The understanding of the fixation of mutations within human tissues and their subsequent clonal expansion is a considerable problem, of which little is known. We have previously shown that nononcogenic mutations in the mitochondrial genome occur in one of a number of morphologically normal colonic crypt stem cells, the progeny of which later occupy the whole crypt. We propose that these wholly mutated crypts then clonally expand by crypt fission, where each crypt divides into two mutated daughter crypts. Here we show that (i) mutated crypts in the process of fission share the same mutated mitochondrial genotype not present in neighboring cytochrome c oxidase-positive crypts (the odds of this being a random event are >2.48 ؋ 10 9 :1); (ii) neighboring mutated crypts have the same genotype, which is different from adjacent cytochrome c oxidase-positive crypts; (iii) mutated crypts are clustered together throughout the colon; and (iv) patches of cytochrome c oxidase-deficient crypts increase in size with age. We thus demonstrate definitively that crypt fission is the mechanism by which mutations spread in the normal human colon. This has important implications for the biology of the normal adult human colon and possibly for the growth and spread of colorectal neoplasms.fission ͉ mitochondria

SummaryHuman intestinal stem cell and crypt dynamics remain poorly characterized because transgenic lineage-tracing methods are impractical in humans. Here, we have circumvented this problem by quantitatively using somatic mtDNA mutations to trace clonal lineages. By analyzing clonal imprints on the walls of colonic crypts, we show that human intestinal stem cells conform to one-dimensional neutral drift dynamics with a “functional” stem cell number of five to six in both normal patients and individuals with familial adenomatous polyposis (germline APC−/+). Furthermore, we show that, in adenomatous crypts (APC−/−), there is a proportionate increase in both functional stem cell number and the loss/replacement rate. Finally, by analyzing fields of mtDNA mutant crypts, we show that a normal colon crypt divides around once every 30–40 years, and the division rate is increased in adenomas by at least an order of magnitude. These data provide in vivo quantification of human intestinal stem cell and crypt dynamics.

Fluorescence lifetime imaging (FLIM) is a functional imaging methodology that can provide information, not only concerning the localisation of specific fluorophores, but also about the local fluorophore environment. It may be implemented in scanning confocal or multi-photon microscopes, or in wide-field microscopes and endoscopes. When applied to tissue autofluorescence, it reveals intrinsic excellent contrast between different types and states of tissue. This article aims to review our recent progress in developing time-domain FLIM technology for microscopy and endoscopy and applying it to biological tissue.