Skeletal Muscle

More than 600 skeletal muscles arise from myoblasts, which originate from mesodermal cells located in pairs of somites that flank the developing notochord and neural tube. Of the three germ layers in the early embryo, the paraxial mesoderm gives rise to somites. A somite may be subdivided into the dorsomedial (epaxial) domain, which

150 200 Gestational Age

Fig. 1 Pattern of weight change for equine, bovine, ovine, and porcine fetuses. The fetal period represents approximately the last 80 85% of the gestational time period.

150 200 Gestational Age

Fig. 1 Pattern of weight change for equine, bovine, ovine, and porcine fetuses. The fetal period represents approximately the last 80 85% of the gestational time period.

generates the muscles of the back, and the ventrolateral (hypaxial) domain, which gives rise to the abdominal, intercostal, and limb musculature. Distinct classes of proliferative myoblasts are present during embryonic, fetal, and adult skeletal muscle development.1-3-1 Ultimately, these myoblasts undergo terminal differentiation, which involves both permanent withdrawal from the cell cycle and expression of muscle-specific genes. Differentiation is followed by fusion of myoblasts to form multinucleated myotubes, or immature muscle fibers. Primary myotubes represent the first muscle fibers to form, and provide a scaffolding upon which other myoblasts align and fuse to form secondary myotubes. The process of secondary myotube, or muscle fiber, formation persists for most of the fetal period. Essentially all muscle fiber formation occurs prenatally in domestic animal species.[4] Small muscles will have fewer muscle fibers than large muscles within an animal, but the number of muscle fibers also varies by species, genotype, and fetal nutrient availability. Individual muscle fibers are encased in a collagenous matrix referred to as endomysium. The complement of secondary muscle fibers surrounding each primary muscle fiber represents a fasciculus, or muscle fiber bundle, which is surrounded by a connective tissue border called perimysium. The connective tissue border surrounding the entire developing muscle is the epimysi-um. Connective tissue septa of muscle are contiguous with the dense connective tissue of tendons, which attach muscle to bone. Similar partitioning of individual cells and clusters of cells by connective tissue septa contributes to the organization and support of most organs.

Myotubes synthesize and accumulate muscle proteins, which are assembled into highly organized contractile structures referred to as myofibrils. During maturation of myotubes into muscle fibers, centrally located nuclei migrate to the periphery of muscle cells as myofibrils assemble and form a centrally located contractile apparatus. Muscle fiber growth is achieved primarily by increased fiber length and diameter, and this coincides with an increase in myofibril content. Myofibrils increase in length by addition of sarcomeres to the ends of myofibrils, and in number by longitudinal splitting. Growth in skeletal muscle fiber length accompanies bone growth and typically precedes extensive increases in muscle fiber diameter, which do not occur until the last trimester of gestation.

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