A comparative morphological study of two human facial muscles : the orbicularis oculi and the corrugator supercilii

Facial muscles have two unique functions: as sphincters and dilators controlling the orifices of the face, and as movers of the skin of the face to produce facial expressions. It was hypothesized that human facial muscles sharing the same innervation and embryology, but having different functions, would posses morphologic differences in architecture, histology, cytochemistry and ultrastructure. To test this hypothesis, two periorbital muscles, the palpebral part of the orbicularis oculi (OO) and the corrugator supercilii (CS), were compared. The OO is a sphincteric muscle, whereas the CS is a muscle of facial expression. Whole muscle samples from human cadavers and biopsies from cosmetic surgery procedures were used. Quantitative measures of fiber sizes, fiber shapes and fiber-type distributions were performed along with measures of capillary area per unit of contractile area (capillary index). Qualitative analyses of nerve and motor end-plate distributions were also undertaken. Architectural differences were elucidated by stereoscopic dissection, conventional histological stains, and electron microscopy. Innervation patterns and motor end-plate regions were demonstrated with a pararosanaline (PIA) stain, and with an antibody to neurofilament protein. Fiber-type profiles were visualized by immunofluorescent microscopy using antibodies to fast and slow myosin. The OO was shown to differ significantly from the CS on the basis of fiber shapes, sizes and types. The OO fibers were small, rounded and 89% of them were type II. The CS fibers were larger, pleomorphic in shape, and only 49% of them were type II. The capillary index of the CS was 2.4 times that of the OO. The innervation of the OO was more intricate and arose from the deep surface of the muscle, whereas that of the CS was less ordered and arose from the superficial surface. Ulfrastructurally, the CS and OO shared many features, but also demonstrated differences in mitochondrial content and distribution, as well as motor end-plate structure. The observed differences between the two muscles support the contention that the function of human facial muscles influences their morphology.Medicine, Faculty ofGraduat