Satellite cells

Satellite cells are mononuclear progenitor cells found in mature muscle between the basal lamina and sarcolemma. Satellite cells are able to differentiate and fuse to augment existing muscle fibres and to form new fibres. These cells are involved in the normal growth of muscle, as well as regeneration following injury or disease.

In undamaged muscle, the majority of satellite cells are quiescent; they neither differentiate nor undergo cell division. In response to mechanical strain, satellite cells become activated. Activated satellite cells initially proliferate as skeletal myoblasts before undergoing myogenic differentiation.

Genetic markers of satellite cells
All satellite cells express a number of distinctive genetic markers including Pax7 and syndecan 4. Quiescent satellite cells can also express markers common to stem cells.

Activated satellite cells express myogenic transcription factors, such as MyoD and Myf5. They also begin expressing muscle-specific filament proteins such as desmin as they differentiate.

Function in muscular repair
When muscle cells undergo injury, quiescent satellite cells are released from beneath the basal lamina. While normally in a post-mitotic state, they become activated and re-enter the cell cycle. Proliferating myoblasts then undergo myogenic differentiation, becoming post-mitotic, and form new myotubes and fuse with existing muscle fibres. This leads to repair of the injured site.

The presence of fibroblasts during muscle repair can lead to the formation of scar tissue that can lead to impaired muscle function.

Plasticity
Upon minimal stimulation, satellite cells in vitro or in vivo will undergo a myogenic differentiation program.

Some research is underway to investigate the potential for satellite cells to differentiate into other cell types. One application for this would be cardiac cell therapy, should muscle satellite cells be able to be converted into viable cardiac muscle cells.

Satellite cells can also be converted in vitro into bone and cartilage cells, via the application of osteogenic bone morphogenetic proteins or transforming growth factor-beta.

Regulation
Satellite cells are regulated by myogenic regulatory factors, such as MyoD, Myf5, myogenin, and MRF4. MEF2 family transcription factors can also promote myogenic differentiation.

There is also research indicating that satellite cells are negatively regulated by a protein called myostatin. Increased levels of myostatin up-regulate a cyclin-dependent kinase inhibitor called p21 and thereby induce the differentiation of satellite cells.

Satellite cells are commonly linked to aid in muscular hypertrophy.