Aberrant crypt foci (ACF) constitute putative preneoplastic lesions originally described in experimental animal models of colorectal cancer (253,254,255). ACF have been proposed as intermediate biomarkers in carcinogenesis studies (253,254,255,256,257). Their detection involves examining colons cleaned with Krebs-Ringers solution. The colons are cut open along their
longitudinal axis, fixed flat in buffered formalin, and then placed in a Krebs-Ringers solution containing 0.2% methylene blue for approximately 30 minutes. This preparation is then placed on a glass slide, mucosal side up, and examined under a light microscope at a magnification of 40× with transillumination (253). ACF consist of clusters of abnormally large, darkly staining, slightly elevated mucosal crypts (Fig. 14.69).