Tendinous muscle insertions (scleromuscular junctions of the recti muscles) in patients with ocular alignment problems.

Background: The purpose of this study was to prove the hypothesis whether the scleromuscular junction of extraocular recti muscle is tendinous. Patients and Methods: Muscle samples of the 41 extraocular recti muscles of 33 patients and 4 muscle-/eye-matched samples from 2 postmortem eyes, were processed for light/electron microscopy and immunohistochemistry with antibodies against desmin, smooth-muscle actin and muscle regulating proteins like myf3 and myf4 (myogenin), tenascin C and for 8 samples against collagens I to IV. Results: Histological examination of the muscle samples confirmed a thick collagen-structured tissue, specific for muscle tendon; without appearance of muscle tissue. This was confirmed by immunohistochemistry with antibodies against desmin, smooth-muscle actin, myf3 and myf4 (myogenin) and for eight samples with collagens I to IV. Anti-tenascin C marker was only strongly positive in the connective tissue of the blood vessel walls. Electron microscopy demonstrated collagen bundles composed of parallel oriented fibrils with a moderate amount of ground substance. Conclusions: The absence of contractile fibers at the sclerotendinous junction is an entirely normal finding in humans and cannot be related to ocular alignment pathogenesis.