2.2. Fibril forming and associated collagen types in skeletal muscle

The epimysium consists mainly of type I collagen with minor amounts of type III collagen (Light & Champion 1984, Järvinen et al. 2002). Equal amounts of both collagen types are found in the perimysium. In the endomysium, type III collagen is the predominant form and only trace amounts of type I collagen are found (Light & Champion 1984, Järvinen et al. 2002). Slow muscles contain more type I collagen than type III collagen, the proportion of type III collagen being greater in fast muscles (Miller et al. 2001). Collagens I and III are fibril forming and serve as a supportive structure in muscle tissue. They attach myocytes to each other and muscle bundles to each other (Järvinen et al. 2002). Also nerves and capillaries are surrounded and attached to muscle by collagen (Järvinen et al. 2002). Type V collagen is also fibril forming and can be found in the endo- and perimysium in smaller amounts than the collagen types I and III (Light & Champion 1984). Collagens III and V are known to copolymerize with type I collagen and they may have a role in collagen fiber diameter regulation (Birk & Mayne 1997, Fleischmajer et al. 1990a,b, Birk et al. 1990). Type V collagen is postulated to form the core of the fibrils, and collagens I and III copolymerize around this core (Fig. 2) (Aumailley & Gayraud 1998). Type II and XI collagens, which are also fibril forming, are detected in skeletal muscle only at mRNA level (Sandberg et al. 1993). Because type V and XI collagens form heterotypic molecules it is proposed that instead of being separate collagen types, they can be considered as a single kind of collagen (Kleman et al. 1992, Mayne et al. 1993, Prockop & Kivirikko 1995). Fibril associated collagens with interrupted triple helixes (FACIT) types XII and XIV are located only in the perimysium (Listrat et al. 2000), although their distribution is closely related to type I collagen and they are also found in the epimysium and endomysium during embryonic development (Wälchli et al. 1994). These FACIT collagens associate with the surface of interstitial collagen fibrils and possibly act as molecular bridges among fibrils or between fibrils and other components of the ECM (Fig. 2) (Wälchli et al. 1994). Although mRNAs of the other members of FACIT subfamily (IX, XVI, XIX, XXI) are detected in skeletal muscle, the respective proteins have not been found (Lai & Chu 1996, Bönnemann et al. 2000, Sumiyoshi et al. 2001, Chou & Li 2002).