Fig. 6. Somite-derived cells replace lateral-plate mesoderm-derived SMCs at E10.5. (A-D) lacZ staining of E9.5 Meox1-cre/R26 reporter mice at two anterior-posterior levels between the forelimb and hindlimb regions. The reporter gene was expressed in myodermatome and in sclerotome in cells surrounding the dorsal part of the aorta. lacZ-expressing cells were found in close proximity to the aortic lumen on the dorsal and lateral sides of the vessel but not on the ventral or ventrolateral sides. (E-J) lacZ staining of E10.5 Meox1-cre/R26 reporter mice at three anterior-posterior levels between the forelimb and hindlimb regions. The reporter gene was homogenously expressed in dorsal mesoderm, but lacZ-expressing cells had also migrated along the ventrolateral walls of the aorta and replaced the lacZ-negative cells that were found in the ventral wall of the aorta at E9.5 (E-H). Meox1-cre/R26-expressing cells were still confined to the dorsal and lateral walls of the aorta in the posterior part of the embryo (I,J), which indicates that the migration of somite-derived cells along the ventrolateral walls of the aorta is initiated in the anterior part of the animal and progresses towards the posterior end. Arrows, Meox1-cre/R26-expressing peri-endothelial cells; arrowheads, lacZ-negative peri-endothelial cells; nt, neural tube; md, myodermatome; sc, sclerotome; da, dorsal aorta; hg, hindgut. Scale bars: 50 µm.