Fine Structure of Extraocular Muscle in Rabbit

It has been established that the extraocular muscles of several mammalian species possess two different types of muscle fibers, twitch and slow fibers, with distinct differences in their respective fine structures (1-8). In this investigation we have examined the fine structure of isolated and identified extraocular muscle fibers in rabbit. Though there has been general agreement on the differences between the fine structures of the two fiber types, it remains unclear whether the slow fibers are characterized by an M line. Extraocular muscle fibers in rhesus monkey which have fine structural characteristics similar to those of slow fibers in frog skeletal muscle have been reported by Cheng and Breinin (5) and Miller (6) to lack an M line, whereas in cat such fibers have been described by Hess (7) as clearly possessing an M line. Thus the morphological definition of slow fibers in terms of their fine structural characteristics has varied in previous investigations. According to Hess (1, 7), reliable criteria for the identification of slow fibers are irregular and illdefined myofibrils, a virtually absent T system, zigzag Z lines, and rudimentary postjunctional folds. Page (9) and Miller (6) rely, rather, on the absence of an M line for identification of slow fibers. There is complete agreement, however, that slow fibers have multiple motor nerve endings (1, 2, 6, 7, 9). In the present study, therefore, we first classified extraocular muscle fibers, on the basis of their motor nerve endings, into either singly innervated or multiply innervated fibers. The respective fibers were then examined under the electron microscope. Our findings indicate that the fine structural characteristics of the singly and multiply innervated fibers in the extraocular muscle of rabbit are essentially similar to those previously described for the twitch and slow fibers, respectively, in other species. No M line was observed, however, in the multiply innervated fibers.