Fig. 1

Schematic of the mucus layers in the GI tract as a whole (A), as well as tissue histology of the mucin MUC5B in the submucosal glands of the esophagus (B) as shown by Arul et al. [23], the mucins MUC5AC/MUC6 in the stomach (C) as shown by Ho et al. [24], and the mucin MUC2 in the small intestine (D) as shown by Gustafsson et al. [25]. The schematic (A) shows the esophagus (left), stomach (left middle), small intestine (right middle), and colon (right). The esophagus contains a thin layer of mucin MUC5B. The stomach contains two layers of mucin MUC5AC: a thin layer firmly attached to the epithelium and a thicker, loosely attached layer above. This outer layer also contains “bands” of mucin MUC6. The small intestine and colon both contain the mucin MUC2, but the small intestine only contains a thin, loosely bound layer. The colon is organized similarly to the stomach, with a thin, firmly attached layer and a thicker, loosely attached layer above