Intramuscular innervation of the human masseter muscle: whole-mount Sihler analysis and a field-of-view–based descriptive framework

Abstract

Abstract Prior Sihler-based and landmark-referenced studies have described intramuscular innervation of the human masseter. However, bilateral series-based descriptions focused on within-field arborisation morphology and the distribution of terminal branching within a standardised field of view remain limited. We analysed 60 masseter muscles from 30 adult body donors using a modified Sihler whole-mount staining protocol. A standardised circular field of view was evaluated to (1) classify within-field branching morphology and (2) localise the region of highest terminal arborisation density (HTAD) within the field. Two recurring within-field branching patterns were classified according to predefined morphological criteria: Type I, characterised by a compact branching configuration, and Type II, characterised by a broader fan-like/tree-like branching configuration within the photographed field. Type I predominated ( n  = 24/30; 80%), whereas Type II was less frequent ( n  = 6/30; 20%). The within-field pattern was bilaterally concordant in all individuals. The HTAD region was described using an operational sector-based definition within the photographed field-of-view and may provide a descriptive reference for understanding visible terminal arborisation morphology. These findings support hypothesis generation and highlight the need for future landmark-referenced whole-muscle studies to relate within-field branching patterns to motor entry points, functional correlates and clinically validated applications.