Desmin is not ubiquitously expressed in head and neck muscle fibers.

Desmin, a major muscle-specific intermediate filament protein, is widely regarded as a ubiquitous component of mature skeletal muscle fibers. However, recent observations in human extraocular and palatal muscles indicate that a subset of fibers lack or show very weak desmin expression. The aim of the study was to investigate the presence of desmin in 21 different adult, two toddler, and one fetal human head and neck muscles. The samples were analyzed by immunohistochemistry, and, in one muscle group, also by in situ hybridization for desmin mRNA. Two limb muscles, the biceps brachii and the vastus lateralis, served as controls. In limb muscles, all fibers consistently exhibited desmin immunoreactivity. In contrast, fibers with absent or very weak desmin immunoreactivity were identified across all examined head and neck muscles, with substantial variation between muscle groups. These fibers were predominantly associated with the slow MyHC1 phenotype. A similar pattern was also observed in infant and fetal samples. In longitudinal sections, some fibers showed segmental absence of desmin along their length. In situ hybridization demonstrated absent or markedly reduced desmin mRNA in fibers with absent or reduced desmin immunoreactivity. Taken together, this study provides the first evidence that subsets of myofibers in most human head and neck muscles lack desmin immunoreactivity, indicating that desmin is not ubiquitous in all myofibers in these muscles. These findings reveal previously unrecognized cytoskeletal heterogeneity within human head and neck musculature. The functional significance of fibers lacking or showing very weak desmin immunoreactivity remains to be determined.