Journal of the Neurological Sciences, 104 (1991) 197-202 © 1991 Elsevier Science Publishers B.V. 0022-510X/91/$03.50 197 JNS 03575 Regional differences in the orbicularis oculi muscle: conservation between species Linda Kirschen McLoon and Jonathan D. Wirtschafter Department of Ophthalmology, Universityof Minnesota, Minneapolis, Minnesota (U.S.A.) (Received 28 January, 1991) (Revised, received 15 March, 1991) (Accepted 19 March, 1991) Key words: Facial muscles; Mimic muscles; Fiber typing; Enzyme histochemistry Summary The orbicularis oculi muscle is a complex facial muscle involved in eyelid closure. The central parts ofpretarsal 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. There 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. Introduction The determination of pathological changes in injured and regenerated muscle is important to our understanding of the effects of various diseases or surgical and drug treat- ments on normal muscle fiber structure and function. Histo- logical studies of muscles are often directed at the assess- ment of fiber size and composition in the affected muscle after a specific drug treatment or disease state. One problem with many of these studies is that the histological analysis is based on biopsy material that is randomly removed from the affected muscles even though it is now recognized that muscles are not homogeneous entities. The extraocular muscles, as an extreme case, show marked differences in muscle fiber types in various regions of the muscles (Spencer and Porter 1988). The masseter muscle shows regional and compartmental specialization in terms of fiber Correspondence to: Linda Kirschen McLoon, Ph.D., Department of Ophthalmology, University of Minnesota, Box 493, Mayo Memorial Building, Minneapolis, MN 55455, U.S.A. Tel.: (612) 624-0409; Fax: (612) 626-3119. type content (Bredman et al., 1990). Even the diaphragm has different fiber type and size composition on the thoracic and abdominal surfaces (Kilarski and Sjostrom 1990). Precise knowledge of the fiber sizes and fiber type com- position of individual muscle groups would not only aid in our understanding of pathological processes, it would also help in designing treatment protocols where surgical or chemical manipulations of a specific muscle are required. For example, botulinum toxin is injected into the orbicularis oculi muscle to alleviate muscle spasms in patients with blepharospasm and hemifacial spasm and is becoming the current preferred non-surgical treatment (Scott et al. 1985). Knowledge of regional differences in the orbicularis oculi muscle may alter the location of botulinum injections in these patients. Interpretation of supposed histopathologic changes in the botulinum treated muscles may also be obscured by lack of knowledge as to the basic anatomy of this muscle. The anatomy of the facial muscles is unique, due to the fact that these muscles have a complex spatial arrangement and insert directly into the skin. The orbicularis oculi muscle is described as being composed of palpebral and