Characteristics of tonsil-derived mesenchymal progenitor cells (T-MPCs). (a) Morphology of T-MPCs at initial passage. Two different types of cell morphologies are apparent under phase-contrast microscopy: a fibroblast-like spindle-shaped morphology and a round morphology with large nuclei (monocytic contamination). The morphology of passage 2 bone marrow-derived mesenchymal progenitor cells (BM-MPCs) and T-MPCs in culture is shown. Cultures were observed at day 7 and day 14. T-MPCs (A, B); BM-MPCs (C, D). The two cell types display similar fibroblastic morphologies. Bars = 20 μm. (b) Proliferation kinetics of T-MPCs and BM-MPCs analyzed by MTS (methanethiosulfonate) assay. T-MPCs and BM-MPCs were plated at the same initial density (1 × 105 cells per plate). A difference in the proliferation rates of T-MPCs and BM-MPCs is observed, with T-MPCs proliferating at a faster rate than BM-MPCs throughout the assay period. Values are mean ± standard deviation (n = 9).