Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Skeletal muscle consists of multinucleated fibres which are classified as slow (I) or fast (IIA, IIB, IIX) muscle fibre types which differ in their contractile properties and their expression of contractile protein isoforms. Individual muscles contain characteristic distributions of fibre types which can be identified based on their myosin heavy chain (MyHC) content. While classical studies have demonstrated that the fibre type composition of adult muscles can be altered by extrinsic factors (such as changes in the pattern of innervation), recent studies suggest that different fibre types may be formed from distinct myoblast lineages. The purpose of this study was to test the hypothesis that myoblasts from early and late stages of development represent distinct myoblast lineages, which differ in their developmental potential. The goals of this project were to (a) determine if developmental populations of myoblasts in the rat exhibit different fibre type potentials, (b) determine when lineages are established during myogenesis, and (c) determine if extrinsic factors can modulate their expression.;To address these questions, myoblasts obtained from embryonic day (ED) 14 (embryonic) and ED20 (fetal) rat hindlimbs were grown in culture and analyzed for MyHC expression using a panel of monoclonal antibodies specific for MyHC isoforms. Embryonic myoblasts expressed both embryonic and slow MyHCs while fetal myoblasts expressed embryonic, neonatal fast and adult fast isoforms, suggesting that these populations have different default patterns of expression and may be programmed to form slow or fast fibre types, respectively. To determine if these two populations could fuse with each other and if fusion altered MyHC expression, individual populations were specifically labelled and then co-cultured. Examination of these co-cultures revealed muscle heterokaryons containing nuclei from both embryonic and fetal myoblast populations. Individual nuclei maintained their characteristic MyHC expression as nuclear domains within these muscle heterokaryons indicating that distinct developmental potentials are established prior to fusion.;To determine if external cues could affect the developmental potential of these myoblast populations, both embryonic and fetal myoblasts were injected in the caudate-putamen of adult rats. Myotubes expressing slow, IIA, IIB and IIX MyHCs were observed in both types of injection grafts. However, myotubes which expressed exclusively slow MyHC were only found in the embryonic injection sites, indicating the existence of a "slow only" myoblast population only at early time points in development. Therefore, it appears that fetal myogenic precursor cells are present in embryonic cultures, but only differentiate in vivo.;These studies support the existence of myoblast lineages and suggest that, although environmental cues can modulate fibre type expression, the intrinsic program of the myogenic lineage determines the extent of modulation that can occur. This restriction in developmental potential represents the adaptive range of a myoblast lineage. The "delineation" of muscle precursor cells with different developmental potentials follows myogenic determination and precedes myogenic differentiation. (Abstract shortened by UMI.)



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