Fig. 2. Myogenic effects of p38 inhibition rely on factors present in primary limb mesenchymal cultures. G8 myoblasts (A-D) and C2C12 myocytes (E-H) were tagged using the pMSV-tk-ßgeo retrovirus and were monitored by staining with X-gal or magentagal. When SB202190 is added for 4 days, there is no noticeable change in cell appearance, number or organization of either the G8 cells (compare A with B) or the C2C12 cells (compare E with F). To analyze the effects of SB202190 on the muscle cells in the primary limb mesenchymal cultures, cells were mixed at a ratio of 19:1, primary cells:G8 cells (C,D) or primary cells:C2C12 cells (G,H), and the tagged muscle cells were identified by magenta-gal staining after 12 days. When mixed into the primary cultures, both tagged cell populations become highly organized, resembling MyHC-positive cells from primary cultures that were treated with SB202190 (compare to Fig. 1). In contrast, in the untreated cultures, G8 and C2C12 cells appear to lose their muscle phenotype, becoming less bipolar and more round in appearance. Bars, (A,B) 125 µm; (C-D) 500 µm; (E-H) 250 µm.