An enzyme- and immunohistochemical study has been performed on human masseter muscle spindles. Antibodies selective for different myosin heavy chain (MHC) isoforms and M-band proteins (M-protein, myomesin, and MM-CK) were used. The expression of these proteins was determined in the different intrafusal fiber types. Nuclear bag1 and nuclear bag2 fibers expressed predominantly slow-twitch and slow-tonic MHCs. The bag2 fibers in addition contained fetal MHC. Nuclear chain fibers coexpressed embryonic, fetal, and fast-twitch MHCs. The bag2 and chain fibers contained all three M-band proteins, whereas the bag1 fibers contained only myomesin. In general the MHC expression in the human masseter intrafusal fiber types was similar to that previously reported for limb muscles in man as well as for limb and masseter muscles in other species. However, the number of intrafusal fibers per spindle was unusually high (up to 36). This reinforces the idea that masseter muscle spindles have a strong proprioceptive impact during the control of jaw movements.
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