Analysis of the clinical outcomes of fetal bowel dilatation combined with other abnormal ultrasonographic features.

OBJECTIVES: To explore the significance of fetal bowel dilatation combined with other abnormal ultrasound features in the diagnosis of gastrointestinal malformation. METHODS: A retrospective study of fetuses with bowel dilatation was performed, from August 2012 to October 2015. All the cases were identified from the ultrasound database and all observations of the relationship of prenatal abnormal abdominal ultrasound features and intestinal malformation were performed through the infancy stage. RESULTS: We found 52 fetuses with prenatal suspicion of bowel dilatation. Of these, 20 cases were surgically confirmed to have intestinal malformation, 13 cases had no abnormal bowel loops after birth, 8 cases had abnormal intestinal features while no surgical intervention was performed after birth, 10 cases were lost to follow-up and 1 fetus died in utero at 34 weeks of gestation. Forty cases with full data were divided into three groups, including Group A (Small bowel dilatation combined with other features vs. Isolated small bowel dilatation), Group B (Colonic bowel dilatation combined with other features vs. Isolated colonic bowel dilatation) and Group C (Bowel dilatation combined with other features vs. Isolated bowel dilatation). The intestinal malformation occurrence rates were 73.33% vs. 31.25% in Group A, 50% vs. 25% in Group B, and 70% vs. 30% in Group C. These results suggest that malformation occurs at a lesser frequency in simple bowel dilatation versus bowel dilatation in combination with other abnormal ultrasound features (p = .026), similarly in simple small bowel dilatation versus small bowel dilatation in combination with other abnormal ultrasound features (p = .032). CONCLUSIONS: Prenatal bowel dilatation in combination with other abnormal ultrasound features, especially small bowel dilatation in combination with other abnormal ultrasound features, detected in the second and third trimesters, tended to indicate intestinal malformation, which contributes to enhance the accuracy of prenatal diagnosis of intestinal malformation.

Prenatal diagnosis of antenatal midgut volvulus: Specific ultrasound features.

OBJECTIVE: To assess specific, direct, and indirect prenatal ultrasound features in cases of fetal midgut volvulus. METHODS: Retrospective case series of neonatal volvulus, based on postnatal and prenatal imaging findings that occurred from 2006-2017. Prenatal and postnatal signs including the specific "whirlpool sign" were computed. Postnatal volvulus was confirmed by pathology examination after surgery or neonatal autopsy. RESULTS: Thirteen cases of midgut volvulus were identified. Though not a specific sign, a decrease in active fetal movements was reported in eight patients (61.5%). The prenatal whirlpool sign was directly seen in 10 cases, while an indirect but suggestive sign, a fluid-filled level within the dilated loops, was present in five cases. No intestinal malrotation was observed. Pregnancy outcomes were two terminations of pregnancy, both associated with cystic fibrosis, one early neonatal death, three prenatal spontaneous regressions, and seven favorable outcomes after neonatal surgery with resection of midgut atresia. CONCLUSIONS: Identification of the whirlpool sign or of a fluid-filled level within the dilated loops improves the accuracy of ultrasound findings for suspected volvulus. In the absence of total volvulus (in cases of intestinal malrotation) or association with cystic fibrosis, the prognosis appears good.

Novel variant of reversed midgut rotation - retro-arterial proximal jejunum and transverse colon: a case report and review of the literature.

BACKGROUND: Reversed rotation of the midgut is the rarest variation of midgut malrotations, which are congenital disorders that result from aberrant rotation and fixation of the midgut during embryological development. Common complications of these disorders are small bowel obstruction by volvulus or peritoneal bands, usually occurring in early infancy. CASE PRESENTATION: A 23-year-old Caucasian woman presented with recurrent abdominal pain. A contrast-enhanced multidetector computed tomography study revealed a novel variant of reversed rotation of the midgut. Besides the specific finding of a retro-arterial transverse colon, we also found the proximal jejunum to cross posterior to the mesenteric root, a variation that has not been reported in the literature so far. In this case, substantial symptomatic relief was achieved with conservative management. CONCLUSIONS: The hypothesis of a double reversed rotation of the pre-arterial segment of the umbilical loop around the superior mesenteric artery axis provides a possible explanation for this anomaly. There is no evidence-based consensus on the management of patients presenting with non-symptomatic or mildly symptomatic intestinal malrotations. In this case, radiologic and clinical presentations excluded acute small bowel obstruction, and surgical intervention was avoided.

Drosophila left/right asymmetry establishment is controlled by the Hox gene abdominal-B.

In Drosophila, left/right (LR) asymmetry is apparent in the directional looping of the gut and male genitalia. The dextral orientation of the organs depends on the activity of a single gene, MyosinID (myoID), whose mutation leads to a fully inverted LR axis, thus revealing the activity of a recessive sinistral pathway. Here, we present the identification of the Hox gene Abdominal-B (Abd-B) as an upstream regulator of LR determination. This role appears distinct from its function in anteroposterior patterning. We show that the Abd-Bm isoform binds to regulatory sequences of myoID and controls MyoID expression in the organ LR organizer. Abd-Bm is also required for the sinistral pathway. Thus, when Abd-B activity is missing, no symmetry breaking occurs and flies develop symmetrically. These findings identify the Hox gene Abd-B as directing the earliest events of LR asymmetry establishment in Drosophila.

Embryonic exposure to propylthiouracil disrupts left-right patterning in Xenopus embryos.

Antithyroid medications are the preferred therapy for the treatment of Graves' disease during pregnancy. Propylthiouracil (PTU) is favored over methimazole (MMI) due to potential teratogenic concerns with MMI. This study was to determine the teratogenic potential of MMI and PTU using a validated Xenopus tropicalis embryo model. Embryos were exposed to 1 mM PTU (EC(50)=0.88 mM), 1 mM MMI, or vehicle control (water) from stages 2 to 45. Treated embryos were examined for gross morphological defects, ciliary function, and gene expression by in situ hybridization. Exposure to PTU, but not MMI, led to cardiac and gut looping defects and shortening along the anterior-posterior axis. PTU exposure during gastrulation (stage 8-12.5) was identified as the critical period of exposure leading to left-right (LR) patterning defects. Abnormal cilia polarization, abnormal cilia-driven leftward flow at the gastrocoel roof plate (GRP), and aberrant expression of both Coco and Pitx2c were associated with abnormal LR symmetry observed following PTU exposure. PTU is teratogenic during late blastula, gastrulation, and neurulation; whereas MMI is not. PTU alters ciliary-driven flow and disrupts the normal genetic program involved in LR axis determination. These studies have important implications for women taking PTU during early pregnancy.

Peritoneal bands: a review of anatomical distribution and clinical implications.

The complexity of embryological development of the gastrointestinal tract and mesentery provides a platform for the formation of a wide variety of variant veils, folds, and membranes, collectively termed peritoneal bands. These structures, which represent anatomically unabsorbed portions of the omentum and mesentery, although often benign, have the potential to cause clinically significant manifestations in both the neonate and adult. Although these deviant structures may be identified over a broad range of the abdominal cavity, they are most commonly identified in the regions of the duodenum, duodenojejunal flexure, ileocecal junction, and ascending colon. As a result of the diverse location of these variant structures, clinical manifestations are highly variable, ranging from acute presentations of intestinal necrosis as a result of strangulated midgut volvulus to chronic, vague abdominal pain. This article seeks to highlight the importance of a thorough anatomical understanding of the distribution of the various abnormal peritoneal folds, bands, and ligaments, which may result from aberrations in embryonic gastrointestinal development and their respective clinical implications. Moreover, to advance the knowledge of peritoneal bands, this article discusses the appropriate diagnostic studies and treatment interventions required for these variant structures.

Abnormal development of intrinsic innervation in murine embryos with anorectal malformations.

INTRODUCTION: Constipation, soiling, and incontinence are common problems after definitive repair of anorectal malformations (ARMs) in children. We studied the expression of substance P (SP), vasoactive intestinal peptide (VIP), and c-kit in the rectum of murine embryos with or without ARMs at later developmental stages. METHODS: On the 9th embryonic day (E9), pregnant Institute of Cancer Research mice were fed etretinate, a synthetic vitamin A analogue (60 mg/kg), whereas controls were fed only with sesame oil. Embryos were excised between E14 and E18, and prepared for histological examination. The SP, VIP, and c-kit expressions were examined by immunohistochemical staining for the SP, VIP, and c-kit antigens, respectively. RESULTS: On E14 and E15, the expression levels of the anti-SP and anti-VIP antibodies in the rectum did not differ between the control and etretinate-treated group. However, as compared to the controls, a decreased SP and VIP immunoreactivity was observed in the circular muscle layer of the rectum between E16 and E18. On the other hand, on E14 and E15, the expression of anti-c-kit antibody in the rectum did not differ between the etretinate-treated and control group. However, c-kit immunoreactivity was slightly higher in the circular muscle layer of the rectum in the controls on E16 and E17, and considerably higher on E18 than that of the muscle layer in the etretinate-treated group. CONCLUSION: At later developmental stages, the expression levels of SP, VIP, and c-kit reduced in the circular muscle layer of the rectum in mice with etretinate-induced ARMs. This result indicates that reduced SP, VIP, and c-kit expression levels in the circular muscle layer may cause severe constipation in children who develop severe ARMs after definitive surgery.

Abnormal notochord branching is associated with foregut malformations in the adriamycin treated mouse model.

Oesophageal atresia (OA) and tracheooesophageal fistula (TOF) are relatively common human congenital malformations of the foregut where the oesophagus does not connect with the stomach and there is an abnormal connection between the stomach and the respiratory tract. They require immediate corrective surgery and have an impact on the future health of the individual. These abnormalities are mimicked by exposure of rat and mouse embryos in utero to the drug adriamycin. The causes of OA/TOF during human development are not known, however a number of mouse mutants where different signalling pathways are directly affected, show similar abnormalities, implicating multiple and complex signalling mechanisms. The similarities in developmental outcome seen in human infants and in the adriamycin treated mouse model underline the potential of this model to unravel the early embryological events and further our understanding of the processes disturbed, leading to such abnormalities. Here we report a systematic study of the foregut and adjacent tissues in embryos treated with adriamycin at E7 and E8 and analysed between E9 and E12, comparing morphology in 3D in 149 specimens. We describe a spectrum of 8 defects, the most common of which is ventral displacement and branching of the notochord (in 94% of embryos at E10) and a close spatial correspondence between the site of notochord branching and defects of the foregut. In addition gene expression analysis shows altered dorso-ventral foregut patterning in the vicinity of notochord branches. This study shows a number of features of the adriamycin mouse model not previously reported, implicates the notochord as a primary site of disturbance in such abnormalities and underlines the importance of the model to further address the mechanistic basis of foregut congenital abnormalities.

Fisiopatología de la enterocolitis necrotizante (ECN) / Pathophysiology of necrotizing enterocolitis (NEC)

La enterocolitis necrotizante (ECN) es la emergencia gastrointestinal más frecuente en recién nacidos (RN)de pretérmino; pese a ello su patogenia es aún motivo de investigación; el tratamiento es difícil y frecuentementeha probado no ser el más adecuado y hasta el momento no se ha encontrado una estrategia de prevención realmenteeficaz.1 A pesar de que el uso de corticoides prenatales podría disminuir su incidencia,2 como observamos ennuestro laboratorio, no tendría la misma acción en los seres humanos. En este contexto de comprensión de las bases biológicas de la génesis de la entidad enmarcamos la presente revisión bibliográfica.

[Intestinal malrotation: genetics features and other congenital malformations in children].

The article is devoted to features of prenatal development of children with intestinal malrotation. Mass, height, head and chest circumferences of children after birth have been studied. In addition all associated malformations, a blood group (ABO system) and the rhesus-factor have been investigated. In comparison to the control group of newborn the disorders of prenatal development have included associated malformations, low anthropometric data after birth, decreasing of pA gene frequency. Notable, children with intestinal malrotation have had high frequency of malformations as in the abdominal and thoracic cavities as outside intestinal innervations: the small pelvis, the facial skeleton, the upper and lower extremity.

Embryonic development of the internal anal sphincter in rats with anorectal malformations.

BACKGROUND/PURPOSE: The embryogenesis of the internal anal sphincter (IAS) in anorectal malformations (ARMs) remains unclear. This study aimed to investigate the development of the smooth muscle in the terminus of the digestive tract in normal and abnormal rats. METHODS: Rat embryos with ARMs were generated by administration of ethylenethiourea to pregnant rats. The normal rat embryos and embryos with ARMs from E13.5 to E21 were serially sectioned in the sagittal plane and stained immunohistochemically using specific antibody to α-smooth muscle actin (SMA). Temporospatial study was carried out on circular muscle of the distal portion of the hindgut. RESULTS: α-Smooth muscle actin immunolabeling cells could not be observed in the hindgut on E13.5, E14, and E14.5. On E15, there were α-SMA immunolabeling circular muscle cells in the hindgut; and the distal portion of the circular muscle was not thickened in the normal and ARMs rats. From E16 onward, the smooth muscle with slight dilated terminus, which was characterized by the features of IAS, could be noted in the primitive anorectum. In the normal group, the circular muscle in the distal portion of the hindgut thickened slightly and became the musculature with shutter-like bundles. In the ARMs group, the α-SMA immunolabeling myogenic precursors of the smooth muscle could be observed in the primitive anorectum as well. The musculature was similar to that in the normal group. On E15 and E16, there was no significant difference in the development of the circular muscle in the 2 groups. Moreover, the terminus of the circular muscle in the hindgut did not reach the orificium fistulae in ARMs rats. From E17 onward, in ARMs rats, the funnel-shaped distal hindgut communicated the genitourinary tract with a narrow fistula; the dilated musculature at this portion thinned gradually and formed an acute angled extremity in the ARMs group rather than formed blunt extremity in the normal group; the terminus circular muscle in the dorsal hindgut reached the orificium fistulae. During the following gestational days, the circular muscle of the hindgut in both normal and ARMs rats continued its own tendency. CONCLUSION: The IAS primordium started to appear at the terminus of the hindgut on E15 in the 2 groups. The IAS in the ARMs group failed to develop as well as that in the normal group. The IAS dysplasia occurred in the late embryonic development (E17-E21).

Embryology of anorectal malformations.

Today, the normal and abnormal development of the hindgut is still a matter of speculation. However, as the result of recent studies in appropriate animal models, most embryologic events that finally lead to abnormal hindgut development are better known than in the past: (1) the process of maldevelopment starts in early embryonic stages; (2) the cloacal membrane is always too short in its dorsal part, thus, the dorsal cloaca is missing; and (3) as a result, the hindgut remains attached to the sinus urogenitalis, forming the recto-urethral fistula. In the past, an impaired process of septation was believed to be the main cause of abnormal hindgut development. In contrast to this, our results indicate that the development of the septum is more passive than active. Furthermore, the results of our studies in normal and abnormal development indicate that (1) the embryonic cloaca never passes through a stage that is similar to any form of anorectal malformation in neonates, including the so-called "cloacas" in female embryos, and (2) to explain abnormal development, studies in abnormal embryos are mandatory.

Hoxd-13 expression in the development of hindgut in ethylenethiourea-exposed fetal rats.

PURPOSE: Hoxd-13, as one of the most posterior genes among Hox genes, was reported to play a critical role in the development of the most posterior alimentary canal in vertebrates. This study investigated the expression pattern of Hoxd-13 in the hindgut development of the normal and ethylenethiourea (ETU)-exposed rat embryos with anorectal malformations (ARMs) to find out the possible role of Hoxd-13 in the hindgut development and anorectal morphogenesis. MATERIAL AND METHOD: The ETU murine model of ARMs was used via ETU 1% (125 mg/kg) on gestational day (gD) 10. Embryos were harvested via cesarean delivery on gD13 to gD21. Temporal and spatial expression of Hoxd-13 was evaluated in the normal fetal rats (n = 215) and ARMs rats (n = 218) using immunohistochemistry staining, reverse transcriptase polymerase chain reaction, and Western blot analysis. RESULTS: Immunohistochemistry staining revealed that Hoxd-13 expression was confined to the epithelium of the hindgut, cloacal membrane, and urogenital sinus as well as the mesenchyme of the urorectal septum at all gestations in the normal group; however, in the ARMs group, the signal specific for Hoxd-13 was weak in the epithelium of the hindgut and cloacal membrane as well as the mesenchyme of the urorectal septum. Western blot analysis and reverse transcriptase polymerase chain reaction revealed that the level of Hoxd-13 expression was significantly decreased in the ARMs embryos compared with that in the normal embryos on gD13 to gD16 (P < .05) rather than on gD18 to gD21. CONCLUSIONS: The aberrations in spatiotemporal expression pattern of Hoxd-13 on gD13 to gD16 suggested that Hoxd-13 may be an essential inductive signal for normal development of the hindgut, and altered expression may contribute to the abnormal development of the hindgut and accordingly lead to ARMs.

Pyridoxine might not have a preventive effect on the retinyl palmitate-induced viscerocranial anomalies.

Viscerocranial anomalies are induced in the presence of various teratogens. Vitamin A-induced cleft palate formation is one of the most frequently used experimental models in these studies. However, the underlying mechanisms are not yet fully understood. Several studies have shown that exogenous vitamin A disrupts the fusion of the palatal shelves by increasing the expression of epidermal growth factor receptor (EGFR). More recently, pyridoxine (vitamin B6) has been reported to have a potentially protective effect in regard to viscerocranial malformations. Therefore, in this study, we aimed to investigate whether pyridoxine has a preventive effect on retinyl palmitate-induced viscerocranial anomalies. The frequency of gross malformations induced by retinyl palmitate, the natural form of vitamin A, has been studied in a dose dependent manner. Low doses of retinyl palmitate (100 mg/kg) exposure on embryonic day (ED) 10 caused no gross anomalies in the rat fetuses. Teratogenic effects were observed only after exposure to higher dosages (1000 mg/kg) and primarily targeted the developing eyes and palates. On the other hand, co-administration of 10mg/kg pyridoxine, at ED 9 and 10, significantly increased the frequencies of anomalies, even in the moderate dosage (500 mg/kg) group. In all cleft palates, sustained expression of EGFR in the medial edge epithelium was detected by immunohistochemistry. These results show that co-administration of pyridoxine has an inductive rather than protective effect on the formation of viscerocranial malformations after exposure to hypervitaminosis-A.

Spatiotemporal pattern analysis of transcription factor 4 in the developing anorectum of the rat embryo with anorectal malformations.

PURPOSE: As a member of the transcription factors family, transcription factor 4(Tcf4) is known to influence gene expression in endodermally derived tissues including lung, liver, pancreas, stomach, and intestine. However, it remained unknown if this capability is active during anorectal development in the normal and anorectal malformations (ARM) rat embryos. MATERIALS AND METHODS: In this study, ethylenethiourea (ETU)-induced ARM model was introduced to investigate the expression pattern of Tcf4 during anorectal development using immunohistochemical staining, reverse transcriptase polymerase chain reaction (RT-PCR), and Western blot analysis. RESULTS: Immunostaining revealed that Tcf4 expression showed space-dependent changes in the developing anorectum: in normal embryos, Tcf4 protein is initially expressed in the dorsal endoderm of urorectal septum (URS) and hindgut on embryonic day 13 (E13). Additionally, separate expression domain develops intensively on the dorsal CM on E14. On E15, positive cells are then detected in the fused tissue of URS, and prominently in the anal membrane. In the ARM embryos, however, the epithelium of the cloaca, URS, and anorectum was negative or faint for Tcf4. In Western blot and RT-PCR, time-dependent changes of Tcf4 protein and mRNA expression were remarkable during the anorectal development: on E14, E14.5, and E15, the expression level reached the peak; after E16, Tcf4 expression gradually decreased. In contrast, in ARM embryos, spatiotemporal expression of Tcf4 was imbalanced during the anorectal morphogenesis from E13 to E16. CONCLUSIONS: These data implied that the downregulation of Tcf4 at the time of cloacal separation into rectum and urethra might be related to the development of ARM.

Embryonic development of the striated muscle complex in rats with anorectal malformations.

PURPOSE: Many patients with anorectal malformations (ARMs) continue to have postoperative anal dysfunction. The striated muscle complex (SMC) is one of the most important factors that influence defecation function. To explore the development of SMC in ARMs, the authors investigated the pelvic muscle development in rat embryos affected with ARMs. METHODS: Anorectal malformation embryos were induced by ethylenethiourea on the 10th gestational day (E10). Normal rat embryos and embryos with ARMs from E13 to E21 were serial-sectioned in the sagittal, transverse, and coronal planes, stained with H&E and immunohistochemistry staining using specific antibodies to myogenin. Temporal and spatial sequence was carried out on SMC. RESULTS: On E16, in normal group, SMC appeared fibroid structure in normal rats; SMC arose from bulbocavernosus muscle and ran backward, parallel to the perineal skin, and loosely surrounded the anal canal and urethra. Although in ARM rats the rectum was absent, the location and appearance of SMC were similar to the normal group. On E18, in normal group, SMC musculature became much thicker than on E16 and SMC gave off 2 branches outside anterior to the rectum. Striated muscle complex surrounded the rectum more tightly. However, in ARM rats, obvious changes of SMC could be noted. In detail, SMC in ARMs were characterized by abnormal location, appearance, and path. Striated muscle complex shifted obviously cephalad, ventrally, and medianward and converged inferior to the rectal terminus and posterior to the urethra. The distance between SMC musculature and the perineal skin increased. This structure surrounded the fiberlike tissue posterior to the urethra. Under high-power view, there was connective tissue among intermuscular bundles, and the structure was disordered. During the following gestational days, SMC in normal and ARM groups continued their own tendency, respectively. CONCLUSIONS: This study illustrated the development of the SMC in normal and ARM rats. On E16, the location and appearance of SMC in ARM rats were similar to the normal rats, and at this time, the ectopic rectal orifice could be noted. From E18 on, the maldevelopment of SMC could be observed in ARM rats. These observations suggested that the morphological changes of SMC take place after the occurrence of abnormal anorectum.

The contribution of the sonic hedgehog cascade in the development of the enteric nervous system in fetal rats with anorectal malformations.

OBJECTIVE: This study was designed to determine the expression of Sonic hedgehog (Shh) and its downstream genes during development of the enteric nervous system (ENS) in ethylenethiourea (ETU)-exposed fetal rats with anorectal malformations (ARMs). MATERIAL AND METHODS: Anorectal malformations were induced by 1% ETU (125 mg/kg) given on gestational day 10, and the litter was harvested at term. The fetal anorectum and rectosigmoid region, including any communication with the urinary tract, were collected for gene expression studies and immunofluorescence study of the ENS. Gene expression of Shh cascade was performed using reverse transcription and real-time polymerase chain reaction (PCR). The myenteric plexuses of the ENS in normal and ARM rats were visualized with fluorescent antibodies. RESULTS: Reverse transcription-PCR confirmed expression of Shh and its target genes in all parts of the ARMs. Quantitative PCR demonstrated that levels of expression of the genes of the Shh cascade were low in the ARMs. The immunoreactivity of neuromarkers was markedly reduced in high ARMs and slightly reduced in low ARMs. CONCLUSION: This study demonstrates reduced expression of Shh and its target genes in ARMs in ETU-exposed fetal rats. Neurons in the myenteric plexus were decreased in high and low types of ARMs. Our results support a role for the Shh cascade during development of the ENS during hindgut development.

An unconventional myosin in Drosophila reverses the default handedness in visceral organs.

The internal organs of animals often have left-right asymmetry. Although the formation of the anterior-posterior and dorsal-ventral axes in Drosophila is well understood, left-right asymmetry has not been extensively studied. Here we find that the handedness of the embryonic gut and the adult gut and testes is reversed (not randomized) in viable and fertile homozygous Myo31DF mutants. Myo31DF encodes an unconventional myosin, Drosophila MyoIA (also referred to as MyoID in mammals; refs 3, 4), and is the first actin-based motor protein to be implicated in left-right patterning. We find that Myo31DF is required in the hindgut epithelium for normal embryonic handedness. Disruption of actin filaments in the hindgut epithelium randomizes the handedness of the embryonic gut, suggesting that Myo31DF function requires the actin cytoskeleton. Consistent with this, we find that Myo31DF colocalizes with the cytoskeleton. Overexpression of Myo61F, another myosin I (ref. 4), reverses the handedness of the embryonic gut, and its knockdown also causes a left-right patterning defect. These two unconventional myosin I proteins may have antagonistic functions in left-right patterning. We suggest that the actin cytoskeleton and myosin I proteins may be crucial for generating left-right asymmetry in invertebrates.

Type ID unconventional myosin controls left-right asymmetry in Drosophila.

Breaking left-right symmetry in Bilateria embryos is a major event in body plan organization that leads to polarized adult morphology, directional organ looping, and heart and brain function. However, the molecular nature of the determinant(s) responsible for the invariant orientation of the left-right axis (situs choice) remains largely unknown. Mutations producing a complete reversal of left-right asymmetry (situs inversus) are instrumental for identifying mechanisms controlling handedness, yet only one such mutation has been found in mice (inversin) and snails. Here we identify the conserved type ID unconventional myosin 31DF gene (Myo31DF) as a unique situs inversus locus in Drosophila. Myo31DF mutations reverse the dextral looping of genitalia, a prominent left-right marker in adult flies. Genetic mosaic analysis pinpoints the A8 segment of the genital disc as a left-right organizer and reveals an anterior-posterior compartmentalization of Myo31DF function that directs dextral development and represses a sinistral default state. As expected of a determinant, Myo31DF has a trigger-like function and is expressed symmetrically in the organizer, and its symmetrical overexpression does not impair left-right asymmetry. Thus Myo31DF is a dextral gene with actin-based motor activity controlling situs choice. Like mouse inversin, Myo31DF interacts and colocalizes with beta-catenin, suggesting that situs inversus genes can direct left-right development through the adherens junction.