Skeletal muscle satellite cells are quiescent mononucleated myogenic cells, 
located between the sarcolemma and basement membrane of 
terminally-differentiated muscle fibres. These are normally quiescent in adult 
muscle, but act as a reserve population of cells, able to proliferate in 
response to injury and give rise to regenerated muscle and to more satellite 
cells. The recent discovery of a number of markers expressed by satellite cells 
has provided evidence that satellite cells, which had long been presumed to be a 
homogeneous population of muscle stem cells, may not be equivalent. It is 
possible that a sub-population of satellite cells may be derived from a more 
primitive stem cell. Satellite cell-derived muscle precursor cells may be used 
to repair and regenerate damaged or myopathic skeletal muscle, or to act as 
vectors for gene therapy. CELL FACTS: (1) Number of cells in body: 2 x 10(7) to 
3 x 10(7) myonuclei/g, 20-25 kg muscle in average man; 2 x 10(5) to 10 x 10(5) 
satellite cells/g, i.e. approximately 1 x 10(10) to 2 x 10(10) satellite cells 
per person. (2) Main functions: repair and maintenance of skeletal muscle. (3) 
Turnover rate: close to zero in non-traumatic conditions-high in disease or 
severe trauma.