In chicken embryo limb buds, the first myofibers begin to form by E3 �C E4, and these primary myofibers are of at least two distinct types: a fast type that expresses only fast MyHC and a fast/slow type that co-expresses both fast and slow MyHCs. This initial diversification of fast and fast/slow myofibers does not depend on functional innervation. Embryonic chicken limbs also contain distinct types of myoblasts that are committed to form either fast or fast/ slow myotubes. As fetal development begins on,E8, a distinct population of fetal myoblasts appears and secondary myofibers are formed alongside the primary fibers in the limbs. To begin to determine the possible role of Prdm1 in avian myogenesis, we have now analyzed the pattern of Prdm1 expression in cultures of myogenic cells obtained from the somites, embryonic limbs, and fetal limbs of developing chickens. We found that Prdm1 was expressed in all of these Ipsapirone different types of myogenic cells and was not limited to differentiated muscle cells that expressed slow MyHC. In addition, we found that antisense knockdown of Prdm1 inhibited MyHC expression in somite cultures, suggesting that Prdm1 has a functional role in chicken myogenesis. Having detected Prdm1 at E4 and E12 in regions of the embryo in which myogenesis occurs, we next used immunocytochemistry to determine the expression pattern of Prdm1 in cultures of cells obtained from these different regions and developmental stages. Specifically, to study Prdm1 expression in the different types of somitic, embryonic, and fetal myoblasts, we examined cultures of cells prepared from E4 somites, E4 limbs, and E12 limbs. As described below, we found Prdm1 to be expressed in each of the different types of somitic, embryonic limb, and fetal limb myogenic cells. In cultures of TCS-PIM-1-4a somite-derived cells, we found that the MyHCexpressing cells were mononucleate and that all of these myocytes co-expressed fast MyHC, slow MyHC, and Prdm1.In particular, after 1�C2 days of differentiation, all somite-derived myocytes immunostained with both mAb F59, which reacts with all fast class MyHC isoforms and mAb S58, which reacts strongly with the slow MyHC2 and slow MyHC3 isoforms and weakly with the slow MyHC1 isoform.