Figure 1.

Schematic depiction of the small intestinal crypt compartment and the 4 main driving signaling pathways for maintenance of the stem cell niche. All relevant intestinal cell types and signaling molecules are represented in the grey box. In summary, Paneth cells and mesenchymal cells are producers of Wnt ligands, which directly bind to frizzled receptors expressed by intestinal stem cells. Secretory precursors of the transit-amplifying compartment secrete Notch ligands, which bind to Notch receptors expressed by absorptive precursors. Hedgehog signals are produced by epithelial cells and act directly on Hedgehog receptor-expressing mesenchymal cells, preserving them to produce BMPs, which bind to their receptors expressed by epithelial cells counteracting the Wnt pathway. TA, transit-amplifying.