HISTOCHEMICAL INVESTIGATIONS IN MATURATION OF RAT SKELETAL MUSCLE AFTER WHOLE-BODY GAMMA-IRRADIATION AT FOURTH DAY OF LIFE

In the course of the normal development of rat muscle, myotubes gradually disappear between the 1st and 14th days of extra-uterine life. In the muscles of the 14-day-old rat there are only isolated myotubes (0-3% of fibres). After the 8th day of life, clear differentiation of the two main metabolic types of muscle fibre has been observed. Rats treated with 300 r whole-body γ-irradiation at the 4th day of life showed at the 10th-14th days of life an abundance of myotubes which showed features of both metabolic types of fibres (15-30% of all fibres). At this period a decrease in phosphorylase a activity was observed in some groups of muscle fibres. Otherwise the differentiation of the metabolic fibre types in the muscles of irradiated rats was the same as in control animals. The myotubes, which persisted abnormally after irradiation, were of both metabolic types, in contradistinction to the finding in experimentally denervated muscles at the same stage. The fact that the differentiation into metabolic fibre types was not affected by irradiation suggests that the persistence of myotubes is not due to the effect of irradiation upon the innervation of muscle but that it is due to an effect upon the muscle cells themselves. Despite the observed persistence of myotubes at the 14th day the development of muscle tissue in the irradiated animals subsequently continued in exactly the same way as in non-irradiated controls. © 1969.