The contribution of neural crest cells to the nuchal bone and plastron of the turtle shell

The origin of the turtle plastron is not well understood, andthese nine bones have been homologized to the exoskeletal componentsof the clavicles, the interclavicular bone, and gastralia. Earlierdata from our laboratory showed that the plastral bone-formingcells stained positively for HNK-1 and PDGFR, two markers ofskeletogenic neural crest cells. We have now shown that theHNK-1⁺ cells are also positive for p75 and FoxD3, affirmingtheir neural crest identity. These cells originate from thedorsal neural tube of stage-17 turtle embryos, several daysafter the original wave of neural crest cells have migratedand differentiated. Moreover, we have demonstrated the existenceof a staging area, above the neural tube and vertebrae, wherethese late-emigrating neural crest cells collect. After residingin the carapacial staging area, these cells migrate to formthe plastral bones. We also demonstrate that one bone of thecarapace, the nuchal bone, also stains with HNK-1 and with antibodiesto PDGFR. The nuchal bone shares several other properties withthe plastral bones, suggesting that it, too, is derived fromneural crest cells. Alligator gastralia stain for HNK-1, whiletheir ribs do not, thus suggesting that the gastralial precursormay also be derived from neural crest cells.