Basaloid tumor of the sigmoid colon.

A second case of basaloid carcinoma arising in the midsigmoid colon is reported. By light microscopy the tumor was seen to be composed of islands of small, poorly differentiated cells separated by cellular connective tissue. Also seen were small foci of keratinized cells, Ultrastructure study confirms the largely basal character of the tumor and also shows some cells containing tonofilament bundles, representing a more squamoid differentiation. The similarity of this tumor to basaloid tumors arising in the transitional epithelium of the anal canal is discussed, and the suggestion that the tumor arises from a pluripotential basal cell is made.