Abstract

To investigate the influence of growth and aging on some characteristics of skeletal muscle, the slow soleus and fast extensor digitorum longus (EDL) muscles were examined throughout the lifespan of the rat. Muscle fibre-types and capillaries were identified histochemically from frozen, transverse sections of whole muscles. Total fibre numbers, fibre sizes, fibre-type proportions and the capillary/fibre ratio (C/F) were subsequently analysed. The ten-fold increase in body mass during the first year was accompanied by approximately proportional changes in skeletal muscle mass, due to fibre hypertrophy rather than hyperplasia. Indeed, there was an unexplained reduction by 50 in the number of fibres in the EDL muscle over this period. In old age, total fibre numbers remained stable in both muscles, indicating that the observed muscle wasting was due to fibre atrophy alone. Dramatic increases in the proportion of S.O. fibres at the expense of F.O.G. fibres in the growing soleus were largely reversed in old age, and were probably activity-dependent. Less pronounced developmental increases in the F.O.G. population of the EDL muscle were probably unrelated to the contractile activity of this muscle. Developmental changes in C/F were mainly related to fibre size rather than speed of contraction. Verf.-Referat