A study of the histological changes in the growing muscles of beef animals

Summary. 1. Histological studies have been made of the growth of the longissimus dorsi (LD) and semitendinosus (ST) muscles of eight Hereford and eight Friesian steers between the ages of 11 days and 24 months. 2. It was found that the average sarcomere lengths of the muscle fibres varied considerably from animal to animal and muscle to muscle, due to postural shortening during rigor. The lengths of the constituent filaments of actin and myosin were, however, constant within a narrow range (1.30 and 1.50 μ, respectively). Hence it is possible to define a standard sarcomere length for reference, as the length when the actin filaments from each half of the sarcomere just touch in the middle. This length is 2.60 μ, after correction for fixation. Cross-sectional areas and diameters of fibres were then corrected accordingly. 3. After correction in this way, it was found that the internal diameters of the muscle fibres increased from about 15 μ at 1 month to about 45 μ at 24 months. For the first 12 months of growth the cross-sectional areas of the fibres kept in step with the cross-sectional area of the muscle, but after that time the former parameter increased more rapidly than the latter, so that fibres appeared to be lost from the muscle as it aged further. In the ST muscles, this loss amounted to about 40% at 24 months. 4. The collagen content of the muscles was higher at the calf stage than later. From 6 months onwards however this content remained nearly constant (0.43 and 0.77% of the wet weight, for LD and ST muscles, respectively). These changes were easily observable in the histological sections. 5. The elastin content, both by semiquantitative histological estimation and by an exact chemical method, was found to be very low in the LD muscles (about 3% of the total connective tissue content), but much higher in the ST muscles (about 37%). In other muscles of prime cuts, such as biceps femoris, the elastin content is, however, of the same order as in the LD, so that its contribution to the toughness of most cuts of meat is probably insignificant. 6. More detailed conclusions of the results are given at the end of the paper.