Myosin Heavy Chain-based Fiber Types in the Adult Human Cricopharyngeus Muscle

The cricopharyngeus (CP) muscle is a major component of the upper sphincter of the esophagus. Its physiology is complex; a variety of reflexes maintain CP sustained contraction except during swallowing, when it relaxes to allow a food bolus to pass into the esophagus. In order to understand CP function, we previously studied the normal adult human CP and found that it has an unusual layered structure, with a slow inner and fast outer layer. In addition, a majority of its muscle fibers express unusual myosin heavy chain (MHC) isoforms (slow-tonic, alpha-cardiac, neonatal, and embryonic) as well as the major MHC isoforms (types I, IIa, and IIx). In this study, autopsied adult human CP muscles were studied with immunocytochemical techniques to determine the patterns of MHC coexpression in CP muscle fibers. The results show that CP fibers were hybrids expressing from two to six MHC isoforms. Ten different combinations of MHC isoforms were identified in CP fibers, with the most common (54%) containing three MHC isoforms. The variety of hybrid CP fiber types suggests that the CP is capable of a wide range of contraction characteristics. Determination of MHC expression patterns of the CP muscle fibers is critical for evaluating the contractile properties of the sphincter.