Regional factors affecting proliferation in the large intestine of the rat.

Colonic neoplasia is more frequent in the distal colon than in the proximal colon in spontaneous human disease and in carcinogen-induced tumors in rodents. The possibility that this may reflect regional differences in morphology and in proliferative responses to fasting and refeeding was explored in this study in rats. Scanning electron microscopy revealed that the density of colonic crypts was 36% higher in the distal than in the proximal colon, while light microscopy revealed that distal crypts had 70% more colonocytes than proximal crypts. Thus, the number of colonocytes per unit area in the distal colon is approximately twice that in the proximal colon. Proliferation was assessed by the uptake of bromodeoxyuridine in vivo and showed that regions of the distal colon had greater suppression of proliferation during fasting than the cecum, and greater enhancement of proliferation during refeeding than that observed in the cecum or the proximal colon. Changes in proliferation associated with fasting and refeeding were accompanied by changes in the concentrations of short chain fatty acids, but the data did not support the hypothesis of a direct relationship between increasing concentrations of short chain fatty acids and enhanced proliferation. Regional differences in morphology and proliferation could be relevant to the greater susceptibility of the distal colon to neoplasia.