Fig. 1. Effects of p38 inhibition on chondrogenesis and myogenesis in PLM cultures. Primary limb mesenchymal cells were plated at high density, and cultures were fixed for detection of cartilage and muscle. Cartilage was detected by alcian blue staining, and muscle cells were identified by immunofluorescence with a mouse MyHC antibody. In untreated cultures, cartilage nodule formation increases over time, whereas the number of MyHC-positive cells declines to only a few by day 10 (A). Exposure of limb mesenchymal cultures to SB202190 (10 µm) (A) inhibits cartilage nodule formation, whereas there is a substantial increase in the presence of MyHC-positive cells in comparison to control cultures. At later stages the SB202190-treated cultures exhibit extensive myocyte fusion accompanied by the organization of the myocytes into parallel arrays (B). Bars, (A) bright field, 1.0 mm; fluorescence images, 500 µm; (B) 65 µm.