Retinoic acid (RA) and BMP4 counteract each other's inhibition of skeletal myogenesis or cardiomyogenesis. P19 cells were mixed with P19[BMP4] or P19[control] cells in the presence of 1% dimethylsulfoxide (DMSO), with or without RA. Panel I: P19[BMP4] cultures treated with RA were fixed on day 9 for immunofluorescence with MF20 antibody (A, B) and counter stained with Hoechst dye (C, D). Magnification is 160x. Panel II: Skeletal myogenesis was quantified for each condition by counting the number of myosin heavy chain+ve bipolar skeletal myocytes, expressed as the percentage of total cells (white bars) and their standard errors (n = 3). MyoD transcript levels (black bars) were quantified by quantitative polymerase chain reaction and expressed relative to control cultures (n = 2), *P < 0.05. Panel III: Cardiomyogenesis was quantified as described for Panel II, n = 4. Panel IV: On days 6 and 9 total RNA was harvested for northern blot analysis and probed with the cDNAs indicated on the right. Panel V: A time course of P19[BMP4] and P19[control] cells aggregated in the presence of DMSO and RA. Total RNA was harvested for northern blot analysis on days 1-4, 6, and 9 and probed with the cDNAs indicated on the right.