Use of electron microscopy to classify canine perivascular wall tumors

The histologic classification of canine perivascular walltumors (PWTs) is controversial. Many PWTs are still classifiedas hemangiopericytomas (HEPs), and the distinction fromperipheral nerve sheath tumors (PNSTs) is controversial. Arecent histologic classification of canine soft tissuesarcomas included most histologic types of PWT, but omittedthose that were termed undifferentiated. Twelve cases ofundifferentiated canine PWTs were evaluated by transmissionelectron microscopy. The ultrastructural findings supported aperivascular wall origin for all cases with 4 categories ofdifferentiation: myopericytic (n = 4), myofibroblastic (n =1), fibroblastic (n = 2) and undifferentiated (n = 5). A PNSTwas considered unlikely in each case based onimmunohistochemical expression of desmin and/or the lack oftypical ultrastructural features, such as basal lamina.Electron microscopy was pivotal for the subclassification ofcanine PWTs, and the results support the hypothesis thatcanine PWTs represent a continuum paralleling the phenotypicplasticity of vascular mural cells. The hypothesis that asubgroup of PWTs could arise from a pluripotent mesenchymalperivascular wall cell was also considered, and may explainthe diverse differentiation of canine PWTs.[...