Histology of colon cancer in Saguinus oedipus oedipus.

Diagnosis of colon cancer in tamarins often requires histologic (microscopic) inspection of the entire colon before minute primary sites can be located in the flat colonic mucosa. In other cases the cancer is easily recognized grossly because infiltration produces local desmoplastic reactions. The cancer does not have a preceding benign polypoid stage. The carcinoma is most typically poorly differentiated. The PAS stain, however, demonstrates that some of the malignant cells always produce mucin. With other special stains and electron microscopy, undifferentiated stem cells, mitochondria-rich absorptive cells, and cells rich in argentaffin granules are demonstrated. Most commonly the carcinomatous cells are distributed in structureless masses, but occasionally they form tubular structures that resemble glands. Preneoplastic epithelial changes in the crypts are disguised by epithelial hyperplastic (reparative) changes without hyperchromatic nuclear changes. Nuclear pleomorphism and enlarged nuclear/cytoplasmic ratios are commonplace in nonmalignant as well as malignant cells. All colon cancers found in this species arise in association with a pre-existing, chronic ulcerating colitis.