Figure 1.

All embryonic germ layers contribute to the tissue layers of GI organs. Divided into three regions, the endoderm (orange) gives rise to the innermost epithelial layer of all GI organs. The stratified squamous epithelium of the esophagus (E) and forestomach (FS) as well as the simple columnar epithelium of the hindstomach (HS) and proximal duodenum (D) of the small intestine originate from foregut endoderm. The junction between the stratified squamous epithelium and simple columnar epithelium is referred to as the squamocolumnar junction (SCJ). Midgut endoderm gives rise to the simple columnar epithelium of the distal duodenum, jejunum (J), and ileum (I) of the small intestine as well as to the simple columnar epithelium of the proximal colon (PC). The simple columnar epithelium of the distal colon (DC) arises from hindgut endoderm. Beneath the epithelium (Ep), the lamina propria (LP), muscularis mucosae (MM), submucosa (SM), vasculature, lymphatics, and muscularis externa (CM, circular muscle; LM, longitudinal muscle) layers develop from mesoderm (green). The enteric nervous system is derived from ectoderm and ganglia reside within the submucosa and the muscularis externa (blue). The gut tube is covered by a mesoderm derived adventitia (A) or serosa (S).