Monaghan and a team of researchers discovered how axolotl flesh cells at the site of an injury respond to retinoic acid, a molecule similar to the retinol in many skin creams. Axolotls, which are aquatic salamanders, biologically control the amount of acid in their limbs to grow the precise amount of a limb lost to injury, Monaghan said. That is how the animals know whether to grow a finger, a hand, or an entire arm, scientists said in a study published Tuesday in the journal Nature.

For centuries, scientists have wondered how salamanders regrow limbs, especially axolotls, which do so at one of the fastest rates, said James Monaghan, biology department chair at Northeastern.

A team at Northeastern University announced Tuesday they discovered a key to limb regeneration in axolotls, the smiley pink salamanders that have become a social media sensation, findings that could one day help scientists regrow human fingers or limbs.

“We solved that piece of the puzzle, in that we discovered a group of enzymes that break down retinoic acid,” said Monaghan.

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James Monaghan, chair and professor of biology at Northeastern, poses for a portrait on Dec. 12, 2024. Alyssa Stone/Northeastern University

Like axolotls, humans also naturally have retinoic acid. Monaghan says the findings could help scientists engineer a similar biological process to one day allow humans to regrow limbs, he said.

“Instead of generating a scar, they will turn on that biological program that made the limb in the first place,” he said.

Outside of scientific laboratories, Monaghan said he hopes axolotl fans young and old will see his team’s research and learn more about their favorite animal, which last month appeared as a McDonald’s Happy Meal toy.

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“I wouldn’t have known 22 years ago it would explode in popularity, and now it’s become this superstar of cuteness,” he said.

Northeastern chair and professor of biology James Monaghan works with axolotls in his lab in the Mugar Life Sciences on Dec. 11, 2024. Alyssa Stone/Northeastern University

An axolotl in the Northeastern lab. Alyssa Stone/Northeastern University

‘Spiderman’ villain The Lizard was inspiration

Growing up in the 1980s and ’90s in Carmel, Indiana, Monaghan watched Spiderman cartoons every day after school with his little brother. His favorite character was Dr. Connors, a scientist who lost an arm, regrew the limb, and became The Lizard, one of the earliest villains in the series.

Those cartoons inspired Monaghan to study limb regeneration, he said.

“Not that I want to be Dr. Connors, but maybe I can be him without the evil part,” Monaghan said, with a laugh.

Northeastern chair and professor of biology James Monaghan works with axolotls in his lab in the Mugar Life Sciences on Dec. 11, 2024. Alyssa Stone/Northeastern University

Since he was 14 years old, Monaghan has held onto a Spiderman blow-up figurine that’s now strung across the ceiling of his lab, which is also home to about 500 axolotls.

What are axolotls?

In the wild, axolotls are a critically endangered salamander native to the lakes and waterways of Mexico City and surrounding valleys. They evolved from a salamander that once walked on land and developed gills, according to scientists.

Culturally, they’re a new internet phenomenon, especially popular with children and Gen Z, spawning an axolotl Build-a-Bear toy, a highly coveted Squishmallow stuffed toy, and a character in the widely played Minecraft video game. There’s also a professional basketball axolotl mascot named Juanjolote for Mexico City’s NBA G League team, the Capitanes.

“The best decision I made in my career is to work on the axolotl,” Monaghan said.

The animals also make great pets, Monaghan said, although the creatures can grow to over a foot long, he warned. He only works with live axolotls at the lab.

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At his home in the Boston area, Monaghan said, his 5-year-old has about 10 axolotl toys that smile down from bookshelves and bed frames.

“It’s pretty surreal to see that it’s become such a cultural icon,” he said.

Claire Thornton can be reached at claire.thornton@globe.com. Follow Claire on X @claire_thornto.