Skeletal muscle fiber hyperplasia

Area of Science:

• Exercise physiology
• Muscle biology
• Cellular adaptation

Background:

• Skeletal muscle enlargement in adults is traditionally attributed to fiber hypertrophy (increased size).
• Emerging evidence suggests muscle fiber hyperplasia (increased number) also plays a role in adult muscle mass increases.
• This phenomenon is relevant to both animal models and human athletes.

Purpose of the Study:

• To investigate the cellular mechanisms underlying skeletal muscle enlargement.
• To determine the contribution of muscle fiber hyperplasia to muscle mass increases.
• To examine evidence for fiber hyperplasia across different experimental models.

Main Methods:

• Utilized three models of muscle enlargement: compensatory hypertrophy, stretch overload, and exercise.
• Employed direct muscle fiber counting via nitric acid digestion.
• Used indirect fiber counting through histological cross-sections.
• Analyzed the expression of embryonic myosin isoforms.
• Investigated the role of satellite cells.

Main Results:

• Direct fiber counts revealed significant increases (9-52%) in fiber number following exercise and stretch overload.
• Indirect histological methods suggested fiber hyperplasia (10-82%) across all three models.
• Embryonic myosin isoform expression indicated new fiber formation in stretch-overloaded muscle.
• Satellite cells were implicated in muscle fiber hyperplasia during stretch and exercise.

Conclusions:

• Muscle fiber hyperplasia contributes to skeletal muscle enlargement in adult animals.
• Evidence supports fiber hyperplasia in response to exercise, stretch, and compensatory hypertrophy.
• Satellite cells are involved in the process of muscle fiber hyperplasia.