Myf-5 is transiently expressed in nonmuscle mesoderm and exhibits dynamic regional changes within the presegmented mesoderm and somites I-IV

Myf-5 is one of four myogenic regulatory factors that play important roles in skeletal muscle development. This study provides detailed analysis of My f-5 expression during early chick development using an in situ hybridizatio n technique that has been optimized to detect low level Myf-5 transcripts. This facilitated detection of heretofore unrecognized dynamic changes in My f-5 expression patterns. Myf-5 expression is first detected at stage 3 in t he primitive streak and exhibits transient low-level expression in nonmyoge nic mesoderm. Myf-5 is later expressed in the presegmented mesoderm (psm) i n a reiterating pattern that is coordinated with somitogenesis and also col ocalizes with the Notch ligand C-Delta-1. In somites (S) I-IV, Myf-5 expres sion exhibits dynamic regional changes, and in somites rostral to S IV, Myf -5 is expressed at higher levels in muscle precursors in the dorsomedial so mite. Semiquantitative comparison of Myf-5 mRNA levels in the psm and in my otome-containing somites indicates about a 10-fold difference. The expressi on pattern of Myf-5 differs from that of MyoD, which we find is expressed o nly in the dorsomedial somite. These data reveal that Myf-5 is expressed at low levels several stages before muscle differentiation occurs and suggest that only a subset of cells that initially express Myf-5 will upregulate i ts expression and differentiate as muscle. (C) 2001 Academic Press.