Figure 1.

Idiopathic pericarditis, dog. (a) Pericardial sac of dog that underwent subtotal pericardiectomy, diagnosed with chronic idiopathic pericarditis. Serosa is variably thickened by multifocal-coalescing whitish areas of fibrosis; multifocal areas of hemorrhages and a focal area of gray-yellowish fibrin exudation at the inner/cardiac side of the serosa are present, 40×. (b–d) Idiopathic pericarditis, dog, H&E. (b) Mesothelial papillary hyperplasia, simple to branching papillae (arrow) lined by cuboidal mesothelial cells supported by a small axis of fibrovascular stroma infiltrated with inflammatory cells, 100×. (c) Fibrinous pericarditis, pericardium is covered by a thick mat of fibrin (asterisk) and expanded by abundant fibrosis in which multifocal aggregates of inflammatory cells are embedded, 100×. (d) Fibrous pericarditis, pericardium is replaced by abundant fibrosis (asterisk), 100×. (e) Idiopathic pericarditis, dog, Masson’s trichrome. Mature collagen is stained in blue, 100×. (f) Control pericardial tissue appeared as normal pericardium with the inner, cardiac surface covered by a single layer of flat mesothelial cells; the parietal pericardium was composed of poorly vascularized fibrous tissue and overlies mediastinal adipose tissue, 40×.