Adult Human Upper Esophageal Sphincter Contains Specialized Muscle Fibers Expressing Unusual Myosin Heavy Chain Isoforms

Liancai Mu ^1*, Jun Wang ¹, Hungxi Su ¹ and Ira Sanders ¹

¹ Department of Otolaryngology, The Mount Sinai School of Medicine, New York, New York (LM,JW,HS), and Alice and David Jurist Institute for Biomedical Research, Hackensack University Medical Center, Hackensack, New Jersey (IS)

^* To whom correspondence should be addressed. E-mail: lmu03{at}yahoo.com.

Submitted on August 30, 2006
Accepted on 13 October 2006

	Abstract

The functional upper esophageal sphincter (UES) is composed of the cricopharyngeus muscle (CP), the most inferior part of the inferior pharyngeal constrictor (iIPC), and the upper esophagus (UE). This sphincter is collapsed and exhibits sustained muscle activity in the resting state; it only relaxes and opens during swallowing, vomiting, and belching. The tonic contractile properties of the UES suggest that the skeletal muscle fibers in this sphincter differ from those in the limb and trunk muscles. In this study, myosin heavy chain (MHC) composition in the adult human UES muscles obtained from autopsies was investigated using immunocytochemical and immunoblotting techniques. The results showed that the adult human UES muscle fibers expressed unusual MHC isoforms such as slow-tonic (MHC-ton), -cardiac (MHC-), neonatal (MHC-neo), and embryonic (MHC-emb) which coexisted with the major MHCs (i.e., MHCI, IIa, and IIx). The MHC-ton and MHC- were coexpressed predominantly with slow type I MHC isoform, whereas the MHC-neo and MHC-emb coexisted mainly with fast type IIa MHC. A slow inner layer (SIL) and a fast outer layer (FOL) in the iIPC and CP were identified immunocytochemically. The MHC-ton and MHC- containing fibers were concentrated mainly in the SIL, whereas the MHC-neo and MHC-emb containing fibers were distributed primarily to the FOL. Identification of the specialized muscle fibers and their distribution patterns in the adult human UES is valuable for better understanding the physiological and pathophysiological behaviors of the sphincter.

Key Words: upper esophageal sphincter, cricopharyngeus muscle, inferior pharyngeal constrictor, upper esophagus, fiber types, myosin heavy chain isoforms, immunocytochemistry, immunoblotting, swallowing

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