Satellite cells are small mononuclear progenitor cells with virtually no cytoplasm found in mature muscle. They are found sandwiched between the basement membrane and sarcolemma (cell membrane) of individual muscle fibres, and can be difficult to distinguish from the sub-sarcolemmal nuclei of the fibres. Satellite cells are able to differentiate and fuse to augment existing muscle fibres and to form new fibres. These cells represent the oldest known adult stem cell niche, and 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.
It should be noted that the field of satellite cell biology suffers from the same technical difficulties as other stem cell fields. Studies rely almost exclusively on Flow Cytometry and Flourescence Activated Cell Sorting (FACS) analysis, which gives no information about cell lineage or behaviour. As such, the satellite cell niche and is relatively ill-defined and it is likely that it consists of multiple sub-populations.
The process of muscle regeneration involves considerable remodeling of extracellular matrix and, where extensive damage occurs, is incomplete. Fibroblasts within the muscle deposit scar tissue, which can impair muscle function, and is a significant part of the pathology of muscular dystrophies.
Unfortunately, it seems that transplanted satellite cells have a limited capacity for migration, and are only able to regenerate muscle in the region of the delivery site. As such systemic treatments or even the treatment of an entire muscle in this way is not possible. However, other cells in the body such as pericytes and hematopoietic stem cells have all been shown to be able to contribute to muscle repair in a similar manner to the endogenous satellite cell. The advantage of using these cell types for therapy in muscle diseases is that they can be systemically delivered, autonomously migrating to the site of injury. Particularly successful recently has been the delivery of mesoangioblast cells into the Golden Retriever dog model of Duchenne muscular dystrophy, which effectively cured the disease.
There is some 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.