The junction between the midgut and hindgut co-localizes with the rectosigmoid junction

Textbooks locate the junction between the midgut and hindgut where the vascular beds of the superior (SMA) and inferior (IMA) mesenteric arteries meet. In a previous study, we observed that the formation of the midgut corresponded with a pronounced thinning of its dorsal mesentery. We re-investigated, therefore, the location of the distal boundary of the midgut, making use of 3D reconstructions of serial sections of 36 human embryos between 4 and 13 weeks of development. Using the boundaries of the thin mesentery of the midgut as a criterion, the midgut-hindgut junction corresponds in 10-week and older foetuses with the rectosigmoid junction. In addition, we established that the 3D orientation of the trunk of the IMA (between its aortic root and first branching node) also identifies the position of the midgut-hindgut junction in the gut. The growth rate of the early colon is exponential, whereas that of the rectum is linear. Initially, the foetal colon has ascending and descending limbs only, of which the descending limb grows fastest. The mesentery of the ascending colic limb adheres to the ventral surfaces of the duodenum, stomach and dorsal pancreas shortly after the hernial return into the abdomen during the 10th week, which rules out an effect of differential growth on the position of the junction. We, therefore, postulate that the rectum is the sole descendant of the embryonic hindgut. The rectum is unique in that its differentiation follows a caudocranial direction. Vascular connections between the perfusion areas of the SMA and IMA expand to form the first colic arterial arcade only at 10 weeks.