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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.
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