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Pufferfish beak originates from stem cell tweak

The unusual beak of the pufferfish is produced by small tweaks to the normal programme of tooth development, research reveals this week.

The beak, ideal for crunching through prey such as shellfish, is composed of elongated teeth that are fused together. A team of researchers, including Museum scientists, have uncovered how the pufferfish creates this structure.

Dr Ralf Britz, fish researcher at the Museum says, 'Pufferfish beaks have fascinated anatomists since the time of Georges Cuvier and Richard Owen in the early nineteenth century, but their development has only been recently unravelled.

'We previously demonstrated that the beak is formed by multiple replacement generations of only four teeth, which elongate along the jaw. Our current study identifies the source of the tooth-forming cells and their spatially restricted distribution in the pufferfish's jaw.'  

The discoveries will help scientists understand how teeth are produced in a wide range of vertebrates (animals with backbones), including humans.

A close-up of the pufferfish Tetraodon lineatus, showing its beak, which is fused to the jaw

Building a beak

While a young pufferfish's first set of teeth are set in a normal row along the jaw, the teeth that replace them when they wear down are very different.

Only a single replacement tooth forms in each quarter of the jaw. It elongates and eventually stretches across the entire jaw. New bands of replacement teeth then form underneath, in the jaw cavity. They make a stack that fuses together as the teeth grow, creating the beak.

Stages of tooth development in a pufferfish, showing clockwise from top left: the change from teeth, to bands, to the beginnings of a beak. The teeth and bands are stained red.

The regeneration game

Most fish, unlike humans, produce new generations of teeth throughout their life. This requires a source of stem cells, which have the potential to develop into many different cell types and can be programmed to transform into the cells that build a tooth.

The stem cells are usually located in the jaw, just below where the teeth will form. In the pufferfish, however, the stem cells are located outside of the jaw. Their access to the cavity in which the replacement teeth form is restricted to a single small opening in each quarter of the jaw.

Stem cells are able to enter via these openings and move up through the jaw cavity to reach the four sites where the new teeth will grow.

Growing bands

The next step in pufferfish tooth replacement is to alter the shape of the four teeth. The genes that control how the teeth form cause them to elongate rather than grow as simple teeth.

The first set of pufferfish teeth (T1) grow as normal. But later sets (R1, R2, R3) grow as single bands that elongate from sites near the front of the jaws. Stem cells are able to get to these sites through a single opening (grey arrows) in each quarter of the jaw, near the front edge of the jaw joint (white arrow).

The researchers were able to show that the bands grow by elongation from the four sites by interfering with the normal chemical signaling processes of tooth development. When the correct signals were blocked, the new teeth did not elongate along the jaw but just grew near the four original sites.

An ancient plan

The research team are interested in the pufferfish beak because although its final shape is unusual, it fundamentally uses the same tooth-formation programme that all vertebrates have used for over 400 million years.

Dr Gareth Fraser from the University of Sheffield, who led the research, says, 'The fact that all vertebrates regenerate their teeth in the same way, with a set of conserved stem cells, means that we can use these studies in more obscure fishes to provide clues to how we can address questions of tooth loss in humans.'

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