Box I.

Intestine cellular physiology.

A brief review of the gut physiology is provided here with specific focus on the important role of cellular microenvironment in organ-on-a-chip systems. The gut possesses an immensely diverse cellular system spanning different anatomical sections, including the lumen, epithelium, and sub-epithelium (Fig. 2). The microbiome consists of hundreds of species of bacteria, viruses, yeasts, and fungi that live in the lumen under the condition of the convective flow of the digested food and the fluctuating mechanical motion, termed peristalsis, that propels the food forward.72–74 Lumen is characterized by an anaerobic environment, resulting from the oxygen consumption of microbes during food digestion.74,75 The environmental variables themselves vary widely along the lumen; for example, the colon features a slower flow and less acidic pH compared to the small intestine, causing a denser microbial community.26,76 Encircling the lumen are epithelial cells, a monolayer of closely adjoined cells shaped naturally into a 3D topography, consisting of villi (peaks) and crypts (valleys).77 The bottom of the crypt is made of stem cells performing tissue renewal during which most differentiated cells migrate toward villi.78 The differentiated epithelial lineages include columnar enterocytes (absorptive cells), which are responsible for food absorption, goblet cells, which produce a protective mucin layer, Paneth cells, which excrete antimicrobial peptides, and endocrine cells, which release gastrointestinal hormones.77 Besides these roles, the tight junctions provided by the epithelium provide structural integrity for the gut tissue.67 Beneath the epithelium is a niche consisting of structural and cellular components that are essential for maintaining its homeostasis.79 The underlying ECM provides physical support for the epithelium and contains a structured network of collagen fibers, integrins, fibronectin filaments, laminins, and glycosaminoglycan.79 The cellular components in this area, termed lamina propria, are diverse comprising structural elements (fibroblasts, fibrocytes, and vascular endothelial cells) in addition to blood and immune cells.80 Lamina propria is nutritious, oxygenated, and rich in blood and lymphatic capillaries, which carry the absorbed nutrients off the gut.67,81 Overall, the structure of the gut cellular system varies widely in terms of composition, architecture, physicochemical cues, and function across the organ's anatomy. It is clear that without a holistic approach to model cellular microenvironments, it is impossible to capture the physiological and functional complexity of the gut.