Introduction: Canine perivascular wall tumours (PWTs) includehaemangiopericytoma (HEP), angioleiomyoma/sarcoma and glomustumour. HEP remains a controversial entity due to the lack of specificmorphological and immunohistochemical features. The aim of thisstudy was to characterize cases of undifferentiated canine cutaneousPWTs by transmission electron microscopy (TEM).Materials and Methods: Samples from skin masses were fixed in2.5% gluteraldehyde, post-fixed inOsO4 and embedded in epoxy resin.Semi-thin and ultrathin sections were stained with toluidine blue andcounterstained with uranyl acetate and lead citrate, respectively.Results: The evaluation of key cellular features (cytoplasmic processes,external lamina, pinocytic vesicles, subplasmalemmal densities,myofilaments, fibronexus junctions) allowed the identification of thefollowing categories: haemangiopericytoma-like undifferentiatedPWT (6/12), haemangiopericytoma (3/12), myopericytoma (1/12),undifferentiated fibroblastic-like PWT (1/12) and undifferentiated angiomyofibroblastoma(1/12). One myopericytoma diagnosed by TEMhad a benign behaviour, while two hemangiopericytomas recurred.Conclusions: The main problem with immunohistochemicaldiagnosis and classification of canine PWTs is loss of specific differentiationmarkers by neoplastic cells and the phenotypic plasticity ofvascular mural cells (pericytes and perivascular fibroblasts) reflectingtheir origin from vascular wall resident mesenchymal (stem) cells(perivascular niche). This problem may be solved by TEM analysisin association with an ultrastructural diagnostic algorithm.[...]