Regional differences in the orbicularis oculi muscle: conservation between species.

The orbicularis oculi muscle is a complex facial muscle involved in eyelid closure. The central parts of pretarsal and preseptal regions of the palpebral part of the orbicularis oculi muscle in rabbit and cynomolgus monkey lower eyelids were examined histologically and were analyzed for muscle fiber number, muscle fiber cross-sectional area and fiber type composition. Distinct regional differences were seen in the muscle fiber composition in these two regions of the muscle. The pretarsal portion of the muscle, that closest to the eyelid margin, was quite homogeneous and almost completely composed of type 2 fibers. These fibers were the smallest in cross-sectional area. Type 2 fibers also predominated in the preseptal portion of the muscle, but this region contained between 10 and 20% type 1 fibers. They appeared to be a gradient in muscle fiber size, whereby the fiber size increased as a function of the distance from the eyelid margin. The same pattern of regional differences were found in both rabbit and monkey orbicularis oculi. Thus, there is a clear conservation of these regional differences in these two species. While the developmental significance is unknown, the identification of this pattern may facilitate the evaluation of chemomyectomy agents for treatment of eyelid spasms in humans and allow a more accurate analysis of biopsy material from this muscle.