Flat adenomas of the colon in rats are associated with mucosal lymphoid nodules

The occurrence of lymphoid nodules associated to colonic adenomas and adeno carcinomas was investigated in 276 tumors induced in 242 Sprague-Dawley rat s, Colonic neoplasms were divided into 2 groups: Group I had neoplastic tis sue at the luminal aspect of the colon (rr = 234) and Group II, had histolo gically normal colonic mucosa on top (rt = 42), In Group I, a lymphoid nodu le was associated to 28.6% of the 84 adenomas, to 8.4% of the 119 adenocarc inomas arising in adenomas and to 9.7% of the 31 carcinomas without remnant adenomatous tissue. A lymphoid nodule was found in 35.6% (21/59) of the fl at (i.e. non-polypoid) adenomas and carcinomas but only in 9.1% (16/175) of the exophytic (i.e. polypoid) adenomas and carcinomas, When only adenomas were considered, lymphoid nodules were present in 50.0% (17/34) of the flat adenomas, but only in 14.0% (7/50) of the exophytic adenomas, Flat colonic adenomas seem to evolve preferentially from the minor fraction of the colo nic mucosa which overlays the few existent lymphoid nodules in rats, All 42 adenocarcinomas in Group II were lymphoid nodule-associated, It was diffic ult to classify those tumors into flat or exophytic, When Groups I and TI w ere considered together, 28.6% (79/276) of the induced adenomas and adenoca rcinomas in rats were lymphoid nodule-associated.