Mitochondrial morphometrics of histochemically identified human extraocular muscle fibers

Three fiber types--coarse, granular, and fine--were readily identified in histochemical cryostat sections of human extraocular muscle (EOM). The cryostat retrieval method was utilized to identify these three fiber types in serial electron microscopic thin sections. Using morphometric techniques, five mitochondrial variables (mitochondrial volume fraction, mitochondrial profile size, mitochondrial profile density, and clusters of two or of three or more mitochondrial profiles) were determined for a total of 162 histochemically identified fibers from two regions (orbital and global zones) from six EOMs. Coarse fibers had numerous large-sized mitochondrial profiles, often occurring in clusters. Granular fibers had fewer and smaller-sized profiles scattered across the fiber. Fine fibers had the most numerous, but smallest-sized mitochondrial profiles. Despite significant differences in group (fiber types) means for the mitochondrial variables, no single variable was sufficient for separating fiber types into distinct populations. Although a scattergram plot of two variables was sufficient to separate orbital zone fibers, a computer-generated, multivariate discriminant analysis was needed to separate the global zone fibers into distinct populations. These results will aid future studies on normal and pathological human EOM by providing a morphometric basis for identifying fiber types in the orbital and global zones.