Enzyme-immunohistochemical aspects of muscle fiber type classification in mammals

Skeletal muscles are the most abundant and adaptable tissue in mammalians. They are composed of heterogeneous muscle fibers, in which distinct sets of structural proteins and metabolic enzymes are expressed. The percentages of different muscle fibers in the muscle define its morphological and functional characteristics. In this review, we summarize enzyme-immunohistochemical techniques to present muscle fiber type characteristics and their diversity in somatic skeletal muscles of various animal species. The principal methods to define myofiber properties on the tissue sections are based on the immunohistochemical determination of myosin heavy chain (MHC) isoforms and the myosin ATPase and metabolic enzyme histochemistry Four MHC isoforms (-I, -IIa, -IIx and -IIb) have been detected in somatic skeletal muscles of small mammals. Fibers that co-express more than one MHC isoform simultaneously are labeled as hybrid myofibers and are indicators of muscle fiber transition. The maximal shortening velocity of MHC fibers increases in the following order: -I -IIa -IIx -IIb. On the basis of the myosin ATPase activity myofibers have been classified as types I, IIA, IIB and IIC. Type IIC fibers represent an intermediate type between MHC-I and type MHC-IIa fibers. Most large mammals do not possess fastest MHC-IIb isoform, although some recent studies in pigs and llamas have shown the existence of all three fast MHC isoforms in their skeletal muscles. Additional MHC isoforms are present transitorily during development, and in some highly functionally specialized muscles such as extraocular, laryngeal and masticatory muscles (MHC-extraocular, MHC-m). Embryonic and neonatal MHC isoforms are expressed during muscle development and regeneration. Slow MHC-I myofibers show high oxidative capacity, whereas fast MHC-II myofibers revealed entire spectrum of metabolic enzymes activity with large overlaps between contractile fiber types. Combining the contractile classification with metabolic enzymes activity, myofibers can be basically defined as slow-twitch oxidative (SO), fast-twitch oxido-glycolytic (FOG) and fast-twitch glycolytic (FG). In most cases enzyme and immunohistochemical techniques are not fully interchangeable, which makes combination of different techniques necessary to get a reliable classification of myofibers.