Environmental and Genetic Factors as Sources of Variation in Skeletal Muscle Fibre Number

The understanding of skeletal muscle growth is an important goal in animal science and in human medicine. The purpose of this review is to demonstrate the importance of muscle fibre number for muscle growth and the influence of selected genetic and environmental factors on muscle fibre number. The total number of muscle fibres is mainly determined prenatally when multinucleated myofibres form from myoblasts. In general, muscle fibre number remains almost unchanged during postnatal growth, and it is inversely correlated to the size of the individual muscle fibre. Species-specific differences in muscle mass are primarily due to differences in the number of muscle fibres. With respect to gender, in some cases male muscles were reported to exhibit more muscle fibres than female muscles. Differences in muscle mass obtained by breeding and selection are due to changes in both muscle fibre number and muscle fibre size. Genetic variability and heritability are sufficiently high to use fibre number and fibre size in farm animal selection. Moderate postnatal feed restriction does not influence muscle fibre number, whereas strong feed restriction is able to induce fibre loss. The prenatal period of muscle development is more sensitive to nutritional deficiencies in reducing fibre number. Physical activity has been shown to influence postnatal muscle fibre number. Activity stimuli are able to induce increases, whereas disuse of muscles may be followed by decreases in muscle fibre number. The postnatal application of growth promoters induces no changes in muscle fibre number, whereas the prenatal period seems to be more sensitive to hormonal factors.