The adaxial cell phenotype of oating head and sonic you embryos is similar: failed caudal myf5 and myoD maintenance and delayed terminal differentiation. myf5 (A, B, E, F, I, J) and myoD (C, D, G, H, K–P) mRNAs are reduced in adaxial cells, but not elsewhere in homozygous h embryos (B, D, F, H, J, L, P), in which notochord fails to mature, compared to wild-type or heterozygous siblings. As in syu embryos, h embryos show good adaxial expression at tailbud stage, but h additionally has ectopic mesodermal midline myf5 and myoD mRNA (arrows, B, D). At 5 somites, much myf5 mRNA is lost in adaxial cells of anterior presomitic mesoderm (F), and myoD mRNA-containing cells are converging into the midline (H). This trend is accentuated at 10 somites, when both myf5 and myoD mRNAs (J, L) appear reduced in anterior presomitic adaxial cells, except for rare clusters of cells that appear to reexpress in the anterior midline (arrows, F, H, J, L). By 17 somites, myoD mRNA is undetectable in anterior presomitic mesoderm of h (P) and syu (M) embryos, but nevertheless is detected at wild-type levels anking the notochord anlagen in the tip of the tailbud (arrows, M, O, P). In contrast, this expression is entirely lost in no tail embryos (arrow, N). Arrowheads mark the most posterior left somite boundary in each panel. Wholemount immunohistochemical detection of sarcomeric MyHC (Q, R) reveals that, whereas adaxial cell terminal differentiation precedes or is contemporaneous with somite formation in siblings, differentiation is substantially and variably delayed in syu (Q) or h (R), with the caudalmost detectable MyHC being in older somites (arrows). Bar, 200 μm in A–P and 177 μm in Q, R.
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