Morphometric and Immunohistochemical Characteristics of the Adult Human Soft Palate Muscles

Abstract

The soft palate is the only structure that reversibly separates the respiratory and gastrointestinal systems. Most species can eat and breathe at the same time. Humans cannot do this and malfunction of the soft palate may allow food to enter the lungs and cause fatal aspiration pneumonia. Speech is the most defining characteristic of humans and the soft palate, along with the larynx and tongue, plays the key roles. In addition, palatal muscles are involved in snoring and obstructive sleep apnea. Considering the significance of the soft palate, its function is insufficiently understood. The objectives of this study were to document morphometric and immunohistochemical characteristics of adult human soft palate muscles, including fiber size, the fiber type, and myosin heavy chain (MyHC) composition for better understanding muscle functions. In this study, 15 soft palates were obtained from human autopsies. The palatal muscles were separated, cryosectioned, and stained using histological and immunohistochemical techniques. The results showed that there was a fast type II predominance in the musculus uvulae and palatopharyngeus and a slow type I predominance in the levator veli palatine. Approximately equal proportions of type I and type II fibers existed in both the palatoglossus and tensor veli palatine. Soft palate muscles also contained hybrid fibers and some specialized myofibers expressing slow-tonic and embryonic MyHC isoforms. These findings would help better understand muscle functions.

Keywords: fiber types; immunocytochemistry; myosin heavy chain isoforms; respiration; soft palate muscles; speech; swallowing.

Figure 1.

Schematic diagram of the human soft palate muscles, showing tissue sampling sites (enclosed).

Figure 2.

Cross-sections from human tensor veli palatini (TVP) (A-B) and from human biceps brachii (C-D) stained for NOQ7-5-4D (A, C) and succinic dehydrogenase (SDH) (B, D). Note that the fast type II fibers in the TVP had a higher level of SDH activity than those in the biceps brachii. Bar = 50 µm.

Figure 3.

Cross-sections of human levator veli palatini stained with monoclonal antibodies NOQ7-5-4D specific to type I myosin heavy chain (MyHC) (A) and SC-71 to type IIa MyHC (B), showing the fiber types I, IIA, and I/IIA hybrid fibers (asterisks). Bar = 50 µm.

Figure 4.

Relative percentages of the myosin heavy chain-based fiber types in adult human MU, PP, PG, TVP, and LVP muscles. Abbreviations: MU, musculus uvulae; PP, palatopharyngeus; PG, palatoglossus; TVP, tensor veli palatini; LVP, levator veli palatini.

Figure 5.

Cross-sections of adult human tensor veli palatini (TVP) stained with monoclonal antibodies NOQ7-5-4D (A), ALD-58 (B), SC-71 (C), and F1.652 (D). Note that the TVP contains muscle fibers expressing myosin heavy chain (MyHC)-ton (B) and MyHC-emb (D) isoforms. MyHC-ton was coexpressed with MyHCI, whereas MyHC-emb coexisted with MyHCII. Bar = 25 µm.

Figure 6.

Relative percentages of the MyHC-ton and MyHC-emb containing fibers in adult human MU, PP, PG, TVP, and LVP muscles. Abbreviations: MyHC, myosin heavy chain; MU, musculus uvulae; PP, palatopharyngeus; PG, palatoglossus; TVP, tensor veli palatini; LVP, levator veli palatini.