The neural markers present in the normal circulating monocytoid cells able, in pathological situations, to transdifferentiate into different mesenchymal-type cells, confirm the hypothesis previously raised that these cells derive from the neural crest. In culture, the normal cells display a great plasticity very reminiscent of microglial cells in culture. Almost a quiescent cell in normal individuals, this monocytoid cell shows its division potentialities in pathological situations of fibrosis and cancer (chondrosarcoma) where it is found to spontaneously proliferate. While the normal neofibroblasts are rapidly recognized and destroyed by fibrophagic T-lymphocytes, the pathological cells escape this control and, as a result, they accumulate in vitro giving rise to a tissue sometimes organized as nodules. Although basically the transdifferentiation process is similar in all the pathological situations of fibrosis and cancer studied so far, the end-result phenotype evokes the pathology the patient is suffering from. It evokes osteoblasts in a case of osteomyclosclerosis, chondroïdocytes in a case of chondrosarcoma, myelofibroblasts in a case of fibrosis of lung and kidney in a patient under ciclosporine treatment. Hence, this circulating monocytoid cell is a multipotent cell with great division potentiality. These are characteristics of stem/preprogenitor cells. Since this circulating monocytoid cell also bears the neural markers we called it a monocytoid ectomesenchymal stem/preprogenitor cell. Therefore, the existence of an ectomesenchymal system is discussed here.