Here’s what you’ll learn when you read this story:

• Mammals aren’t exactly known for their limb-regenerating abilities, but some of them, including humans, have the ability to regenerate fingertips, skin, bone, and nerves.
• A new study moves beyond the cellular lens of this phenomenon and instead examines the extracellular matrix (ECM), the non-cellular structure that surrounds cells in tissues.
• The research shows that regeneration relies on a balance between the ECM and hyaluronic acid, a kind of sugar molecule.

While our big brains might be the envy of the animal kingdom, many vertebrates seemingly simpler than ourselves retain impressive regenerative abilities. Zebrafish, for example, can completely regenerate damaged or diseased eyes, while salamanders can regrow entire limbs and even parts of their brain. At first glance, this seems like an evolutionary ability largely abandoned by mammals, but if you know where to look, even our class of fauna can still regrow body parts in surprising ways.

One of the more head-scratching abilities of humans, especially younger children, is to effectively regrow amputated fingertips. Unlike other body parts, this particular area can regrow not only damaged skin, but also bone, tissue, and nerves. Scientists have wondered if nail stem cells, which sit right near the cuticle, could be one reason for this unique ability, since it’s possible that they send a signal toward a wounded area that activates regrowth of bones and nerves. Now, scientists at Stanford University have published a new study in the journal Science in which they looked closely at the extracellular matrix (ECM), the non-cellular structure that surrounds cells in tissues, and discovered that the key to regeneration came down to the balance between this collagen matrix and hyaluronic acid, a kind of sugar molecule.

“Tissue regeneration is rare in mammals, but the digit tip can regrow after amputation, whereas injuries beyond the nail do not. How the microenvironment drives divergent outcomes remains unclear,” the authors wrote. “We found that the extracellular matrix and tissue mechanics govern the amputation response in mouse digits. Nonregenerative regions were stiffer and contained dense, organized collagen, whereas regenerative regions were soft and enriched in hyaluronic acid.”

The researchers used mice to investigate how the ECM impacted the regenerative ability of distal (near the finger tip) and proximal (near the knuckle) amputation. Previous research looked into how cells or molecular pathways could impact this regeneration, but the signals inside the microenvironment of the ECM remained poorly understood. So using single-cell RNA sequencing, they found that osteo-lineage cells produced this much-needed acid in blastema (undifferentiated cells that congregate near a wound) along with HAPLN1 and aggrecan—proteins and sugars that were largely absent when examining wounds that scarred over without regrowth.

To test this hypothesis, the authors treated various mice with hyaluronidase (along with an additional chemical inhibitor) to suppress the sugar. They found evidence that “strongly suggested an indispensable role” for hyaluronic acid in distal regeneration by maintaining tissues that are soft and fluid—unlike the stiff, collagen-heavy environments of scarred tissue. Additionally, they found that the HAPLN1 protein, when applied to fibroblasts in nonregenerative digit stumps, softens tissues and reduces scarring and even triggered bone regeneration, which is another strong indicator of hyaluronic acid’s starring role in regenerating fingertips.

“By demonstrating that the ECM acts as a mechanochemical signalling hub, we show that ‘retuning’ the physical environment from a stiff, collagenous state to a soft, HA-rich state can unlock regenerative potential,” the authors wrote. “This shift in perspective—from targeting cells to engineering the matrix—offers a promising new toolkit for regenerative medicine and the treatment of fibrotic disease.”