An analysis of factors influencing post-natal growth and development of the muscle fibre

Abstract

1. An investigation is described in which the effect of both technical and physiological factors on muscle fibre size was examined. Size was determined by measuring the cross-diameter of 16,450 individual fibres by means of an ocular micrometer. In cases where an animal was represented by a single muscle sample, 100 fibres were measured, otherwise fifty measurements were recorded per sample. 2. The material consisted of muscle samples always taken from the same position along the length of the muscle and immediately afterwards fixed for varying periods of time in 10% formalin. Samples were obtained from various sources, principally from experiments conducted in the past at the School of Agriculture, Cambridge, but also from slaughterhouses and contemporary investigations. These latter samples were treated in the same manner to ensure consistency. 3. From the results of an investigation on the effect of formalin fixation on muscle fibre diameter, carried out on samples obtained from a mature buck rabbit, it was tentatively concluded that although shrinkage does occur the effect is only slight; the difference between measurements of fresh compared with fixed fibres being non-significant statistically. Shrinkage apparently ceased as soon as the fixative had penetrated the sample, since no further changes could be detected after prolonged fixation. 4. The same material was used to study the effect of heat. Application of heat (boiling) for a matter of seconds caused an immediate, highly significant, shrinkage.of individual fibres, but continued heattreatment resulted in no further changes. Though heat caused the fibre to lose its characteristic striated appearance, there were no signs of fibres disintegrating or of the sarcolemma becoming detached from its protoplasmic contents. 5. From the results of previous investigations it was deduced that the measurement of 100 fibres per sample provides a reliable mean value and a representative indication of the dispersion in a given muscle, or at a given age. Results obtained in the present study demonstrated a slight tendency for larger fibres to be measured as the period of observation proceeds, a factor which should be guarded against. 6. The effect of species on muscle fibre diameter was examined by comparing fibres of m. gastrocnemius of the rabbit, the pig, the sheep and cattle at birth and maturity. Only male animals were included in the comparison. It was shown that no relation exists between muscle fibre size and body size at either age. At birth the rabbit and sheep had rather similar-sized fibres, while those of the pig and cattle were respectively smaller and larger in diameter. At maturity the pig had the largest fibres, followed in decreasing order by the rabbit, cattle and sheep. The size of muscle fibres at maturity was accounted for by the degree of post-natal development in body weight of the particular species. 7. The effect of breed was studied on two groups of steers, the one consisting of Dairy Shorthorns and Dairy Shorthorn-crosses, and the other of Friesians and Friesian-crosses. Samples were available from m. longissimus dorsi for each of thirty-four animals: 100 fibres were measured per sample. The Friesians and their crosses had significantly thicker muscle fibres than the pure- and cross-bred Dairy Shorthorn steers. The largest differences existed between the respective pure-bred animals; differences between Friesian × Angus and Dairy Shorthorn × Angus, and Friesian × Hereford and Dairy Shorthorn × Hereford, though fairly distinct, were, however, not statistically significant. 8. The effect of age was investigated on a group of forty-one lambs of different nutritional status and sex, and ranging in age from birth to 290 days. Muscle fibre diameter was shown to increase in general with age, while a consistent decline in the coefficient of variation was regarded as indicative of the fact that muscular growth during post-natal life occurs essentially by hypertrophy of individual fibres, there being no increase in the number of fibres after birth. 9. Correlating changes in muscle fibre diameter with corresponding changes in weight, indicated that muscular growth is primarily a function of physiological age, and not strictly one of chronological age. Though highly significant correlations were established between mean fibre diameter and body and carcass weights, the strongest correlation was shown to exist between the former variate and muscle weight. However, a correlation of an even higher order was obtained between the square of muscle fibre diameter and muscle weight. It was attempted, by means of linear regressions, to indicate the contribution of length growth of the fibres to increments in muscle weight. The need for further investigation is, however, apparent; insufficient data in the present study made it impossible to elucidate this point altogether. The relationships were confirmed by an analysis of twenty lambs of the same breed, all slaughtered at 112 days of age; the heavier lambs had larger fibres than their lighter counterparts, very nearly proportional to differences in weight of muscle. 10. Sex differences in muscle fibre diameter could very nearly be accounted for on a basis of muscle weight alone at birth and at a carcass weight of 13·6 kg. At 290 days of age, high-plane wethers had but slightly thicker fibres than their female counterparts, despite a significantly heavier musculature. This was ascribed to differences in length of muscle (as shown by bone measurements), and also to differences in composition of the muscle. The results of chemical analyses were presented to prove that the muscles of wethers at that age contain greater amounts of intra-muscular (chemical) fat, hence the apparent increases in the weight of muscle could not be accounted for by an increase in the diameter of component fibres. 11. The effect of nutrition was studied in both lambs and mature...