Abstract
The intestinal tract is lined by a single layer of epithelium that is one of the fastest regenerating tissues in the body and which therefore requires a very active and exquisitely controlled stem cell population. Rapid renewal of the epithelium is necessary to provide a continuous physical barrier from the intestinal luminal microenvironment that contains abundant microorganisms, whilst also ensuring an efficient surface for the absorption of dietary components. Specialised epithelial cell populations are important for the maintenance of intestinal homeostasis and are derived from adult intestinal stem cells (ISCs). Actively cycling ISCs divide by a neutral drift mechanism yielding either ISCs or transit-amplifying epithelial cells, the latter of which differentiate to become either absorptive lineages or to produce secretory factors that contribute further to intestinal barrier maintenance or signal to other cellular compartments. The mechanisms controlling ISC abundance, longevity and activity are regulated by several different cell populations and signalling pathways in the intestinal lamina propria which together form the ISC niche. However, the complexity of the ISC niche and communication mechanisms between its different components are only now starting to be unravelled with the assistance of intestinal organoid/enteroid/colonoid and single-cell imaging and sequencing technologies. This review explores the interaction between well-established and emerging ISC niche components, their impact on the intestinal epithelium in health and in the context of intestinal injury and highlights future directions and implications for this rapidly developing field.
Highlights
The luminal surface of the gastrointestinal tract is covered by a single layer of epithelial cells that forms a continuous physical barrier to intestinal contents that include microbiota and dietary factors
Future directions: Single-cell sequencing has highlighted a high degree of heterogeneity within intestinal mesenchymal cell populations, the contribution of these populations to intestinal stem cells (ISCs) niche and crypt-villus axis homeostasis still needs resolution which will enable more clarity in the currently evolving telocyte/sub-epithelial myofibroblasts (SEMFs) nomenclature
Future studies need to address the integration of multiple signals from well-defined mesenchymal sub-populations and their communication mechanisms with different crypt-villus axis regions and cell types of the intestinal epithelium, which will help to further elucidate processes responsible for re-forming the ISC niche upon extreme mucosal damage and identify novel approaches to treat intestinal disease
Summary
The intestinal tract is lined by a single layer of epithelium that is one of the fastest regenerating tissues in the body and which requires a very active and exquisitely controlled stem cell population. Cycling ISCs divide by a neutral drift mechanism yielding either ISCs or transitamplifying epithelial cells, the latter of which differentiate to become either absorptive lineages or to produce secretory factors that contribute further to intestinal barrier maintenance or signal to other cellular compartments. The mechanisms controlling ISC abundance, longevity and activity are regulated by several different cell populations and signalling pathways in the intestinal lamina propria which together form the ISC niche. The complexity of the ISC niche and communication mechanisms between its different components are only starting to be unravelled with the assistance of intestinal organoid/enteroid/colonoid and single-cell imaging and sequencing technologies.
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