Morphological changes in the masseter muscle and its motoneurons during postnatal development

Area of Science:

• Developmental Biology
• Neuroscience
• Muscle Physiology

Background:

• Masticatory activity patterns influence masseter muscle morphology.
• Rat masseter muscle activity shifts from sucking to biting around 3 weeks post-birth.
• This study hypothesizes that this masticatory activity alteration impacts masseter muscle and motoneuron development.

Purpose of the Study:

• To investigate the morphological changes in rat masseter muscle fibers and motoneurons.
• To correlate these changes with the developmental shift in masticatory activity from sucking to biting.
• To determine the influence of altered mastication on muscle and motoneuron morphology.

Main Methods:

• Electromyography confirmed the shift from sucking to biting in rat masseter muscle activity.
• Horseradish peroxidase was used to label motoneurons innervating the masseter muscle.
• Histochemical analysis measured muscle fiber and motoneuron diameters; myosin heavy chain (MHC) isoform composition was analyzed.

Main Results:

• Masseter muscle fibers exhibited rapid growth up to 42 days post-birth, followed by gradual growth until 280 days.
• A significant decrease in neonatal type MHC occurred between 21 and 42 days post-birth.
• Motoneurons showed rapid growth by 42 days post-birth, with no significant further growth observed.

Conclusions:

• The transition from sucking to biting significantly influences the morphological development of masseter muscle fibers.
• Motoneuron development innervating the masseter muscle is not significantly affected by this change in masticatory activity.
• The study highlights a differential impact of masticatory activity on muscle fibers versus their motoneurons.