FIG. 2.

Schematics of the gut cellular composition and structure. The microbiota lives in the lumen, which features an anaerobic condition and a convective fluid flow. Epithelial cells surround the lumen's outer surface and form 3D villi and crypts (the inset shows the cellular composition of a crypt as described in Box I). Below the lumen is the mucosa, which embeds aerobic blood and lymph arteries in lamina propria; this area is populated by a diverse cellular makeup ranging from endothelial cells to immune cells.