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Ultrastructural findings in 350 endomyocardial biopsy specimens and 59 autopsy or explanted hearts from cardiac transplant recipients are reviewed. Myocyte degeneration can be readily distinguished from necrosis by the technique described. Vascular changes of endothelial activation, endothelial cell damage, and basement membrane reduplication can be readily identified. In addition, the myofilament composition of ischemic hearts in patients with allograft coronary artery disease is distinctive: There is a disproportionate loss of actin over myosin, giving a coarse appearance to the myofilaments. These changes are useful in further defining the morphologic features associated with rejection and ischemia in cardiac transplant recipients.

A case of myocardial paraprotein deposition in a patient with Waldenstrom's macroglobulinemia is presented. Routine light microscopy revealed diffuse widening of interstitial regions by pale eosinophilic material that had a grayish cast on trichrome stains and was Congo red negative. Immunofluorescence showed strong immunoglobulin M and kappa light chain staining around blood vessels but predominantly around myocytes. Ultrastructural examination revealed massive deposition of flocculent electron-dense material around myocytes and occasionally around blood vessels.

The report describes a malignant gastrointestinal stromal tumor occurring in the duodenum in a 71-year-old woman. The neoplasm showed both epithelioid and spindle cell patterns by light microscopy. The ultrastructural features were diagnostic of nerve sheath origin. The tumor had numerous wrapping processes joined by junctions and surrounded by axons. No features of smooth muscle differentiation were identified. Immunocytochemistry was inconclusive. The differential diagnosis of such neoplasms is discussed.

Thymoma is the most common tumor of the anterior-superior mediastinum, especially in middle-aged or older adults. Microscopically, thymomas can be differentiated from other tumors with which they can be confused by the finding of a mixed population of cells, including neoplastic thymic epithelial cells with numerous processes surrounding activated-appearing lymphocytes. Thymomas can be classified as benign or malignant, and the majority of those that are malignant appear cytologically benign and are locally invasive. Cytologically benign thymomas have been classified as being lymphocyte rich, epithelial cell rich, or spindle cell type. This classification has not been found to be prognostically useful. A new method classifies these tumors as being cortical, medullary, or mixed. This new classification appears to have prognostic significance. Malignant thymomas that are cytologically malignant are uncommon. Such tumors usually are squamous cell carcinomas. Other types include sarcomatoid carcinoma, clear cell carcinoma, basaloid carcinoma, and mucoepidermoid carcinoma. Because other tumors can occur in the anterior mediastinum, electron microscopy and/or immunocytochemistry is helpful in making the distinction. The differential features by ultrastructural and immunocytochemical analysis are reviewed.