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Compensatory hypertrophy was induced in the rat extensor digitorum longus muscle by extirpation of the synergistic tibialis anterior. Injections of 3H-thymidine (2 muCi/g body weight) was given at 24, 48, and 72 h after the operation. In animals killed immediately after the last injection both satellite cell nuclei and true muscle nuclei were labeled, whereas in animals killed 4 weeks later only true muscle nuclei were labeled. The results are interpreted as indicating incorporation of newly formed satellite cells into the fibers during compensatory hypertrophy.

The appreciation and utilization of Histochemistry in Biology varied in the course of time, according to the development of new and more reliable techniques and according to the possibility of checking the results on the basis of rigorous biochemical research. It is well known that the need of histochemistry is best understood when the problem is that of individualizing single cells or groups of cells whose physiology (and biochemistry) is different from that of neighbouring other cells of the same nature, and when it is impossible or not desirable to fractonate such kind of cells or their operative substructures. One of the field of animal biology in which biochemistry has been very successfully applied is that of the study of the differentiation of skeletal muscle fibers, which derive from the fusion of many separate myoblastic cells with possible different genetic potentialities. In this connection, it appears sufficiently clear that the differentiation (or modulation) of fibres in fast and slow, red and white is largely dependent on epigenetic influences and particular on that of the type of neuron with which the fiber is connected. We have sufficient experimental evidence of change in type of muscle fibres following change of the neuronal part of the neuromyone (cross reinnervation, etc.). It is also possible that the neurotic influence is translated only as pattern of excitation and in so far it could be imitated by other sort of stimulation; on the other hand, it is also possible, particularly in young animals, that the state of the muscles and the physiological utilization of them in the whole complex animal behaviour (modifiable by experiment) could influence the differentiation of neurons in the fast and slow types. In any case, any established physiological differentiation among muscle fibres, particularly that reflected by different kinds of contractile proteins and of their ATPase activity, corresponds to a simmetrical differentiation, among motoneurons. In the case of pathology, the histochemical study of muscle fibres has become an unavoidable mean of diagnosis. But some criticism should not be neglected when we face the problem of inferring the nature (neurogenic or non neurogenic) of a myopathy only from the distribution of the lesions among the fiber types of a diseased muscle.

Electron microscopic evidence is presented that the early response to denervation ("simple atrophy") of the semitendinosus m. of the frog is characterized by a greater prominence of the sarcoplasmic reticulum and by the presence, in the interfibrillar spaces, of mitochondria which are more numerous and smaller than in normal muscle. In contrast with the dynamic changes of the sarcoplasmic structural components, the myofibrils showed a progressive decrease in diameter after denervation and throughout the period studied. By carrying out tissue fractionation experiments, the yield of microsome-protein was found significantly greater in the denervated muscles, as compared with the contralateral controls, in this initial stage. Under the conditions attending the overdevelopment of the sarcoplasmic reticulum (SR), denervated semitendinosus m. incorporated valine-C(14) into proteins more actively than the control pairs. The denervated muscles also showed an increase in the number of freely scattered and membrane-bound ribosomes and of polyribosomes, suggesting a more active synthesis of the SR membranes. Pronounced atrophy of the myofibrils, disorganization of the SR, and an increased number of ribonucleoprotein particles lying in the enlarged interfibrillar spaces were the main ultrastructural features of "degenerative atrophy" in frog muscle in the late periods after denervation. The probably adaptive character of the early changes occurring on denervation of frog muscle is discussed.