On the cellular regulation of growth and development in skeletal muscle

1. Fibres of skeletal muscle in different mammalian species vary more in number and in their rates of growth than in their ultimate breadth, and they grow more slowly in cattle and man than in rats and mice. Cells of large mammalian species probably divide comparatively slowly in pre-natal life but do so for longer, and thus they attain greater numbers than do their counterparts in smaller mammals. Such cells include the precursors of muscle, and common mechanisms may therefore limit rates of growth before and after muscles form. If some metabolic processes are slower in mammals destined to be large, corresponding trends in age-related cellular changes which ultimately suppress mitotic activity may cause differences between species in the overall size of muscles and in that of other tissues. This is probably an oversimplification. 2. It is difficult to decide how far the rate of growth and the final diameters of muscle fibres reflect the number of myoblasts which initially fuse into myotubes and the number of myoblasts which are subsequently incorporated into individual fibres. New nuclei are probably added with age along the length of a fibre, but it is uncertain whether they then synthesize ribosomes which produce contractile protein. It seems likely that fibres elongate to different extents by adding myoblasts terminally. 3. There is some evidence that myofibrils grow throughout the depth of a fibre by adding new myofilaments to their surface, but there is none that is convincing to the effect that myofibrils form de novo at a fibre's periphery. Ribosome-like structures distributed in the sarcoplasm between myofibrils have been described, and their numbers decline in comparison with those of the myofibrils during growth. Thus, fibres possibly attain their maximum breadth when the loss of superficial filaments from myofibrils exceeds the capacity of ribosomes to replace them. The evidence is inconclusive as to whether myofitrillar protein is broken down and replaced at rates which vary within a muscle or between muscles differing in physiological properties. Sarcoplasmic proteins appear to be replaced more rapidly than those in myofibrils. It is also speculated that muscle proteins are synthesized and degraded more slowly in species which take longer to develop. 4. Observations, with the microscope suggest that new ribosomes appear in cells which are becoming myoblasts. Whether the ribosomes subsequently break down is not established. The evidence that I-somes occur in muscle is inconclusive, as is that for the existence of messenger RNA and its selective synthesis when muscle is forming in the embryo. 5. A decline in the synthesis of RNA occurs as myotubes appear and contractile protein begins to accumulate. The significance of this phenomenon is not known, and in more mature muscle some RNA also appears to fluctuate in a fashion which is unrelated to rates of controlling protein synthesis. Such RNA may occur at the periphery of fibres or in satellite cells. In some instances it may be formed by cells of the connective tissue and capillaries. There are indications that the growth of muscle does not require the continued transport of new RNA and ribosomes into the body of a fibre. 6. As regards the existence of polyribosomes in muscle and the activity of muscle ribosomes in zlitro, most relevant phenomena can be explained if the ribosomes are aggregated, inter ah, by newly completed protein and if observed variations in activity are some function of the residual amounts of nascent protein which remain on the ribosomes. The morphological appearance of ribosomes in myoblasts is difficult to reconcile with the notion of ribosomes linked by messenger RNA. There is also some rather inconclusive evidence that the sarcoplasm varies in the effectiveness with which it supports protein synthesis by ribosomes. 7. Muscle fibres differ markedly in the number of mitochondria which they exhibit in histological sections and in the rate at which the homogenized fibres catalyse the processes of aerobic respiration which occur in mitochondria. It is uncertain how far such variation is determined by the properties of myoblasts and myotubes, by the nature of subsequent contractile activity and by dilution of the mitochondria as myofibrillar protein accumulates. In part, the tendency of fibres richest in mitochondria to be comparatively small may reflect the diversion of energy sources and oxidizable precursors of protein into energy-generating pathways. However, such fibres perhaps also possess fewer nuclei and fewer functional ribosomes. 8. Within a given animal, variation between fibres in the activity of sarcoplasmic enzymes becomes most pronounced after the myoblast stage. Assuming that these sarcoplasmic proteins are increasing by dissimilar amounts, genes in different fibres are perhaps varying in activity, but this has not been studied. It may be that the intermittent and increasingly forceful contractions of developing fast-phasic fibres simply cause them to accumulate increased amounts of amino acids in the pool from which protein is synthesized, so that a generalized stimulation of protein synthesis follows. Sarcoplasmic protein should then accumulate more than myofibrillar protein relative to starting quantities. This is a consequence of sarcoplasmic protein turning over faster. However, in addition, one must postulate that sarcoplasmic enzymes vary in stability between fibre types. It also remains to assess whether such differences reflect the presence of different molecular forms of each enzyme and whether the latter possess dissimilarities of amino-acid sequence or of molecular configuration. Similar unsolved problems arise regarding the ATPase activity of myosin in developing muscles and its variation between fibres.