Fig. 3. Whole-mount ß-galactosidase staining of Myf5-I transgenic mouse embryos and serial transverse sections. (A) An E9.5 embryo (26 somites) illustrating that activation of the transgene occurs correctly in the epaxial domain of somites but is delayed by 5-6 somites. The red arrowhead indicates the most recently formed somite, and the black arrowhead points to the first somite expressing the transgene. Sections confirm that Myf5-I expression is properly restricted to the epaxial myotome along the entire axis but does not reach into the hypaxial domain during early stages. Expression in branchial arches occurs in the correct cells. (B) Myotomal expression of Myf5-I is maintained in an E12.5 embryo and, in addition, the transgene is now also expressed in the muscle anlagen of fore- and hindlimbs. This expression persists in all limb muscles at E13.5 (C). At this stage, the ventral myotome of tail somites shows robust transgene activity.