Переводчик Федоров Егор Николаевич

To understand the impacts of genetic variants affecting skeletal muscle traits in livestock, it is important to consider the normal functions and development of skeletal muscle. Skeletal muscle is superbly adapted to its functional roles. Muscles undergoing slow but continuous contractions in the post-natal environment, such as various postural muscles, are characterized by predominance of slow twitch fibers (type 1 fibers), which do not tire easily and are reliant on oxidative metabolism. They have abundant mitochondria and appear reddish in color. In contrast, muscles requiring rapid contraction generating substantial force, such as some locomotory muscles, have a greater proportion of fast twitch fibers (type IIb and IIx fibers). These muscles largely depend on glycolytic metabolism for energy generation and are whiter and have fewer mitochondria (MacIntosch et al., 2006). The number of fibers and fiber type composition are set during early to mid fetal development. During late fetal development and the rapid post-natal growth period there is extensive muscle fiber hypertrophy driven by fusion of mononuclear muscle satellite cells with multinucleated myofibers (MacIntosch et al., 2006). Satellite cells are located between the sarcolemma and basal membrane of muscle fibers, and while normally quiescent in adult muscle, upon muscle injury, they proliferate and fuse with myofibers thereby aiding repair. Muscle fibers are composed of a number of highly organized proteins, principally polymeric actin, myosin, and their ancillary proteins (Tellam, 1985; MacIntosch et al., 2006).

The transition from the ruminant fetal environment to that of the newborn is associated with major physiological changes. Skeletal muscle must rapidly adapt to meet the new demands of locomotion and postural support against gravity, whilst using different sources of energy than those available in the uterine environment. These adaptations are of great importance to newborn ruminants as they stand, walk, and run within a few hours of birth. Ovine skeletal muscle undergoes a major transition late in its development, which ensures newborn skeletal muscle is largely characterized by adult skeletal muscle fiber types (Byrne et al., 2010b). Thus, skeletal muscle is subject to a robust developmental program upon which is built responsiveness to environment cues. Mutations affecting muscling (i.e., muscle thickness relative to skeletal dimensions) typically