Expression of CD34 and Myf5 defines the majority of quiescent adult skeletal muscle satellite cells

Skeletal muscle is one of a several adult postmitotic tissues that retain t he capacity to regenerate. This relies on a population of quiescent precurs ors, termed satellite cells. Here we describe two novel markers of quiescen t satellite cells: CD34, an established marker of hematopoietic stem cells, and Myf5, the earliest marker of myogenic commitment. CD34(+ve) myoblasts can be detected in proliferating C2C12 cultures. In differentiating culture s, CD34(+ve) cells do not fuse into myotubes, nor express MyoD. Using isola ted myofibers as a model of synchronous precursor cell activation, we show that quiescent satellite cells express CD34. An early feature of their acti vation is alternate splicing followed by complete transcriptional shutdown of CD34. This data implicates CD34 in the maintenance of satellite cell qui escence. In heterozygous Myf5(nlacZ/+) mice, all CD34(+ve) satellite cells also express p-galactosidase, a marker of activation of Myf5, showing that quiescent satellite cells are committed to myogenesis. All such cells are p ositive for the accepted satellite cell marker, M-cadherin. We also show th at satellite cells can be identified on isolated myofibers of the myosin li ght chain 3F-nlacZ-2E mouse as those that do not express the transgene. The numbers of satellite cells detected in this way are significantly greater than those identified by the other three markers. We conclude that the expr ession of CD34, Myf5, and M-cadherin defines quiescent, committed precursor s and speculate that the CD34(-ve), Myf5(-ve) minority may be involved in m aintaining the lineage-committed majority.