Muscle hyperplasia

Abstract: In this chapter we are concerned with the processes of muscle fiber hypertrophy (increase in size) and hyperplasia (increase in number) during post-embryonic growth, i.e., after the initial phase of myogenesis. A brief outline is given of methods used to measure these processes. We then discuss the main features of hypertrophic growth, and distinguish between two quite distinct phases of hyperplastic growth. The first phase of hyperplastic growth generally takes place during at least part of larval life and completes the formation of the main muscle layers which were initiated during embryonic myogenesis. As this process generates new fibers along a distinct germinal layer we have named it “stratified” hyperplasia. In fish which grow to a large final size this is followed by a second and quite different hyperplastic process. As new fiber production is now disseminated across the whole myotome this results in a mosaic of fiber diameters when the muscle is cut in transverse section and we have, therefore, called this process “mosaic” hyperplasia. Mosaic hyperplasia results in a large increase in total fiber number during juvenile growth, and is therefore very important for commercial aquaculture species; it is lacking in species which remain small. We also discuss various biotic and abiotic factors which can affect muscle growth, especially in the context of intensive aquaculture, and have attempted to identify some areas where new investigative approaches are needed.

Abstract: Heart regeneration is limited in adult mammals but occurs naturally in adult zebrafish through the activation of cardiomyocyte division. Several components of the cardiac injury microenvironment have been identified, yet no factor on its own is known to stimulate overt myocardial hyperplasia in a mature, uninjured animal. In this study, we find evidence that Neuregulin1 (Nrg1), previously shown to have mitogenic effects on mammalian cardiomyocytes, is sharply induced in perivascular cells after injury to the adult zebrafish heart. Inhibition of Erbb2, an Nrg1 co-receptor, disrupts cardiomyocyte proliferation in response to injury, whereas myocardial Nrg1 overexpression enhances this proliferation. In uninjured zebrafish, the reactivation of Nrg1 expression induces cardiomyocyte dedifferentiation, overt muscle hyperplasia, epicardial activation, increased vascularization, and causes cardiomegaly through persistent addition of wall myocardium. Our findings identify Nrg1 as a potent, induced mitogen for the endogenous adult heart regeneration program.

Abstract: Growth of white axial muscle fibres of ten species of freshwater teleosts from five families (Cyprinidae, Centrarchidae, Percidae, Salmonidae, Esocidae) possessing widely different growth rates and ultimate sizes have been studied. The dynamics of muscle increase (i.e. increase in fibre numbers and/or diameter) appears to determine the ability for rapid somatic growth and large ultimate size in teleosts. Thus, the largest and fastest growing species (smallmouth bass, lake whitefish, rainbow trout, muskellunge) show evidence of sustained recruitment of muscle fibres to a large size, in contrast to the smaller and slower growing species (bluntnose minnow, longnose dace). Pumpkinseed, bluegill and yellow perch are all intermediate in fibre growth dynamics, growth and ultimate size between the smaller and larger species. Moreover, the ability of teleosts to grow rapidly and attain a large ultimate size is dependent on the body length at which recruitment of new muscle fibres into the growing axial muscle ceases. The regression equation, y=– 0.29 + 2.26 (x), showing the relationship (r = 0.95) between ultimate body length, x, and fork length at cessation of recruitment, y, indicated that for these teleosts, recruitment tends to cease when the fork length reaches about 44% of the ultimate body length. Possible mechanisms to account for this relationship are proposed, and the role of the ultimate fibre diameter in posing limits to the ultimate size of the species is discussed.

Abstract: The growth of red and white muscle was investigated in the rainbow trout, using fish from 2.2 cm to 71 cm in length. In the white muscle, fibre hyperplasia, initially, accounted for all muscle growth but its relative contribution decreased as the contribution from fibre hypertrophy increased. At and above about 65 cm there was no hyperplasia, and this corresponded approximately to the stage when there were no more fibres in the smallest size class (less than 20 microns). The results for the red muscle are more variable and hence more difficult to assess. Although red fibre hyperplasia may continue throughout growth, at 34 cm there are no fibres in the smallest size class, which possibly indicates no new fibre formation beyond this stage.