Fiber-type differentiation by myosin immunohistochemistry on paraffin-embedded skeletal muscle. A useful adjunct to fiber typing by the adenosine triphosphatase reaction.

The myofibrillar adenosine triphosphatase reaction has been traditionally used to delineate fiber types in skeletal muscle biopsy specimens. At a pH of 9.4, the type 2 fibers are stained and the type 1 fibers remain unstained, thus permitting a clear distinction between the two fiber types and allowing morphometric assessments of fiber size, fiber diameter, and fiber ratios. However, this histochemical reaction requires frozen muscle tissue and is not feasible in situations when only paraffin-embedded tissue is available. We describe a useful alternative to determine fiber types in skeletal muscle specimens by myosin histochemistry on formalin- or Bouin-fixed, paraffin-embedded tissue. Our experience with over 300 muscle samples from both surgical and autopsy specimens indicates that myosin immunohistochemistry also provides accurate assessments of fiber types, fiber sizes, and fiber ratios. The most significant advantage of this method is its applicability to nonfrozen material, thus permitting fiber-type distinction in situations when frozen tissue is unavailable and facilitating retrospective analysis on archival material.