Muscle fiber types of human extraocular muscles: A histochemical and immunohistochemical study

PURPOSE. TO classify muscle fibers of human extraocular muscle (hEOM) and t o compare them to previous studies on hEOM, as well as to nonhuman EOM clas sification schemes and skeletal muscle fiber types. METHODS. Muscle fibers cut in different muscle planes were followed on cons ecutive cross sections and typed with regard to their oxidative profile in combination with their myosin-immunohistochemical characteristics. RESULTS. Three zones were observed. In the global layer three muscle fiber types were observed: global layer singly innervated granular fibers, 79.4 /- 8.1 mu m (perimeter [values at midmuscle region] +/- SD); 59%; global la yer singly innervated coarse libers (80.3 +/- 10.8 mu m; 21%); and global l ayer multiply innervated muscle fibers (4.1 +/- 9.7 mu m; 21%). Two muscle fiber types were detected in the orbital layer. orbital layer singly innerv ated muscle fibers (54.1 +/- 8.5 mu m; 83%) and orbital layer multiply inne rvated muscle fibers (53.5 +/- 7.6 mu m; 17%). Three muscle fiber types wer e differed in the marginal zone: marginal zone singly innervated muscle fib ers (83.1 +/- 15.8 mu m; 56%), marginal zone multiply innervated low oxidat ive muscle fibers (84.4 +/- 23.3 mu m; 7%), and marginal zone multiply inne rvated high oxidative muscle fibers (88.4 +/- 14.5 mu m: 37%). Coexpression s of developmental myosin heavy chain isoforms and fast myosin heavy chain isoform were detected mainly in the marginal zone. CONCLUSIONS. hEOMs resemble mammalian EOM with regard to their organization . However, in addition to an inner global layer and an orbital layer an ext ernal marginal zone was described for the first time in hEOM in the present study.