Tumors of the alimentary tract.

Electron microscopy offers substantial aid in the diagnosis of gastrointestinal tract tumors, especially small-cell carcinoma and enterochromaffin-cell tumors. Tumors that are most difficult to classify by light microscopy--that is, poorly differentiated ones--may also have ultrastructural features by which the cell of origin can be determined. As always, the well-preserved tissue is more likely to retain diagnostic features than is the inadequately preserved.