 In striated muscle cells, paxillin is important in costamerogenesis, or the formation of costameres, which are specialized focal adhesion-like structures in muscle cells that tether Z-disc structures across the sarcolemma to the extracellular matrix.The current working model of costamerogenesis is that in cultured, undifferentiated myoblasts, alpha-5 integrin, vinculin and paxillin are in complex and located primarily at focal adhesions.During early differentiation, premyofibril formation through sarcomerogenesis occurs, and premyofibrils assemble at structures that are typical of focal adhesions in non-muscle cells; a similar phenomenon is observed in cultured cardiomyocytes.