HISTOCHEMICAL STUDIES ON THE HISTOGENESIS OF RAT MEDIAL PTERYGOID MUSCLE

Abstract

The cellular differentiation and histogenesis of the histochemical architecture of the medial pterygoid muscle of rats were studied by histochemical staining for SDH. From day 18 of gestation to day 18 after birth, the development of two kinds of muscle fibers could be distinguished: fibers with fine formazan crystals diffusely distributed in the sarcoplasm (SS) and fibers with large formazan particles diffusely distributed in the sarcoplasm (SL). After day 18 some muscle fibers showed marked progress of cellular differentiation and became similar to adult muscle fiber types. After day 23 most muscle fibers could be classified into three types; Type A, Type B, and Type C according to Stein and Padykula (47). Until day 2 after birth, regions with high and low percentage distribution of SL fibers (Psl) were distinguishable. By day 3 after birth the organization of the primordial muscle structure observed in cross section had become similar to that of adult muscle and significant differences of Psl values were found in different regions in cross sections. Thereafter the Psl values of each muscle region increased, especially from day 6 to 18. On day 23 after birth the distribution patterns of percentage of Type B plus Type C fibers (Pbc) and the percentage of Type C fibers (Pc) in cross sections became similar to that in young adult muscle. Moreover, the range of Psl, Pbc and Pc values of primary fasciculi in each region showed significant changes during postnatal development.