Electron microscopy of intimal lesions in the pulmonary trunk of a dog with Dirofilaria immitis.

Protruding intimal lesions in canine pulmonary trunk induced by naturally occurring heartworm disease were further characterized at light and electron microscopic levels. The pulmonary trunk was dissected free, fixed and subdivided into many specimens which were routinely prepared for scanning electron microscopy (SEM). After SEM observation, selected samples were embedded in Epon-Araldite and sectioned for correlated light (LM) and transmission electron microscopic (TEM) observation. SEM observation revealed protruding lesions of various sizes and shapes on the luminal surface. These ranged from papillae to complex lamellar projections extending far into the lumen. This suggests a progressive growth pattern. A layer of cells covered most of the lesion surface and between the lesions. These cells had a rounded conformation and no uniform orientation on the luminal surface. Other areas of the lesion surface appeared fibrous and devoid of cells. LM and TEM sections of the SEM specimens revealed three types of surfaces on the protruding fibro-muscular lesion. The lesion was primarily covered by cells which were (1) endothelial or (2) non-endothelial in nature. The non-endothelial surface cells were separated from each other and did not have the typical characteristics of endothelial cells. The third region was acellular and had the fibrous appearance of extracellular matrix. The acellular and non-endothelial portions formed a significant amount of the lesion surface. Previous reports have indicated these lesions were covered by typical endothelium. This report indicates that further characterization and quantitation of the cells covering the lesion surfaces is necessary.