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Mast cell tumours (MCTs) are often diagnosed by cytology based on the identification of purple intracytoplasmic granules with methanolic Romanowsky stains, including May-Grünwald-Giemsa (MGG). In clinical practice, aqueous rapid stains (RS) are commonly used, but mast cell granules may not stain properly. Aim of this prospective study was to investigate the frequency of MCT hypogranularity with RS and its potential implications in tumour identification, cytological grading assessment and recognition of nodal metastatic disease. Cytological preparations of canine primary MCTs and metastatic lymph nodes with subsequent histopathological confirmation were included. For each case, good-quality smears were stained with both MGG and RS and comparatively assessed. Eleven of 60 (18.3%) primary MCTs were hypogranular with RS; 9 of them were histologically high-grade tumours and in 3 cases (5%) a definitive MCT diagnosis could not be made. Accuracy in cytological grading assessment (85%) did not differ between RS and MGG. Thirteen of 28 (46.4%) metastatic lymph nodes were hypogranular with RS and 3 independent observers failed to identify nodal MCT metastases in 7% to 18% of RS-stained smears. This study confirms that, in limited cases, RS can be ineffective in staining MCT granules, particularly in high-grade tumours, thus making diagnosis more dependent on experience and quality of preparations. In dubious cases, methanolic stains should be applied. The use of RS is discouraged for the search of nodal metastases, as the identification of isolated mast cells can be more challenging.

Introduction - Von Brunn’s nests and cysts are submucosal benign urothelial processes, related to irritative stimuli. They are common findings in the urinary bladder of human beings. Very few cases have been described in dogs and cats. Materials and Methods - We report macroscopic, histopathological and, in one case, immunohistochemical features of these rare forms in the left ureters of two cats, after ureterectomy due to urinary obstruction. Results - In both cases, macroscopical findings were coherent with an isolated, nodular and compact mass of 2.5 cm diameter. Microscopic examination showed considerable thickening of the muscular wall due to multifocal fibrosis, multifocal lacking of the lining epithelium in transverse sections of the ureteral lumen, and foci of urothelial islands in the lamina propria without any cytological atypia or mitosis and with occasional central cystic cavities. Multifocal areas of oedema, hyperemia/hemorrhage, angiogenesis and mild to moderate widespread lymphoplasmacytic infiltrates were also present. The cystic cavities contained macrophages full of lipofuscin, hemosiderin, erythrocytes and eosinophilic amorphous material. In one case, urothelial islands showed intense immunoreactivity for pan-cytokeratin (CK AE1/AE3). Inflammation was poor in the other case, where the urothelium appeared multifocally dilated with focal mucous metaplasia. In both cases, such findings and the anamnesis allowed us to hypothesize that the presence of epithelial islands was consistent with a hyperplastic condition, known as von Brunn’s nests and cysts. Conclusions - It is important to know the histological features of von Brunn nests, since it is not always easy to distinguish these islands from a well-differentiated urothelial carcinoma.