Ancient fish scales and vertebrate teeth share an embryonic origin.
By Dental Tribune International.
CAMBRIDGE, UK: Two new studies focusing on the evolutionary origin of teeth and of vertebra have provided further evidence of the human connection to marine organisms. One study has confirmed the long-held assumption that teeth and the scales of sharks and skates have a common evolutionary origin and the other has highlighted backbone formation as relating to common ancestry of all jawed vertebrates.
Owing to the strong similarities between fish scales and tooth structure, it has long been assumed that the two are related; however, various studies in the past have shown that zebrafish scales are more closely related to the musculoskeletal system than to the tissues that form teeth. Owing in part to these conflicting theories, Dr J. Andrew Gillis of the University of Cambridge and lead author of one of the two studies investigated this further.
“The scales of sharks and skates are very different from the scales of zebrafish [and other] types of bony fish,” said Gillis. “We wondered if they may be a different type of scale, and have a different embryonic origin more closely related to teeth.”
Teeth are formed from a special group of cells located at the crest of the neural tube, the embryonic tissue that eventually becomes the brain and spinal cord. Gillis hypothesised that these neural crest cells also formed the scales of sharks and skates.
By injecting the neural tube of embryonic skates with a fluorescent dye that adhered to neural crest cells, the researchers could see the dye that remained in the cells as they divided and moved from the neural tube to form different parts of the skate’s body. The cells that became scales were marked by fluorescent dye, indicating they came from the neural crest cells.
Gillis went on to explain that jawed vertebrates once had bony armour on the outside of their bodies composed of dentine and under that layer was a bonier type material. This suggests that over time the armour was lost in different fishes, with the dentine becoming the scales of sharks and skates.
In the second paper, Dr Katharine Criswell, also from the University of Cambridge, and Gillis focused on the evolution of vertebral tissue by studying spinal development in skates. These cartilaginous fishes are a distant relative of both land vertebrates and ray-finned fishes.
“The backbone provides protection to the spinal cord and support for the limbs and head. But despite this important role, the shape and construction of the backbone varies considerably between different animals,” said Criswell. “This variation made us wonder how the earliest vertebrates built their backbones.”
The spines of all land vertebrates, including humans, form only from a group of cells called somites. Ray-finned fishes, however, form their spines from both somites and chordoblast cells. Using fluorescent cell tracking, Criswell determined that the spines of skates form from somites alone. Since two of the three related groups use only somites in vertebral development, Criswell concluded that the common ancestor of all jawed vertebrates had a backbone derived from somites.
“It is interesting to think that backbone development in sharks and skates is more similar to humans than it is to bony fishes like zebrafish and salmon. It refutes the old notion of sharks and skates as ‘primitive’ fishes,” said Criswell.
The studies, titled “Trunk neural crest origin of dermal denticles in a cartilaginous fish” and “Embryonic origin of the gnathostome vertebral skeleton”, were published in the Proceedings of the National Academy of Sciences of the United States of America and the Proceedings of the Royal Society B, respectively, on 20 and 22 November.