The concept of “stem cell pathology” is to establish the role of the stem cells by exploring their contribution to lesion development. The somatic stem cells are present in the body. Malignant fibrous histiocytoma (MFH; recently named “undifferentiated pleomorphic sarcoma”) includes pluripotential undifferentiated mesenchymal stem cells as a cell element. An antibody (A3) generated by using rat MFH cells as the antigen labels somatic stem cells such as bone marrow stem cells and immature endothelial cells and pericytes, as well as immature epithelial cells in epithelialization. By using A3 and other antibodies recognizing somatic stem cells, it is considered that myofibroblasts appearing in rat fibrotic lesions are developed partly from immature hepatic stellate cells in hepatic fibrosis, immature pancreatic stellate cells in pancreatic fibrosis, pericytes/endothelial cells in neovascularization in injured tissues, as well as via the epithelial-mesenchymal transition. These progenitors may be in the stem cell lineage. In this review, the author introduces the histogenesis of MFH and the characteristics of myofibroblasts appearing in fibrosis, based mainly on the author’s studies.