400 Million-Year-Old Questions
(December 5th, 2017) What do fish scales and teeth have in common? Quite a lot, say researchers from the University of Cambridge. For instance, their embryonic origin.
For a long time, there’s been a cloud of mystery around the embryonic development of some fish. The reason? Most of these fall within the cartilaginous category, like sharks and skates for example, which give birth to live young. The idea of killing a pregnant wild animal for the sake of research is just not acceptable.
Taking advantage of one exception to this rule – the egg-laying little skate (Leucoraja erinacea) - Andrew Gillis and his team from the University of Cambridge, UK, developed a method for manipulating and observing early embryos inside their eggs. The use of fluorescent markers “allows us to label specific cell populations in the early embryo, and then follow them through development to see what they give rise to in a hatchling”, says Gillis. “These sorts of experiments now allow us to address longstanding questions about the embryonic origin of different tissues and organs in cartilaginous fishes”.
One of these challenges relates to a possible connection between teeth and fish scales. The discussion has been around for a while but so far there’s no hard evidence either way. This may sound a little far-fetched but the Cambridge team discovered that scales in cartilaginous fish and teeth have a common origin: neural crest cells. “We were able to trace the embryonic origin of the scales of cartilaginous fishes back to the neural crest - the same cell population that gives rise to teeth”, says Gillis.
At first glance this may sound somewhat weird but a closer look at scales of primitive fish dissipates any doubts. These small spiky scales – now present in only a few living species, including sharks and skates – bear a striking resemblance to jagged teeth. “The tissue similarities between teeth and these dentine-containing scales led to the hypothesis that teeth could have evolved from scales that moved into the mouth at the origin of jaws”, says Gillis.
The authors believe these skin ‘denticles’ are the left-overs of the earliest mineralised skeleton of vertebrates forming a thick armour plating, which consisted of a bottom bony layer and a top dental layer (dentine and enamel). About 400 million years ago, this combination of bone and dentine armour would have served them well as protection against predators. “We often think of dentine as being associated just with teeth, but the first vertebrate fossils to have dentine were jawless fishes - they lacked both jaws and teeth - so it seems likely that dentine first evolved in the earliest vertebrates to help strengthen their bony armour”, adds Katharine Criswell, who was also involved in the study.
Over the course of evolution, this armour shrunk from large plates to smaller scale-like structures. Curiously, while sharks and skates chose to keep the tooth-like dentine outer layer; other species opted for the bony under layer. While we will never get a definite answer, it’s interesting to speculate about why it happened this way. “One possibility is that the nature of scales has changed in response to aspects of the fish’s environment - i.e. depending on whether they need heavy scales, light scales, flat scales, pointy scales, etc for protection or swimming efficiency, they’ve modified their scale structure accordingly. But this is just speculation”, says Gillis.
For Gillis and Criswell, this study was just the beginning. “Neural crest cells are one of the defining features of vertebrate animals, and in order to understand their ancestral repertoire, we need to know what they give rise to in lots of different vertebrate lineages”, says Gillis. “Over the next few years, we hope to map out all of the neural crest derivatives of the little skate, so that we may compare these with better understood systems, like mouse and chicken”.
The mystery surrounding the scales is also not completely solved and Gillis is keen to study another primitive example: the bichir (Polypterus), one of the only living lineages that has actually kept both bony and dentine layers in their scales. This “would allow us to test the embryonic origin of both of these tissue layers in the same animal. I would love to collaborate with my colleagues that work with this animal, to further test the embryonic origin of these different types of scale tissues”.
“It would be great to follow the development of both mesodermal and neural crest cells in a bichir to test our prediction that the outer layer is derived from neural crest cells and the inner layer is derived from mesoderm”, concludes Criswell.
Photo: Leucoraja erinacea by Katharine & Robert Criswell