Trunk neural crest origin of caudal fin mesenchyme in the zebrafishBrachydanio rerio

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Abstract
The origin of fin mesenchyme and the cells forming the distal fin ray skeleton in fishes is unknown, but is often assumed to be the neural crest. To test this possibility, a series of experiments have been done using a vital cell marking technique to label candidate progenitor cell populations in early zebrafish embryos. Injections of the fluorescent membrane probe DiI were made in discrete focal points into the dorsal aspect of the caudal neural keel before trunk neural crest cell emigration at that axial level in embryos at the 15-20 somite stage. As controls, injections were made into both paraxial, unsegmented mesoderm, and somitic mesoderm before somite elevation, ventral and lateral to the dorsal keel injection site. Only after injection into the neural keel site were labelled cells subsequently found in the fin mesenchyme (along with labelling of other cells types traditionally regarded as crest derived). These data provide the first experimental evidence that early fin bud mesenchyme contains a contribution from the trunk neural crest, and support the hypothesis that the dermal skeleton depends upon a crest-derived mesenchyme for its formation. We suggest that a causal link between neural crest-derived mesenchyme and the development of the exoskeleton has been evolutionary conserved throughout the neural crest of teleosts but has been lost in the trunk crest of higher vertebrates.