Sunday, 4 October 2015

On The Origin Of Enamel

The white layer of the tooth is the enamel
Teeth are nothing without enamel. At the centre is a cavity filled with soft pulp and nervous tissue. The enamel - the hardest substance produced by the human body - is what gives teeth their bite.

The mineral portion of enamel, hydroxyapatite, is similar to that found in bone. It is deposited on a matrix of three unique proteins. The hydroxyapatite, in conjunction with the protein matrix is what makes enamel so tough.

But where did it come from? Palaeontological studies have shown that teeth originated as scales which later migrated inside; while the jaw derived from the gill arches in once jawless fish. Yet scales do not have a covering of enamel.

Certain types of living archaic fish, such as the gar, have dermal denticles. These are tooth-like scales which have a covering of ganoine. This is tough and mineralised. The protein component of ganoine contains two out of the three proteins found in our own enamel, suggesting that the two substances are closely related. Palaeontological studies have shown that ganoine was once much more prevalent in bony fish. Genes for enamel-like substances are expressed in the mouth and in the skin of fish with dermal denticles. So did enamel originate in the mouth, the skin or both?

An artist's impression of Psarolepis
A recent study focusing on two fossil fish provides an answer. The two species are Psarolepis from China and Andreolepis from Sweden, the former a lobe-finned fish, the latter a ray-finned fish.

In Psarolepis the scales and the denticles of the face are covered with enamel, but there is no enamel on the teeth. In Andreolepis only the scales are covered in enamel.

'Psarolepis and Andreolepis are among the earliest bony fishes, so we believe that their lack of tooth enamel is primitive and not a specialisation. It seems that enamel originated in the skin, where we call it ganoine, and only colonised the teeth at a later point,' said Per Ahlberg from Uppsala University.

This is the first study to combine palaeontological and genetic data to investigate the origin of a particular tissue. The research group responsible for the study have stated that they plan to use the same approach to investigate the origin of other tissues. The science of evolutionary development can reveal the origin of broad patterns of evolution and of high level structures like body plans, but it can equally provide insight into the development of more specific structures.