Morphogenesis and evolution of vertebrate appendicular muscle.

Two different modes are utilised by vertebrate species to generate the appendicular muscle present within fins and limbs. Primitive Chondricthyan or cartilaginous fishes use a primitive mode of muscle formation to generate the muscle of the fins. Direct epithelial myotomal extensions invade the fin and generate the fin muscles while remaining in contact with the myotome. Embryos of amniotes such as chick and mouse use a similar mechanism to that deployed in the bony teleost species, zebrafish. Migratory mesenchymal myoblasts delaminate from fin/limb level somites, migrate to the fin/limb field and differentiate entirely within the context of the fin/limb bud. Migratory fin and limb myoblasts express identical genes suggesting that they possess both morphogenetic and molecular identity. We conclude that the mechanisms controlling tetrapod limb muscle formation arose prior to the Sarcopterygian or tetrapod radiation.