Fig. 2. (A) Normal skeletal muscle in transverse section showing muscle fibres separated by endomysial connective tissue and small profiles of vascular supply. Part of a muscle spindle is visible toward the bottom left of the micrograph. Satellite cells are impossible to identify at this magnification. (Original micrograph x140; Toluidine Blue staining.) (B) Normal muscle in longitudinal section, some 10 min following traumatic injury. Fibres are broken and show hypercontracted myofibrils. There is extravasation of red blood cells from a damaged vessel into the interstitial space. (Original micrograph x140; Toluidine Blue staining.) (C) Micrograph of a section of skeletal muscle from a muscle of a patient with Duchenne muscular dystrophy. While there are some fibres of muscle present (to the left of the field), much of the field is filled with dense collections of collagen bundles and adipocytes (central and right of the field). (Original micrograph x140; Toluidine Blue staining.) Much of the pathology literature on DMD refers to muscle being replaced over time by adipose and connective tissues as muscle fibres are damaged and the disease progresses. However, there is a distinct possibility that the adipocytes and fibroblasts may have differentiated from stem cells in the satellite cell position on fibres, since this alternate differentiation can be observed in tissue culture and in aging muscle (Jozsi et al., 2001; Shefer et al., 2004; Taylor-Jones et al., 2002). Other descriptions of `transdifferentiation' are reported between vascular smooth muscle and skeletal muscle and may relate to the apparent pool of stem cells in the mesangioblast compartment in development (Cossu and Bianco, 2003; Galli et al., 2005; Graves and Yablonka-Reuveni, 2000; Sampaolesi et al., 2003).