Fig. 1

The gut in health and critical illness. In conditions of health (a), intestinal stem cells proliferate in the crypt (gray and orange), divide into daughter cells, and migrate up in a single-cell layer to the top of the villus. The majority of epithelial cells are enterocytes (white and orange), although there are also goblet cells, enteroendocrine cells, and tuft cells present. The epithelium is surrounded by a continuous mucus layer (gray). This acts as a barrier to luminal microbes (red and green) which are also recognized by secretive IgA (light red). Permeability is also mediated via the tight junction (inset) where a complex machinery between epithelial cells acts as a selective barrier allowing solutes and water through but preventing movement of larger molecules. In critical illness (b), proliferation is decreased and apoptosis is increased leading to a shorter villus length. The mucus layer is damaged and no longer uniform. Along with changes in the tight junction resulting in hyperpermeability, gut barrier function is compromised and bacteria are able to translocate (red rods representing bacteria are present in the lamina propria)