Histological and ultrastructural studies on the ringbands in human extraocular muscles.

Ringbands are a typical finding in the extraocular muscles of individuals of twenty years and older. The number of these structures increases distinctly after the 60th year of life. In optimally fixed muscles the ringbands are separated from the sarcolemma by a space containing the so-called sarcoplasmic masses. These masses were composed of vaculated mitochondria, dilated tubules of the sarcoplasmic system and, in some fibers, of fragments of myofibrils. Structures from which a neoformation of myofibrils as the origin of the ringbands could be concluded (as proposed by Jonecko, 1967), were never observed in this hypolemmal area. In contrast, the formation of the ringbands seems to be correlated to the ultrastructural alterations of the myofilaments and the looser arrangement of the myofibrils which can first be observed in extraocular muscles of 10-year-old patients and increasingly in older patients (Mühlendyck, 1977). These alterations lead to continous and repeated changes in the orientation of the myofibrils during the extension and contraction processes of the muscle fiber. Thus, the myofibrils undergo a constant mechanical stress which they cannot resist indefinetly, leading to disruption of some of the fibrils. The disrupted free end contracts, deviates almost at a right angle from its former course and, while the sarcolemma forms thread-like folds during the contraction of the muscle fiber, it is coiled around the longitudinal undisrupted fibrillar core. This leads to a disfunction of the muscle fiber and aids in explaining the steady decrease of the excursion of the eye after about 30 years of age.