The surface of the colon mucosa of 1,2-dimethylhydrazine-treated F344
rats was examined with the scanning electron microscope. A detailed ex
amination of the mucosal topography revealed foci with one to several
aberrant crypts. These were seen as structures elevated from the backg
round mucosa. The shape of the luminal openings of the aberrant crypts
varied from elongated or tortuous to circular. However, we found no u
ltrastructural variations between the different aberrant crypt foci (A
CF) or between the ACF and the background mucosa. There was no direct
relationship between the size of ACF and the number of aberrant crypts
per focus, which may be explained by the mechanism of crypt fission;
in two aberrant crypts we discovered the formation of a transverse epi
thelial septum, dividing the large crypt into two smaller crypts. The
gross morphology of the ACF observed by scanning electron microscopy a
nd light microscopy was in principle the same.