Simultaneously, the midgut begins to elongate and descend into the umbilical cord as a primary gut loop at around E10.5, and this structure, called the physiological umbilical herniation, exists until around E16.0.

Subsequently, the gut tube, comprising the foregut, midgut, and hindgut, is formed between E9.0 and E9.5. The development of the murine gastrointestinal tract starts at around embryonic day (E) 7.0. In conclusion, DJF formation might be triggered by asymmetric morphology and proliferation along the left-right intestinal axis under the control of retinoic acid and hedgehog signaling. During DJF formation, Aldh1a2 and hedgehog mRNAs were detected at the DJF, and their expression levels differed along the bending axis. The direction of attachment of the dorsal mesentery to the DJF did not correspond to the bending direction of the DJF during flexure formation, and this finding indicated that the dorsal mesentery contributed very little to DJF formation. From the flexure formation to the flexure elongation stage, the DJF started to bend dorsally with counterclockwise rotation along the antero-caudal intestinal axis, indicating that the original right side of the duodenum was rotated towards the dorsal body wall during development of the DJF. From the expansion to the flexure formation stages, the DJF wall showed asymmetric morphology and proliferation along the left-right intestinal axis. During embryonic days 10.75–13.75, DJF formation was morphologically classified into three stages: the expansion stage, flexure formation stage, and flexure elongation stage. We studied the developing mouse duodenojejunal flexure (DJF) to elucidate the mechanism of formation. The mammalian gut undergoes morphological changes during development.

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