Why do rodents’ teeth stay sharp? What is the difference between the mouse and the kangaroo?

Answers to these two questions are given by two recent studies on tooth development and evolution. Scientists at the University of Helsinki have identified the gene responsible for tooth enamel formation, which also explains the sharp incisors of rodents. The second study adds to our knowledge on evolution: the differences between teeth in various species, which have been utilised in evolution research, can be altered through one signalling molecule that regulates development. The research findings will be published this week in two leading scientific journals, Developmental Cell and Nature.

“Enamel gene” explains why rodents’ teeth stay sharp

Scientists at the University of Helsinki Institute of Biotechnology have, in collaboration with their American and Swiss colleagues, identified a novel function for a gene which codes a crucially important signalling molecule, BMP (bone morphogenetic protein). BMP initiates the development of enamel and is, thereby, vital for the formation of enamel.

The scientists have also shown that BMP, together with its specific inhibitor follistatin, explain the special characteristic of rodents’ incisors, their sharp, cutting edge, which stays sharp although the rest of the incisors are worn by gnawing. The sharp edge is caused by the inhibition of BMP signalling molecule in the inner surface of the tooth so that enamel is built only on the outer surface. Thanks to the enamel, the outer surface is harder and will not wear as fast as the inner surface, and a sharp edge is formed at the point where the two surfaces meet.

The fine-tuning of various signalling molecules is the most important mechanism regulating the shape of tissues and organs. The mouse incisor is an illustrative example of how local differences in the activities of the signalling molecules determine the generation of complex structures.

The scientists participating in the study at the University of Helsinki were Xiu-Ping Wang and Marika Suomalainen from the Research Program in Developmental Biology – one of the national Centres of Excellence in Research – and Professor Irma Thesleff, the director of the unit. The results were published in Developmental Cell on 9 November.

One gene brings back teeth of mouse ancestors from 45 million years ago

Tooth fossils are the only remains of many extinct mammals. There are several details in the teeth of different species by which scientists determine the kinship between species. The University of Helsinki evolutionary biologists have shown that several of these characteristics used in evolution research can be changed with one signalling molecule regulating individual development.

The scientists discovered that when the production of a signalling molecule ectodysplasin is increased during development, the mouse teeth develop structures that can be found, for example, in the teeth of the kangaroo. Increasing the secretion of ectodysplasin also causes the mouse to have an extra tooth in a place where the ancestral form of rodents 45 million years ago still had a tooth. If the secretion of ectodysplasin is decreased, however, the mouse’s teeth lose several structures.

The results are beneficial to evolutionary biologists in determining the kinship relations between species, and for developmental biologists when studying why closely related species, such as the chimpanzee and human, have so many structural differences.

The study was conducted by Aapo Kangas, Alistair Evans, Irma Thesleff and Jukka Jernvall from the University of Helsinki Institute of Biotechnology. The study belongs to the field of evolution and developmental biology and its results will be published in Nature on 11 November.