The effect of MSCs on chondrocyte proliferation. (A) GFP staining of the growth plate after GFP-positive MSC infusion. Immunostaining for GFP expression of the growth plate from a GFP-transgenic mouse (positive), a mouse after saline infusion (PBS, negative), or a mouse after GFP-transgenic MSC infusion. Original magnification, ×200. (B) Chondrocyte proliferation was analyzed by MTT Cell Proliferation assay after 3-day coculture with MSCs or chondrocytes or control medium on the transwell plates (n = 3). (C) Sera were collected from MSC-injected and PBS-injected mice 2 days and 7 days after the injection. Chondrocyte proliferation assay was performed at day 0, day 2, day 4, and day 6 after culture with the sera, MSC-conditioned medium, and control medium (P < .001, n = 6). The depicted data are a representative experiment. In total, 38 groups of mice (222 experimental, 154 controls) confirmed these findings. (D) Sera from mice injected with control medium, MSC-conditioned medium, or MSCs were applied into chondrocyte culture. Chondrocytes were cultured for 6 days in serum-supplemented medium followed by the proliferation assay measured as fluorescence intensity (*P < .05, n = 9). All data are mean ± SEM.