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Background and Purpose: Small cells in the range of less than 5 microm have been suspected to lie dormant and to be dispersed throughout the parenchyma of every tissue in the body. In bone marrow, small mesenchymal stem cells with rapidly renewing properties and differentiation potentials have been found. The aim of this study was to isolate and identify small cells from normal and osteoarthritic cartilages and compare their properties with those of small stem cells in bone marrow., Methods: Small cells were isolated from cartilages and bone marrow. Characterization of these small cells was performed by hematoxylin and eosin staining and bromodeoxyuridine labeling. Scanning electron and atomic force microscopy were used for ultrastructural investigation and analysis of the cells., Results: Small cells (< or = 5 microm) from both cartilages and bone marrow were shown to have mitotic activity on the basis of bromodeoxyuridine labeling, with the same replication time of 18 to 20 hours for population doubling. Ultrastructural and atomic force microscopy revealed these cells had spherical shape and were about 0.2 to 2.2 microm in diameter., Conclusions: This study demonstrated the presence of small cells in adult cartilage and bone marrow, which were about 0.2 to 2.2 microm in diameter and exhibited proliferation potential. These cells were able to be activated and differentiate into chondrocytes, and might have differentiation potential similar to small stem cells in bone marrow and thus play an important role in cartilage regeneration.