Postnatal expression of myosin heavy chains in muscle spindles of the rat

The immunocytochemical expression of two myosin isoforms in intrafusal muscle fibers was examined in soleus muscles of neonatal (zero to six days postpartum) and adult rats. Monoclonal antibodies specific for myosin heavy chains of the slow-tonic anterior latissimus dorsi (ALD58) and fast-twitch pectoralis (MF30) muscles of the chicken were used. In adults ALD58 bound to the intracapsular regions of bag1 and bag2 fibers and MF30 bound to the intracapsular regions of bag2 and chain fibers. The extracapsular regions of intrafusal fibers and all extrafusal fibers did not react to ALD58 or MF30. Bag1 and bag2 fibers of neonatal rats expressed immature myosin patterns but chain fibers did not. The adult pattern of immunoreactivity of intrafusal fibers developed by the fourth postnatal day, when the patterns of motor but not sensory innervation in the spindle are still immature. Data suggest that the expression and maintenance of the specific anti-myosin immunoreactivity of intrafusal fibers during postnatal development of rat spindles is dependent upon sensory but not motor innervation. Moreover, afferents might regulate the gene expression responsible for synthesis of myosins isoforms specific to intrafusal fibers only in those myonuclei located within the capsule, but not in the myonuclei in extracapsular regions of intrafusal fibers.