Fig. 8. N-cadherin is not required for the development of the earliest myotome composed of pioneer fibers. (A-D) Electroporation of control GFP to the medial region of newly-formed epithelial somites in 23-25ss embryos. (A) Five hours after transfection, the pioneer cells (arrowhead) express GFP, whereas the dorsal somite (delineated between arrows) is devoid of labeled cells (n=20/24). (B) Dorsal view of a somite 5 hours after transfection. Note the homogeneous rostral-to-caudal distribution of labeled cells. (C) Sixteen hours later, pioneers have dissociated, and are localized preferentially in the rostral half of the segment, forming a triangular shape (arrowhead); in addition, a few partial-length fibers that elongate rostrocaudally (arrow) and localize medially close to the neural tube (NT) are apparent (n=10/10). (D) Twenty-four hours after transfection, full-length myofibers formed (n=13/17). (E-G) Electroporation of cN390-GFP to the medial region of newly-formed epithelial somites in 23-25ss embryos. (E,F) Transverse (E) and dorsal (F) views 5 hours after transfection. Labeled cells are localized to the pioneer region (arrowhead) but not to the prospective DM and DML (between arrows, n=14/17). (G) Twenty-four hours after electroporation, full-length myofibers formed (n=15/16). (H,I) Electroporation of cN390-GFP to the medial region of newly-formed epithelial somites in young 15ss embryos. Twenty-four hours later myofibers had formed normally and already span a significant mediolateral extent of the segment (n=9/10 embryos). (J,K) Electroporation of wtN-cadherin-GFP to the medial region of newly-formed epithelial somites in 23-25ss embryos. As early as twenty hours post-transfection, continuous expression of the protein is compatible with the normal formation of myofibers. In addition, some labeled mesenchymal cells are still apparent in the lateral domain of the somite (arrowheads, n=8/8). H and J depict GFP⁺ cells on a phase contrast background, I and K are GFP only. Note in D,G,H and K that in controls and all experimental treatments, formation of the pioneer fibers occurs as a discrete process leaving no residual GFP labeling of the DML region. The labeled cells observed at 5 hours in the medial domain of panels B and F correspond to the labeled cells in the transverse sections in A and E, respectively (i.e. ventrally located with respect to the future DML). Scale bars: 20 µm for A,C,E.