Immunohistochemical identification of slow-tonic fibers in human extrinsic eye muscles

A fluorescent antiserum against myosin from chicken anterior latissimus dorsi muscle, which stains specifically the multiply innervated slow fibers of birds and amphibians, was applied to frozen sections of human extraocular muscles. A proportion of fibers in oculorotatory muscles were labeled by the antiserum. In contrast, no labeled extrafusal fiber was present in the levator palpebrae or in other body muscles. Serial study of sections stained for acetylcholinesterase and myosin ATPase showed that the labeled fibers in human oculorotatory muscles are multiply innervated and display acid-stable myosin ATPase activity.