Sarcomas display varied degrees of karyotypic abnormality, vascularity and mesenchymal differentiation. We have reported that a strain of telomerized adult human bone marrow mesenchymal stem cells (hMSC-TERT20) spontaneously evolved a tumorigenic phenotype after long-term continuous culture. We asked to what extent our hMSC-TERT20 derived tumors reflected events found in human sarcomas using routine histopathological procedures. Early versus late passage hMSC-TERT20 cultures persistently expressed mesenchymal lineage proteins e.g. CD105, CD44, CD99 and vimentin. However, late passage cultures, showed increased immunohistochemical staining for CyclinD1 and p21WAF1/Cip1, whereas p27Kip1 staining was reduced. Notably, spectral karyotyping showed that tumorigenic hMSC-TERT20 cells retained a normal diploid karyotype, with no detectable chromosome abnormalities. Consistent with the bone-forming potential of early passage hMSC-TERT20 cells, tumors derived from late passage cells expressed early biomarkers of osteogenesis. However, hMSC-TERT20 cells were heterogeneous for alpha smooth muscle actin (ASMA) expression and one out of six hMSC-TERT20 derived single cell clones was strongly ASMA positive. Tumors from this ASMA+ clone had distinctive vascular qualities with hot spots of high CD34+ murine endothelial cell density, together with CD34- regions with a branching periodic acid Schiff reaction pattern. Such clone-specific differences in host vascular response provide novel models to explore interactions between mesenchymal stem and endothelial cells. Despite the lack of a characteristic chromosomal translocation, the histomorphology, biomarkers and oncogenic changes were similar to those prevalent for Ewing's sarcomas. The phenotype and ontogenesis of hMSC-TERT20 tumors was consistent with the hypothesis that sarcomas may arise from hMSC, providing a unique diploid model for exploring human sarcoma biology.