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

Area of Science:

• Developmental biology
• Human embryology
• Gastrointestinal anatomy

Background:

• Traditional textbooks place the midgut-hindgut junction at the confluence of the superior mesenteric artery (SMA) and inferior mesenteric artery (IMA) vascular beds.
• Previous observations noted significant thinning of the dorsal mesentery during midgut formation, prompting re-evaluation of the midgut's distal boundary.

Purpose of the Study:

• To precisely locate the distal boundary of the midgut and the midgut-hindgut junction in human embryos.
• To investigate anatomical markers, including mesenteric vascularization and mesentery thickness, that define this developmental boundary.
• To clarify the embryonic origin of the colon and rectum based on developmental patterns.

Main Methods:

• Utilized 3D reconstructions from serial sections of 36 human embryos aged 4 to 13 weeks.
• Analyzed the thin mesenteric boundaries of the midgut to identify the midgut-hindgut junction.
• Examined the 3D orientation of the inferior mesenteric artery (IMA) trunk as a potential landmark.

Main Results:

• The midgut-hindgut junction was identified at the rectosigmoid junction in fetuses aged 10 weeks and older.
• The 3D orientation of the IMA trunk reliably indicates the midgut-hindgut junction's position.
• The early colon exhibits exponential growth, while the rectum grows linearly; the rectum differentiates caudocranially.

Conclusions:

• The rectosigmoid junction marks the definitive midgut-hindgut junction in later human fetal development.
• The rectum appears to be the sole derivative of the embryonic hindgut, based on its unique differentiation pattern and the timing of colic arcade formation.
• Mesenteric adherence of the ascending colic limb post-herniation rules out differential growth as the primary determinant of the junction's position.