Myofibre number and myofibrillar development in neonatal pigs

Unique features of myogenesis in Egyptian cobra (Naja haje) (Squamata: Serpentes: Elapidae)

During early stages of myotomal myogenesis, the myotome of Egyptian cobra (Naja haje) is composed of homogenous populations of mononucleated primary myotubes. At later developmental phase, primary myotubes are accompanied by closely adhering mononucleated cells. Based on localization and morphology, we assume that mononucleated cells share features with satellite cells involved in muscle growth. An indirect morphological evidence of the fusion of mononucleated cells with myotubes is the presence of numerous vesicles in the subsarcolemmal region of myotubes adjacent to mononucleated cell. As differentiation proceeded, secondary muscle fibres appeared with considerably smaller diameter as compared to primary muscle fibre. Studies on N. haje myotomal myogenesis revealed some unique features of muscle differentiation. TEM analysis showed in the N. haje myotomes two classes of muscle fibres. The first class was characterized by typical for fast muscle fibres regular distribution of myofibrils which fill the whole volume of muscle fibre sarcoplasm. White muscle fibres in studied species were a prominent group of muscles in the myotome. The second class showed tightly paced myofibrils surrounding the centrally located nucleus accompanied by numerous vesicles of different diameter. The sarcoplasm of these cells was characterized by numerous lipid droplets. Based on morphological features, we believe that muscle capable of lipid storage belong to slow muscle fibres and the presence of lipid droplets in the sarcoplasm of these muscles during myogenesis might be a crucial adaptive mechanisms for subsequent hibernation in adults. This phenomenon was, for the first time, described in studies on N. haje myogenesis.

Fiber type composition and postmetamorphic growth of anuran hindlimb muscles

Fiber type composition distinguished by the level of succinic dehydrogenase, mean fiber diameter, and fiber number were determined for the gracilis major and sartorius in ontogenetic series of the anurans Rana pipiens, Bufo americanus, and Xenopus laevis to characterize muscle composition and mode of muscle growth. A correlation can be made between the respective locomotor mode of these anurans, their activity physiology, and the fiber type composition of the propulsive muscles. R. pipiens and X. laevis have hindlimb muscles composed primarily of fast-twitch, fast-fatiguing type 1 fibers. B. americanus's muscles are composed of slow-twitch, slow-fatiguing type 2 and type 3 fibers. Fiber typing is apparently useful in explaining interspecific anuran locomotor capability previously difficult to correlate with body shape. Fiber typing also affords an explantion for reported variations in metabolism during strenuous activity. Postmetamorphically, anuran muscles grow by a combination of hyperplasia and hypertrophy, but the percent fiber type composition remains unchanged. In R. pipiens and X. laevis, the muscles grow primarily by hypertrophy with the addition of relatively fewer fibers. The converse is true for B. americanus. In all three species the combinations result in strong positive allometric increases in muscle cross section. Anuran muscle growth is similar to that reported for fish and different from that associated with mammals.