Unorthodox myogenesis: possible developmental significance and implications for tissue histogenesis and regeneration.

During the last few years several reports have described the occurrence of skeletal myogenesis in cells derived from embryonic, fetal and perinatal tissues that usually do not contribute to skeletal muscle in the adult vertebrate body. After a brief description of current ideas on myogenic determination in higher vertebrates, three examples of this unorthodox myogenesis will be described: 1) the occurrence of myogenesis in chick epiblast cells, cultured in isolation in serum-free medium; 2) the presence of cells endowed with myogenic potential in the embryonic mouse neural tube; and 3) the occurrence of spontaneous or induced myogenesis in mesenchymal cells during fetal and postnatal life. A possible embryological basis for unorthodox myogenesis, in relation to gastrulation and morphogenetic fields, is then presented. It is also proposed that unorthodox myogenesis may represent a compensatory mechanism for higher vertebrates that have lost much of the regeneration potential of lower vertebrates.