The intestinal epithelium is a continuous monolayer of cells that separates the interior of the body from the intestinal lumen. It is composed of five different cell types:
- Enterocytes, whose tight connections create a barrier that prevent random entry of most molecules. The apical membrane controls uptake of molecules through specific channels, transporters, carriers or processes (endocytosis) into and through the monolayer. Enterocytes take part in the digestive and absorptive processes of nutrients which then funnel into blood stream or lymph or metabolise themselves.
- Goblet cells, which continuously secrete mucus protein to maintain the mucus layer functionality.
- Enteroendocrine cells producing hormones and releasing these into circulation, the gut or the enteric nervous system.
- Microfold cells (M-cells) absorbing environmental antigens and presenting them to immune cells.
- Paneth cells, secreting antimicrobial proteins into the intestinal lumen.
The intestinal epithelium is a strong physical barrier. The space in between the cells is sealed by specialised proteins, termed tight junctions and adhesive junctions. This structure allows to discriminate between substances that shall be absorbed and those which shall not (Vancamelbeke & Vermeire, 2017).