, and with seven different immunostainings. The sections were digitalized and, finally, a 3D representation was built from the corresponding images. RESULTS: Myelinated and sensory fibres were detected at the inferior hypogastric plexus (IHP) level. Our analysis showed that most of the afferent sensory fibres come from the urinary and anal sphincters through the anterior and posterior branches of the IHP respectively. A highly positive nitrergic (anti-NOS1) and sensitive (anti-CGRP) labelling was found in the external layer of the urethral sphincter. The 3D representation allowed describing the two components of the innervation system. A sensory-motor regulation loop was found for both sphincters. CONCLUSION: A 3D description of the components of both nervous pathways to the sphincters has been established. Our findings on the innervation of the sphincters tend to question the classical infra/supra levatorian muscle description. The coordinated work of the internal and external layers of the anal and urethral sphincter is probably mediated by multiple roles regulation.

Tracheal Atresia with Segmental Esophageal Duplication: An Unusual Anatomic Arrangement.

An unusual anatomic configuration of segmental tracheal agenesis/atresia with esophageal duplication on autopsy in a fetus that demised in utero at 29 weeks is reported. The mother was scanned initially for a cardiac anomaly at 20 weeks and on follow-up scan at 27 weeks had polyhydramnios and underwent amnioreduction. The final autopsy diagnosis was vertebral, ano-rectal, cardiac, tracheoesophageal, renal, and limb malformations (VACTERL). We discuss the autopsy findings along with the embryological mechanisms and compare the configuration with Floyd's classification for tracheal agenesis. The difficulties in prenatal diagnosis are discussed.

Clarification of mammalian cloacal morphogenesis using high-resolution episcopic microscopy.

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

Fetal growth of the anal sinus and sphincters, especially in relation to anal anomalies.

PURPOSE AND METHODS: The anal sinuses, small furrows above the pectinate line, sometimes form perianal abscesses in adults. We examined the pattern of fetal growth of the anal sinus and sphincters using 22 mid-term (8-18 weeks) and 6 late-stage (30-38 weeks) fetuses. RESULTS: In mid-term fetuses, the external and internal sphincters gradually increased in thickness, depending on specimen size (from 0.2 to 1.5 mm), whereas the anteroposterior diameter of the anal canal at the epithelial junction was relatively stable (0.5-1.0 mm) irrespective of specimen size. Anal canal diameter increased less than twofold between mid-term and late-stage fetuses, from 0.5-1.0 to almost 2 mm, whereas sphincter thickness increased over tenfold, from 0.2-1.5 to almost 3.5 mm. The anal sinus often showed balloon-like enlargement when the sphincter muscle bundles were tightly packed in mid-term, but not in late-stage fetuses. CONCLUSIONS: Large concentric mechanical stress from the sphincters in late-stage fetuses apparently prevented the anal sinus from expanding in a balloon-like manner. Conversely, to avoid anal stenosis, the growing sinuses maintained a luminal space of the anal canal in response to stress from rapidly growing sphincters. The inferiorly extending sinus usually provided temporal double canals separated by a thick column. In the presence of double lumens, anal canal duplication is likely to develop without any abnormalities of the anal epithelium and sphincters.

Is it possible folic acid reduce anorectal malformations ethylenethiourea induced in rats?

PURPOSE: To investigate the effect of folic acid (FA) in an experimental model of anorectal malformations (ARMs) ethylenethiourea (ETU) induced. METHODS: Eight female Wistar rats were divided randomly in two groups. Group A - ETU; Group B - FA+ETU; Dams from group B received daily, since two weeks before pregnancy to the end of pregnancy, FA (50mg/kg) by gavage. Dams from groups A and B, received 1% ETU (125 mk/kg) by gavage on gestational day (GD) 11. Their fetuses were harvested by cesarean section on GD21 and were examined looking for ARMs. The thickness of anal stratified squamous epithelium (ASSE) and intestinal epithelium (IE) were analyzed. p < 0.05*. RESULTS: One hundred and one embryos were harvested. The number of embryos; number of ARMs; mean statistical % (± SD) were determined to be, respectively: ETU - 49 [30;65% (± 24%)] versus FA+ETU - 52 [1;02% (± 3%)] (p = 0.025). AMRs were significantly lower in FA+ETU group than in ETU group (p = 0.025). The thickness (µm) of ASSE (± SD) and IE (± SD) were measured, respectively: ETU - [27.75 (± 0.56) and 18.88 (± 0.93)] versus FA+ETU - [28.88 (± 0.61) and 21.11 (± 0.16)] (p = 0.001). The thickness of IE was significantly enlarged when FA was given (p=0.001). CONCLUSION: Folic acid reduces the number and enlarged the IE of ARMs ETU-induced.

Is it possible folic acid reduce anorectal malformations ethylenethiourea induced in rats?

PURPOSE: To investigate the effect of folic acid (FA) in an experimental model of anorectal malformations (ARMs) ethylenethiourea (ETU) induced.METHODS:Eight female Wistar rats were divided randomly in two groups. Group A - ETU; Group B - FA+ETU; Dams from group B received daily, since two weeks before pregnancy to the end of pregnancy, FA (50mg/kg) by gavage. Dams from groups A and B, received 1% ETU (125mk/kg) by gavage on gestational day (GD) 11. Their fetuses were harvested by cesarean section on GD21 and were examined looking for ARMs. The thickness of anal stratified squamous epithelium (ASSE) and intestinal epithelium (IE) were analyzed. p<0.05*.RESULTS:One hundred and one embryos were harvested. The number of embryos; number of ARMs; mean statistical % (± SD) were determined to be, respectively: ETU - 49 [30;65% (±24%)] versus FA+ETU - 52 [1;02% (±3%)] (p=0.025). AMRs were significantly lower in FA+ETU group than in ETU group (p=0.025). The thickness (µm) of ASSE (± SD) and IE (± SD) were measured, respectively: ETU - [27.75 (±0.56) and 18.88 (±0.93)] versus FA+ETU - [28.88 (±0.61) and 21.11 (±0.16)] (p=0.001). The thickness of IE was significantly enlarged when FA was given (p=0.001).CONCLUSION:Folic acid reduces the number and enlarged the IE of ARMs ETU-induced.

Evaluation of the development of the fetal anal sphincter with tomography ultrasonography imaging.

The aim of the study described here was to examine the potential of tomography ultrasonography imaging (TUI) in evaluation of the fetal anal sphincter. In this prospective cross-sectional study of the fetal anal sphincter with TUI, 326 singleton pregnancies (mean age = 28 y, range: 22-38 y) were scanned at 19-40 wk of gestation. The fetal anal region and ischium were revealed in 320 of 326 patients (98.2%). The normal fetal anal sphincter diameter and ischial space reached maximums of 15 and 39 mm, respectively. The normal fetal anal sphincter diameter and the ischial space were plotted as a function of gestational age (GA) on a linear curve, and the regression equations for normal fetal anal sphincter diameter and ischial space as a function of GA in weeks were obtained. A scatterplot was also created that revealed a significant positive relationship between normal fetal anal sphincter diameter and ischial space. On the basis of these criteria, imperforate anus was diagnosed in one fetus. Ultrasonographic assessment of the fetal anal sphincter and the ischium with TUI is feasible. The reference values reported in this article may be useful in prenatal diagnosis of fetal anal sphincter abnormalities.

Developmental alterations of the C. elegans male anal depressor morphology and function require sex-specific cell autonomous and cell non-autonomous interactions.

We studied the Caenorhabditis elegans anal depressor development in larval males and hermaphrodites to address how a differentiated cell sex-specifically changes its morphology prior to adulthood. In both sexes, the larval anal depressor muscle is used for defecation behavior. However in the adult males, the muscle's sarcomere is reorganized to facilitate copulation. To address when the changes occur in the anal depressor, we used YFP:actin to monitor, and mutant analysis, laser-ablation and transgenic feminization to perturb the cell's morphological dynamics. In L1 and L2 stage larva, the muscle of both sexes has similar sarcomere morphology, but the hermaphrodite sex-determination system promotes more growth. The male anal depressor begins to change in the L3 stage, first by retracting its muscle arm from the neurons of the defecation circuit. Then the muscle's ventral region develops a slit that demarcates an anterior and posterior domain. This demarcation is not dependent on the anal depressor's intrinsic genetic sex, but is influenced by extrinsic interactions with the developing male sex muscles. However, subsequent changes are dependent on the cell's sex. In the L4 stage, the anterior domain first disassembles the dorsal-ventral sarcomere region and develops filopodia that elongates anteriorly towards the spicule muscles. Later, the posterior domain dissembles the remnants of its sarcomere, but still retains a vestigial attachment to the ventral body wall. Finally, the anterior domain attaches to the sex muscles, and then reassembles an anterior-posteriorly oriented sarcomere. Our work identifies key steps in the dimorphic re-sculpting of the anal depressor that are regulated by genetic sex and by cell-cell signaling.