Hemangiopericytoma in dog: an endangered species? Ultrastructural study of canine cutaneous perivascular tumours

ntroductionPerivascular wall tumours (PWTs) reported in dogs are hemangiopericytoma (HEP), angioleiomyoma/sarcoma and glomus tumour. HEP remains a controversial entity due to the lack of specific morphological and immunohistochemical features. The aim of this study was to characterize cases of undifferentiated canine cutaneous PWTs by transmission electron microscopy (TEM).Materials and methodsSamples from skin masses were fixed in 2.5% gluteraldehyde, post-fixed in OsO4 and embedded in epoxy resin. Semi-thin and ultrathin sections were stained with toluidine blue and counterstained with uranyl acetate and lead citrate, respectively.ResultsThe evaluation of “key” cellular features (cytoplasmic processes, external lamina, pino- cytic vesicles, subplasmalemmal densities, myofilaments, fibronexus junctions) allowed the identification of the following categories: hemangiopericytoma-like undifferenti- ated PWT (6/12), hemangiopericytoma (3/12), myopericytoma (1/12), undifferenti- ated fibroblastic-like PWT (1/12), undifferentiated angiomyofibroblastoma (1/12). One myopericytoma diagnosed by TEM had a benign behaviour, while two hemangioperi- cytomas recurred.DiscussionThe main problem in the immunohistochemical diagnosis and classification of canine PWTs derives from the loss of specific differentiation markers by neoplastic cells and from the phenotypic plasticity of vascular mural cells (pericytes and perivascular fi- broblasts) reflecting their origin from vascular wall resident mesenchymal (stem) cells (perivascular niche). This problem may be solved by TEM analysis in association with an ultrastructural diagnostic algorithm.[...]