Figure 1

Ablation of the dorsolateral quarter of epithelial somites (primordium of the ventrolateral dermomyotome lip) interrupted hypaxial myotome growth. A and B: Operation scheme. The dorsolateral quarter of 3 somites (somite III or IV) of a stage 16 chick embryo was ablated. C-F: in situ hybridisation with Pax3. G and H: Transverse section at the level (indicated by a line in D and F) of the embryo in D and F. In the operated side the Pax3 expression was detected only in the epaxial domain (ep), while it was expressed in both epaxial (ep) and hypaxial (hyp) domain of the dermomyotome in the control side. I-L: Immunohistochemistry with MF20. M and N: Transverse section at the level (indicated by a line in J and L) of the embryo in J and L. In the operated side the myosin heavy chain was detected only in the epaxial myotome (ep), while it was expressed in both epaxial (ep) and hypaxial (hyp) domain of the myotome in the control side. C, D, G, I, J, M: Operated embryos after one day reincubation. E, F, H, K, L, N: Operated embryos after two day reincubation. Operated somites are indicated by arrows (one arrow = one segment) in the whole mount. Arrow heads indicate the ventrolateral end of the hypaxial domain of the dermomyotome and myotome. Arrows mark the border between the epaxial and hypaxial domain. Abbreviations: ep epaxial domain, fl forelimb, hl hind limb, hyp hypaxial domain, lat lateral, med medial, nt neural tube.