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Vastus lateralis muscle biopsy of a 9-year-old boy (3)

(A to C) NADH-TR of several different fascicles shows that all small fibers are type I and all large fibers are type II, but some fascicles are composed nearly entirely of small type I fibers. ATPase stains (not shown) confirmed a similar histochemical pattern of fiber types. Serial muscle biopsies of children with congenital muscle fiber-type disproportion frequently show a greater number of type I fibers at older ages than in infancy, as in this case, presumably due to continued conversion over time of some type II fibers to type I. Magnification x 250. (Contributed by Dr. Harvey Sarnat.)

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