Pax3 protein is initially expressed in the dermomyotome of embryonic somites, which gives rise to skeletal muscle. Following myogenesis, Pax3 expression is mostly down-regulated and becomes restricted to a few satellite cells (SCs) of select mature muscles. SCs are activated to form new myonuclei during muscle hypertrophy, regeneration and repair. Intrafusal fibers of muscle spindles are thought to persist in a comparatively immature state as, unlike extrafusal fibers, they maintain small diameters, developmental myosins, Myf5 expression and high SC concentrations. This thesis tests the hypotheses that Pax3 expression is preferentially maintained in SCs of adult skeletal muscle spindles and can be augmented under conditions of SC activation. To study Pax3 through development, immunohistochemical techniques were used to identify SCs by their Pax7 expression, and analyze the proportion of SCs and myonuclei (MN) expressing Pax3 in chicken anterior latissimus dorsi (ALD) muscle excised at 9, 30, 62, and 145 days post-hatch. To induce SC activation, tenotomy was performed on the right ALD muscle of 138-day post-hatch chicks to induce compensatory hypertrophy of the ipsilateral synergistic posterior latissimus dorsi (PLD) muscle. The PLD was analyzed seven days after ALD tenotomy using similar immunohistochemical techniques. This is the first study to show Pax3 expressing SCs within intrafusal fibers of muscle spindles. This thesis demonstrates that throughout development there is a higher percentage of Pax3 expressing SCs within intrafusal fibers of muscle spindles than the surrounding extrafusal fibers that compose the bulk of the muscle. It is also revealed that the proportion of the SC population expressing Pax3 declines with age in both intrafusal and extrafusal fibers. Compensatory hypertrophy of the PLD resulted in a greater percentage of Pax3 expressing SCs in intrafusal and extrafusal fibers than under control conditions. The percentage of SCs expressing Pax3 after PLD overload was similar to that seen in young control muscle. The percentage of Pax3 expressing MN also increased after muscle overload to levels seen in young muscle. A disproportionate decrease in the proportion of SCs expressing Pax3 during development, and a disproportionate increase in the percentage of Pax3 positive SCs as a result of experimentally induced muscle hypertrophy, suggests that Pax3 expression in maturing muscle may be more than just a developmental vestige. Pax3 may be a factor in the activation and differentiation of SCs in maturing muscle.

Pax3 protein is initially expressed in the dermomyotome of embryonic somites, which gives rise to skeletal muscle. Following myogenesis, Pax3 expression is mostly down-regulated and becomes restricted to a few satellite cells (SCs) of select mature muscles. SCs are activated to form new myonuclei during muscle hypertrophy, regeneration and repair. Intrafusal fibers of muscle spindles are thought to persist in a comparatively immature state as, unlike extrafusal fibers, they maintain small diameters, developmental myosins, Myf5 expression and high SC concentrations. This thesis tests the hypotheses that Pax3 expression is preferentially maintained in SCs of adult skeletal muscle spindles and can be augmented under conditions of SC activation. To study Pax3 through development, immunohistochemical techniques were used to identify SCs by their Pax7 expression, and analyze the proportion of SCs and myonuclei (MN) expressing Pax3 in chicken anterior latissimus dorsi (ALD) muscle excised at 9, 30, 62, and 145 days post-hatch. To induce SC activation, tenotomy was performed on the right ALD muscle of 138-day post-hatch chicks to induce compensatory hypertrophy of the ipsilateral synergistic posterior latissimus dorsi (PLD) muscle. The PLD was analyzed seven days after ALD tenotomy using similar immunohistochemical techniques. This is the first study to show Pax3 expressing SCs within intrafusal fibers of muscle spindles. This thesis demonstrates that throughout development there is a higher percentage of Pax3 expressing SCs within intrafusal fibers of muscle spindles than the surrounding extrafusal fibers that compose the bulk of the muscle. It is also revealed that the proportion of the SC population expressing Pax3 declines with age in both intrafusal and extrafusal fibers. Compensatory hypertrophy of the PLD resulted in a greater percentage of Pax3 expressing SCs in intrafusal and extrafusal fibers than under control conditions. The percentage of SCs expressing Pax3 after PLD overload was similar to that seen in young control muscle. The percentage of Pax3 expressing MN also increased after muscle overload to levels seen in young muscle. A disproportionate decrease in the proportion of SCs expressing Pax3 during development, and a disproportionate increase in the percentage of Pax3 positive SCs as a result of experimentally induced muscle hypertrophy, suggests that Pax3 expression in maturing muscle may be more than just a developmental vestige. Pax3 may be a factor in the activation and differentiation of SCs in maturing muscle.