The main functions of the human intestinal epithelium are to form a vital selective barrier between the systemic circulation and the luminal contents of the gut, whilst facilitating digestion and absorption of nutrients. Preservation of barrier function is underpinned by rapid stem cell-driven tissue renewal, and by secretion of a protective mucus layer. These physiological processes are regulated through the bilateral convergence of systemic and luminal signals which enable the gut epithelium to adapt to changes in the gut microenvironment. Not surprisingly, loss of gut epithelial tissue homeostasis precedes the onset of major intestinal conditions such as inflammatory bowel disease and cancer. Until relatively recently, investigation of the molecular and cellular processes that regulate and enact the physiological functions of the of the human gut epithelium in health and disease has been hampered by the lack of appropriate 3D ex vivo model systems.
The human colonic epithelial monolayer invaginates millions of times to form a carpet of crypts that line the gut lumen. At the base of each crypt, a population of 5-10 active LGR5-positive stem cells divides on a daily basis and gives rise to progenitor cells that migrate along the crypt-axis and differentiate into absorptive colonocytes or secretory goblet cells, enteroendocrine cells and tuft cells. Tissue renewal is completed by shedding of differentiated cells from the surface epithelium into the gut lumen. Significantly, the cellular hierarchy of tissue renewal aligns with the topology of the crypt-axis, as do the processes of fluid secretion and absorption.
Secretion of fluid from stem/progenitor cells located in the lower half of the crypt is coupled to the secretion of mucus from crypt-base-goblet-cells which combine to periodically flush the microenvironment of the stem cell niche and the macroscopic crypt lumen. Regulation and coordination of mucus and fluid secretion from collaborating goblet and stem/progenitor cells is achieved in part via the polarised expression of the associated molecular machinery across these gut epithelial cell types.
Recent advances in 3D ‘near-native’ and organoid model systems have made significant progress in recapitulating the topology and the polarity of the molecular machinery that orchestrates intestinal epithelial membrane transport along the crypt-axis. I will showcase the utility of cultured colonic crypts and organoids, in conjunction with fixed clinical specimens, for the study of calcium-dependent excitation-mucus/fluid secretion coupling in the human colon.