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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.)
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