uPA and myoblast proliferation in vitro. Myoblasts were isolated from neonatal WT and uPA^−/− mice and cultured for proliferation experiments as described in “Myoblast cultures.” Cells were incubated in low serum medium without other factors overnight, and then incubated in experimental medium with designated factors added along with 100μM BrdU for 24 hours for assessment of cell proliferation. The number of BrdU⁺ cells was counted in 4 fields observed at 20× and normalized to the total number of cells present in each field. Bars represent mean ± SE; n = 6 to 12 per condition. ^aMean value for uPA^−/− cells significantly smaller than that for WT cells (P < .05). ^bMean value for specific experimental condition significantly larger than that for control condition (low serum or no factors added; P < .05). ^cMean value for specific experimental condition significantly smaller than that for uPA treated condition (P < .05).