The paradigm of epithelium renewal in the intestine.

(A) Cell proliferation and apoptosis in the intestine of zebrafish. Left panel: Cell proliferation assay with proliferating cells stained dark brown. Middle panel: Cell apoptosis assay with apoptotic cells stained green. Right panel: Compartmentalization of epithelium into stem cells, transit amplifying cells, differentiated cells and apoptotic cells. (B) The intestinal epithelium is divided into four components while constructing the model, based on the analogous paradigm of epithelium renewal across teleost, murine and human species. Stem cells maintain their own population through self-renewal, and in the mean time, they produce progenies that will differentiate later on. Transit amplifying cells are directly derived from stem cells and go through rapid expansion. Then they go for cell differentiation and finally apoptosis. Denotation: x₁-population of stem cells; x₂-population of transit amplifying cells; x₃-population of differentiated cells. Note that all populations are normalized against their homeostatic populations in the model.