Following on from Part 1 this article looks at the role of satellite cells in the muscle hypertrophy process.
Muscle is a remarkable living tissue- it has the amazing potential to adapt to a presented stimulus or functional demand. Through the stimulus-overload-adaptation cycle, muscle tissue has the ability to produce more contractile force, store higher levels of substrate, improve endurance or grow in size according to the type of stimulus it is presented with. Additionally none of these adaptations are stable- if the stimulus ceases then the aforementioned adaptations will reverse in order to reduce allostatic load.
Adult muscle cell myonuclei are unable to multiply (postmitotic) therefore during the process of hypertrophy, specific muscle stem cells are required to promote muscle synthesis. These stem cells are referred to as satellite cells (SCs). Satellite cells are located on the outer surface of the muscle fiber, between the sarcolemma and basal lamina (see Diagram 1).
It is thought that satellite cells remain relatively dormant and activate only when a sufficient mechanical stimulus is imposed on the skeletal muscle. Essentially, in the unperturbed state, these cells remain in a non-proliferative, quiescent state. Upon activation, SCs are able to ‘donate’ their nuclei as a means of increasing contractile protein synthesis potential and allowing muscle regeneration. The capacity for nuclei donation is dependent on the integrity of the basal lamina- after the rupture of the basal lamina in response to muscle trauma, SCs migrate to adjacent myofibers in order to adapt tissue bridges. This process is triggered by the stress of muscle damage/overload-induced resistance training. Additionally, exercise-induced myo-trauma initiates an immune response and a subsequent increase in cytokines and other inflammatory markers (Hs-CRP, ILs, TNFa, neutrophils and macrophages etc.).
Research has suggested that SCs not only have a capacity for muscle regeneration but may also contribute to alternative muscle and non-muscle lineages. This is useful in furthering not only our understanding of how ‘bros’ can get bigger arms, but can also improve knowledge of clinical applications in the treatment of diseases such as muscular dystrophy.
The process of muscle regeneration requires the influence of growth factors and a sequence of cellular events, which results in the regulation of the satellite cell population. These are illustrated in Diagram 2.
In response to resistance training, trauma to the muscle results in the release of growth factors that will, in part, regulate the satellite cell population during regeneration and shift the emphasis from protein degradation to synthesis. The purpose of part 3 of this series is to introduce and describe both the signalling pathways and growth factors associated with satellite cell proliferation, differentiation and motility.
Blaawe, B. The role of satellite cells in muscle hypertrophy. J Muscle Res Cell Motil. 24 (10). 2014
Hawke, TJ & Garry DJ. Myogenic satellite cells: physiology to molecular biology. Journal of Applied Physiology Published 1 August 2001 Vol. 91 no. 2, 534-551
Schoenfeld, B. The mechanisms of muscle hypertrophy and their application to resistance training. J Strength Cond Res 24(10): 2857–2872, 2010