HISTOCHEMICAL, ULTRASTRUCTURAL AND CERTAIN BIOCHEMICAL CHARACTERISTICS OF AVIAN SKELETAL MUSCLE FIBERS, WITH EMPHASES ON THE PECTORALIS MUSCLE AND DISUSE ATROPHY

The distribution of fiber types within the pectoralis muscle (PM) of 45 species of carinate birds and two species of ratites is described. The carinate PM consists almost entirely (99-100%) of red (aerobic) and/or white (anaerobic) fast-twitch fiber types. Slow fiber types are rare, and are restricted to the deepest craniodistal fasciculi. There is a higher proportion of red fibers in the deeper areas of the PM in those carinates having both red and white fibers. The numbers and distribution of fast-twitch fiber types represent adaptations to flapping flight. A deep, accessory, belly of the PM found in catharid vultures consists entirely of slow fibers, and may be an adaptation to soaring flight. The ratite PM has a high proportion of widely distributed slow fibers. This might be a primitive or neotenic feature, or simply a consequence of flightlessness. All avian slow fiber types are slow-tonic, as evidenced by their multiple motor endplates.; The effects of moult-induced disuse atrophy on the PM of the giant Canada goose (Branta canadensis maxima) are described. Geese collected during premoult, moult and postmoult periods show no significant differences in body weight. There is, however, a highly significant decrease in the lean dry weight of the PM during moult. The concentrations of intramuscular iron and zinc increase as muscle mass decreases. The total zinc content in the muscle decreases, but the total iron content is unchanged. Red and white fiber types show a significant reduction in their cross-sectional areas during moult. Fiber type transformations do not occur, and the fibers retain their size relationship during atrophy. While the myofibrillar cross-sectional area decreases in both fiber types during moult, the density and number of myofibrils increases significantly. Non-myofibrillar material is lost from the periphery of the fibers during disuse. Natural disuse atrophy is nonpathological, and initiated through longitudinal splitting of the myofibrils. The present study also reveals an extremely high density of muscle spindles in M. coracotriceps of the pigeon (Columba livia), which indicates a role in mechanoreception. This muscle, previously considered vestigial, may act as a mechanoreceptor.