Gastrointestinal stem cells.

Turnover of the epithelial cell lineages within the gastrointestinal tract is a constant process, occurring every 2-7 days under normal homeostasis and increasing after damage. This process is regulated by multipotent stem cells, which give rise to all gastrointestinal epithelial cell lineages and can regenerate whole intestinal crypts and gastric glands. The stem cells of the gastrointestinal tract are as yet undefined, although it is generally agreed that they are located within a 'niche' in the intestinal crypts and gastric glands. Studies of allophenic tetraparental chimeric mice and targeted stem cell mutations suggest that a single stem cell undergoes asymmetrical division to produce an identical daughter cell, and thus replicate itself, and a committed progenitor cell which further differentiates into an adult epithelial cell type. The discovery of stem cell plasticity in many tissues, including the ability of transplanted bone marrow to transdifferentiate into intestinal subepithelial myofibroblasts, provides a potential use of bone marrow cells to deliver therapeutic genes to damaged tissues, for example, in treatment of mesenchymal diseases in the gastrointestinal tract, such as fibrosis and Crohn's disease. Studies are beginning to identify the molecular pathways that regulate stem cell proliferation and differentiation into adult gastrointestinal cell lineages, such as the Wnt and Notch/Delta signalling pathways, and the importance of mesenchymal-epithelial interactions in normal gastrointestinal epithelium and in development and disease.