Masseter muscle fibre changes following reduction of masticatory function

Area of Science:

• Muscle physiology
• Histology
• Neurotoxins

Background:

• Masticatory function is crucial for jaw development and muscle maintenance.
• Botulinum toxin type A (BTX) is used clinically to modulate muscle activity.
• Understanding the histological impact of reduced mastication on jaw muscles is important.

Purpose of the Study:

• To evaluate histological changes in masseter muscle fibers after reduced masticatory function induced by BTX.
• To investigate the effects of BTX injections in masseter and temporalis muscles on muscle fiber characteristics.

Main Methods:

• Sixty male Long-Evans rats were divided into four groups: BTX in masseter, BTX in temporalis, BTX in both, and saline control.
• BTX or saline was injected into the respective muscles.
• After 45 days, muscle mass was recorded, and superficial masseter muscles underwent immunohistochemical analysis.

Main Results:

• Group III (BTX in both masseter and temporalis) exhibited larger fiber sizes compared to other groups.
• A small percentage of type IIa fibers was observed in Group III.
• Reduced masticatory function may lead to decreased muscle fiber size and a shift in fiber subtypes.

Conclusions:

• Reduced masticatory function induced by BTX can alter masseter muscle histology.
• A transition from type IIa to type IIx or IIb muscle fibers may occur with decreased chewing activity.
• These findings contribute to understanding the long-term effects of altered muscle function on jaw musculature.