Fig. 5. (A) A columnar arcade in which plasticity of function or form is not integrated into the design. (original photograph, Agora, Athens, Greece). (B) This micrograph of skeletal muscle regenerating after traumatic injury contrasts with the arcade of Greek columns in A. A tendon courses through the figure vertically (broken line) and is met by myotubes (arrows) forming in the belly of the damaged muscle. In the damaged region, many myogenic cells (progeny of satellite or stem cells) are present, admixed with numerous mononuclear cells that are only partly identifiable as inflammatory, phagocytic, fibroblastic or angiogenic. The myotubes contain numerous central myonuclei, indicating they have been formed in a regenerative process in the 6 days since injury. The tendon and myotubes, vascular and connective tissues are all involved in the regenerative events that support the plasticity of this skeletal muscle. (Original micrograph at x60.) (C) Myotubes forming by regenerative events at 4 days after injury, showing accretion of nuclei and expansion of the myofibre diameter (arrow). (Original micrograph at x140.) (D) Myotubes and mononuclear cells in the bed of a regenerating muscle following injury. In this field, there is a small central area of ossification (arrow), in a region where no osteogenic progenitors would have been identified prior to injury. The formation of bone in a muscle also demonstrates plasticity of satellite cells in taking direction to be precursors for alternate mesenchymal tissues (Deasy et al., 2004); Lee et al., 2000; Shen et al., 2004). (Original micrograph at x140.) (E) A myotube (arrow) forming through addition of myogenic cells, identified (by in situ hybridization) by their expression of myogenin transcripts, a muscle-specific regulatory gene. The myogenic cells are progeny of the satellite or stem cells that were activated by muscle injury. After proliferation, the cells migrate within the regenerating muscle bed, become aligned in the longitudinal axis of the muscle (which is still attached to the tendons and therefore subject to passive tension during locomotion) and fuse. Long filopodial processes can be observed extending toward the myotube from myogenin-expressing cells in the surrounding region of mononuclear cell infiltration. (Original micrograph at x140.) (F) Micrograph showing a branched myotube (arrow), formed in a regenerating muscle during treatment with a NOS inhibitor. The fusion of myogenic precursors to myotubes or remnant stumps of damaged fibres is mediated in part by the level of nitric oxide in skeletal muscle, and with a low level of nitric oxide, the branched myotube phenotype predominates over the typical slender cylindrical myotubes that form in normal regenerating muscle (Anderson, 2000). (Original micrograph at x140.)