Development Repair Healing and Renewal

In skeletal muscle development myoblasts fuse to form multinucleated myofibers

Myoblasts are derived from a self-renewing population of multipotential myogenic stem cells that originate in the embryo from unsegmented paraxial mesoderm (cranial muscle progenitors) or segmented mesoderm of somites (epaxial and hypaxial muscle progenitors). Developing muscle contains two types of myoblasts:

• Early myoblasts are responsible for the formation of primary myotubes, chain-like structures that extend between tendons of the developing muscle. Primary myotubes are formed by nearly synchronous fusion of early myoblasts. Myotubes undergo further differentiation into mature skeletal muscle fibers. Primary myotubes observed in the light microscope exhibit a chain of multiple central nuclei containing myofilaments.

• Late myoblasts give rise to secondary myotubes, which are formed in the innervated zone of developing muscle where the myotubes have direct contact with nerve terminals. Secondary myotubes continue to be formed by sequential fusion of myoblasts into the already-formed secondary myotubes at random positions along their length. Secondary myotubes are characterized by a smaller diameter, more widely spaced nuclei, and an increased number of myofilaments (Fig. 10.12). In the mature multinucleated muscle fiber, the nuclei are all in the peripheral sarcoplasm, just inside the plasma membrane.

Some nuclei that appear to belong to the skeletal muscle fiber are nuclei of satellite cells

Satellite cells are interposed between the plasma membrane of the muscle fiber and its external lamina. They are small cells with scant cytoplasm. The cytoplasm typically

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