A freeze-fracture study of fiber types in normal human muscle.

The sarcoplasmic reticulum (SR) and plasma membranes of Type 1 and Type 2 fibers of normal human muscle were examined by the freeze-fracture technique. Total particle counts in the SR appeared much lower than in other mammals and a packing density of about 1200 particles/micrometer 2 was found in both longitudinal and cisternal components of SR. There was no difference in particle density of Type 1 and Type 2 fibers. In freeze-fracture replicas of plasma membranes several fiber type differences were seen. The surface caveolae were uniformly distributed in Type 1 fibers whereas in Type 2 they were clustered preferentially at the I-band levels. Total density of intramembranous particles was greater in Type 1 fibers (347 +/- 68/micrometer 2 in P-face, 58 +/- 11/micrometer 2 in E face) than in Type 2 fibers (207 +/- 30/micrometer 2 in P-face, 80 +/- 9/micrometer 2 in E-face). There was a striking difference in respect to rectilinear arrays which were virtually absent in Type 1 fibers (0--2/micrometer 2) and numberous (up to 50--70/micrometer 2) in Type 2 fibers.