The normal colonic crypt of Lieberkiihn is surrounded by a mesenchymal sheath, a specialized part of the lamina propria, consisting of fibroblasts tightly applied to the epithelium and of collagen fibers oriented circumferentially to the crypt. Autoradiographic studies of rabbit colon after ³H-thymidine injection demonstrate a “steady state” renewal of the pericryptal fibroblasts and migration to upper portions of the crypt in synchrony with the epithelial migration. This, to our knowledge, is the first detailed account of continuous fibroblast renewal and migration in adult, un-injured tissue. The fibroblast progenitor population, like that of the epithelium, is at the deep one-third of the crypt, where very little collagen is present. Under the absorptive, i.e., surface, epithelium, there is a relatively thick collagen table, suggesting, a functional maturation of the pericryptal fibroblast as it ascends the crypt wall.

These studies, which elucidate the kinetics of a portion of normal colonic mucosa heretofore only occasionally alluded to, suggest that the pericryptal fibroblasts and the epithelium act as a unit to maintain the normal structure, maturation, and function of the crypt of Lieberkühn.