Pull a tooth from this fish and it grows replacements three times faster

01The question

People get two sets of teeth: baby teeth, then adult teeth. Lose an adult tooth and it's gone for good. Many fish and reptiles, however, keep replacing their teeth for their whole lives. Cichlids from Lake Malawi in Africa swap each tooth for a new one roughly every 30 to 40 days, with the next tooth already waiting in the jaw below.

The researchers wanted to know what happens when teeth are lost suddenly. Does replacement speed up, and which cells and chemical signals take charge of building a whole new tooth?

02What they did

A team at Georgia Tech in the US worked with three cichlid species whose teeth look very different: some have a few rows of large, cone-shaped teeth, others hundreds of tiny, three-pointed ones. Under anesthesia, they plucked teeth from one side of each fish's jaw only, so the untouched side served as a comparison.

Two colored dyes told old teeth from new ones. The first marked the bone that already existed before plucking, and the second, given 15 days later, lit up newly forming teeth. In a second experiment the team read which genes were switched on in about 27,000 individual cell nuclei from the jaw (single-cell sequencing) at 1 hour and at 1, 3 and 7 days after plucking.

03What they found

Plucking sped up tooth replacement three to four times, but only on the plucked side of the jaw. After 15 days, about 60% of the teeth on that side were brand new. Remarkably, the speed-up was about the same in all three species, whether a fish had tens of big teeth or hundreds of small ones.

The response started fast. Within an hour, rapid-response genes had switched on and signals linked to inflammation and the immune system were rising. Over the following week the conversation between cells shifted: supporting tissue first signaled to the enamel-making cells, then to immune cells, then the enamel-making cells mostly talked among themselves, and by day 7 the tooth-building signals were back.

Throughout the week the plucked side was busy with signals for building scaffolding (collagen), remodeling tissue, growing new blood vessels and guiding nerves. These are the jobs needed to assemble a living tooth from scratch.

04Why it matters

Many of the cell types that build fish teeth have close counterparts in mouse and human teeth. What mammals seem to have lost is the band of tissue that keeps starting new teeth in adulthood, the successional lamina.

Mapping the step-by-step recipe that lets a fish rebuild a tooth is a starting point for asking whether that ability could ever be reawakened in people. The same approach could also help explain how other body parts, such as hair and feathers, regrow.