Variation in histochemical enzyme profile and diameter along human masseter intrafusal muscle fibers

The histochemical enzyme profile of human masseter intrafusal muscle fibers was analyzed in consecutive serial cross sections along the individual fibers. Two hundred intrafusal fibers in 21 muscle spindles were classified. On the basis of equatorial nucleation, myosin ATPase-staining reactions after alkaline and acid preincubations and diameter, four different populations or types of intrafusal fiber were identified: large-diameter alkaline-stable and acid-stable fibers, bag2; two types of fiber with intermediate-diameter, alkaline-labile and acid-labile fibers corresponding to bag1 and alkaline-labile and acid-stable fibers designated as AS-bag1; and small-diameter alkaline-stable and acid-stable (pH 4.6)-acid-labile (pH 4.3) fibers called chain fibers. Regional variability in staining and diameter along the individual fibers was noted. In general, intrafusal fibers showed stronger oxidative reactions than did extrafusal fibers. The enzyme profile of the human masseter intrafusal fibers differed from that of extrafusal fibers in jaw, limb, and trunk muscles and also from that reported for spindles in limb and trunk muscles in man. The result suggests unique properties of human jaw muscle spindles and the jaw motor system.