Apoptosis, anoikis and their relevance to the pathobiology of colon cancer

The maintenance of a constant number of cells in an adult organism is a tig htly regulated process. This is particularly important in organs where cell s are in a constant rate of renewal during the entire lifespan. In these or gans, cell number homeostasis is the direct consequence of a bal-ance betwe en cell proliferation and apoptosis. The colonic epithelium is an example o f such a site and the high prevalence of colon cancer makes the understandi ng of cell number homeostasis more important to define. Normal colonic epit helium is organized in crypts where cell proliferation, migration, differen tiation and apoptosis are topographically organized in a linear fashion alo ng the crypt axis. Normal colonic crypts are composed of stem cells at the base, a proliferation and a differentiation zone in the lower third of the crypt, a migration zone in the upper two-thirds, and the surface epithelium where senescent cells are eliminated by apoptosis. Globally, apoptosis can be defined as a normal process of cell suicide, critical for development a nd tissue homeostasis. Colonic epithelial cells migrate from the base of th e crypt to the surface epithelium in 6-7 days. The normal architecture of t he crypt is maintained by a balance between cell proliferation at the base and apoptosis at the top of the crypt and surface epithelium.