We have generated a mouse strain carrying a Cre-ER(T2) knock-in allele at the Pax7 locus, the Pax7(CE) allele (Lepper et al., 2009, Nature 460:627-631). Combining Pax7(CE) and the R26R(LacZ) Cre reporter allele, here we describe temporal-specific tamoxifen (tmx)-inducible lineage tracing of embryonic Pax7-expressing cells. In particular, we focus on the somitic lineage. Tmx-inducible Cre activity directed by the Pax7(CE) allele is similar to the endogenous Pax7 expression pattern. The somitic Pax7-expressing cells selectively marked at embryonic day 9.5 (E9.5) give rise to dorsal dermis and brown adipose tissue, in addition to dorsal aspects of trunk muscles and the diaphragm muscle. However, they do not contribute to ventral body wall and limb muscles. After E12.5, marked Pax7-expressing cells become lineage restricted to muscles. Descendants of these early marked Pax7-expressing cells begin to occupy sublaminal positions associated with the myofibers around E16.5, characteristic of embryonic satellite cells. Furthermore, they contribute to adult myofibers and regeneration competent satellite cells in the tibialis anterior muscle, providing evidence that some adult satellite cells are of embryonic origin.
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