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<p>Abstract</p> <p>Background</p> <p>To evaluate the relationship between Aberrant Crypt Foci (ACF) and tumorigenesis, we observed the sequential development from ACF to tumor in the colon of azoxymethane-exposed wistar rats.</p> <p>Methods</p> <p>Sixty wistar rats were sacrificed at different time points after exposure to azoxymethane and the colons were stained with methylene blue for stereomicroscopic analysis.</p> <p>Results</p> <p>We found two types of early lesions: classic ACF and dark ACF. Dark ACF were characterized by dark blue staining, mildly enlarged or small compressed crypts that are not elevated from the surrounding epithelium. Large dark ACF and nascent tumors displayed the same surface morphology. Furthermore, dark ACF grew significantly faster than classic ACF and showed dysplasia without hyperplasia. In contrast, classic ACF showed hyperplasia without dysplasia. Dark ACF has significant higher expression rate of β-catenin (100%) and MMP-7 (81.82%) compared with the expression of β-catenin and MMP-7 in classic ACF (4.84% and 7.87%, respectively).</p> <p>Conclusion</p> <p>Our data indicated that dark ACF is closely related to tumorigenesis while classic ACF is not. Furthermore, Wnt signal pathway was activated during the early period of dark ACF.</p>