Developmental and functional considerations of masseter muscle partitioning

Abstract

The masseter muscle participates in a wide variety of activities including mastication, swallowing and speech. The functional demands for accurate mandibular positioning and generation of forces during incising or a power stroke require a diverse set of forces that are determined by the innate muscle form. The complex internal tendon architecture subdivides the masseter into multiple partitions that can be further subdivided into neuromuscular compartments representing small motor unit territories. Individual masseter compartments have unique biomechanical properties that, when activated individually or in groups, can generate a wide range of sagittal and off-sagittal torques about the temporomandibular joint. The myosin heavy chain (MyHC) fibre-type distribution in the adult masseter is sexually dimorphic and is influenced by hormones such as testosterone. These testosterone-dependent changes cause a phenotype switch from slower to faster fibre-types in the male. The development of the complex organization of the masseter muscle, the MyHC fibre-type message and protein expression, and the formation of endplates appear to be pre-programmed and not under control of the muscle nerve. However, secondary myotube generation and endplate maturation are nerve dependent. The delayed development of the masseter muscle compared with the facial, tongue and jaw-opening muscles may be related to the delayed functional requirements for chewing. In summary, masseter muscle form is pre-programmed prior to birth while muscle fibre contractile characteristics are refined postnatally in response to functional requirements. The motor control mechanisms that are required to coordinate the activation of discrete functional elements of this muscle remain to be determined.

Figure 1

The neuromuscular endplate development in the masseter is delayed compared to development in the tongue and geniohyoid muscles. Frontal cryosections (14 μm thickness) of gestational day 18 mice were immuno/lectin-labeled for nerve (NF, neurofilament), endplate (α-BTX, α-bungarotoxin) and synaptic carbohydrates (VVA, Vicia villosa agglutinin isolectin B4). It can be observed that muscle nerve efferents have reached their target muscle fibers in all the muscles (neurofilament label). Arrows indicate selected endplates for comparison. Well-defined, mature endplates are seen in tongue and geniohyoid muscles while modest endplate development is observed in the superficial and intermediate masseter. VVA-binding specifically at synapses, an indicator of endplate maturation, is distinct in tongue and geniohyoid muscles. However, in masseter, synaptic carbohydrates are found to be diffusely distributed on the muscle fibers. The overlays of immuno/lectin-labeled images further demonstrate the co-localization of VVA-binding at the endplate in tongue and geniohyoid muscle. The scale bar represents 50 μm.