In mouse experiments, scientists unlock the key to scar-free skin healing

blue fibroblasts
Confocal micrograph of a primary human fibroblast cells grown in culture stained for actin, a highly abundant protein in cells that makes up the cytoskeleton of cells (blue). The nucleus is shown in red. Fibroblasts are specialized cells involved in wound healing.

Scars. Like memories, we all have them. When you look at a scar you might see a tumble, a fight, an operation, a reminder of that time you succumbed to youthful stupidity, survived a traumatic incident, cheated death. But these marks aren’t just cosmetic keepsakes. Scar tissue is what happens when skin heals but it doesn’t regenerate. It’s not as strong, it can’t move as well, it can’t grow hair or secrete sweat or sense the environment.

And it has long been thought that scarring, like death and taxes, is an inevitable part of being human.

Now, researchers at Stanford University have decoded the chemical and physical signals that trigger a particular type of skin cell to produce scars. And they have discovered a way to reprogram these cells, transforming them into another cell type capable of regenerating tissues intact. Mice that received this tweak healed from wounds with no scars, scientists reported Thursday in Science. The animals regrew hair, glands, and other critical structures. Their recovery was so complete that an image-classifying algorithm couldn’t tell the healed wound apart from the animals’ healthy, unmaimed skin.

The researchers say the next step is to try to achieve similar skin regeneration in larger, tighter-skinned animals, like pigs, that more closely resemble humans. They are optimistic that the finding could lead to readily available treatments, and the possibility of a scar-free future. They already have identified a drug candidate that has been on the market for two decades to treat certain eye conditions.

“This is really groundbreaking,” said Radhika Atit, a skin biologist at Case Western Reserve who studies skin development and was not involved in the research. “I had goosebumps reading the paper.”

Every year, an estimated 100 million patients acquire scars from surgery. Millions more suffer burns and minor injuries that also leave scars. So scarless wound healing has long been something of a holy grail for scientists, since it was first discovered in fetal lambs 50 years ago.

But accomplishing it involves meeting three critical requirements. First, you have to give skin back all its appendages. Though it might not look like much, skin has a lot going on beneath the surface — follicles for growing hair, glands for secreting sweat and oil, nerves for sensing pain and pressure. Scar tissue doesn’t have any of these things. “It’s basically a hole that’s been plugged with a plaster of epidermis,” said Atit. “It’s just a living Band-Aid.”

When your skin is split or sliced open, you bleed. But pretty quickly, platelets in your blood grab onto the cut edges of the severed vessels, releasing signals to attract more platelets. The reinforcements glom together into a clot that staunches the flow. Immune cells start arriving to dispatch any bacteria or debris in the area. Cells called keratinocytes close the outermost edges of the wound. Then scraggly cells known as fibroblasts begin to fill it in, laying down thick layers of collagen.

In healthy skin, fibroblasts knit collagen fibers into a sort of disorderly basket-weave. That provides the tissue with structure, but it also lends skin tensile strength. So you can grab it and pull it and it doesn’t just shear open.

“You can think of fibroblasts as construction workers,” said Ryan Driskell, a cell biologist at Washington State University who studies skin regeneration and wound healing. “They create a house in which stem cells live, and stem cells give rise to all the other structures of the skin. The quality of the house will define the health of the stem cell.”

During wound healing, fibroblasts hastily lay down collagen fibers parallel to each other instead of cross-wise. The process builds up tissue quickly, but the bonds holding the fibers together are much weaker, resulting in skin that’s thicker but not as strong or flexible. And it doesn’t leave a lot of room for stem cells. For that, you have evolution to thank.

A hundred thousand years ago, healing slowly wasn’t an option. Early hominids didn’t have sterile solutions, stitches, and antibiotics. If you got cut up, the wound would get infected, making you easy prey for a big cat or other predator. Speed-healing was an evolutionary advantage. It didn’t have to be pretty. As long as you could survive and procreate, it gave you an edge.

Overcoming that legacy of natural selection means recreating the lattice-like structure of healthy skin, and the mechanical properties that come with it — the last two requirements for achieving scar-free wound healing, said Atit. The paper in Science nails all three.

The study represents a culmination of discoveries decades in the making. In 1971, a pediatric surgeon in Chicago discovered that when he operated on fetal lambs, their wounds healed without any scarring. Over the next 20 years, scientists found the same remarkable ability in other animals, including sheep, rats, mice, pigs, and monkeys.

In the early ’90s, Michael Longaker, whose lab at Stanford led the new research, was working as a postdoc under pediatric surgeon Michael Harrison at the University of California, San Francisco. Harrison was doing something no one else in the world was — performing surgeries on the unborn. He’d remove fetuses from their mother’s uterus, and, with the umbilical cord still intact, correct various life-threatening defects — patch a hole in the diaphragm, repair a blocked urinary tract — before returning them to the womb. When the babies were born 8 to 12 weeks later, they’d have a little bit of redness around the site of the surgery, but no scars. He asked Longaker to figure out why that might be. And that’s what he’s spent the last 30 years trying to do.

For much of that time, the field focused almost exclusively on stem cells, the cells that make all the skin’s mini-organs. “Fibroblast” was almost a dirty word back then. “The assumption was they were all the same,” said Driskell. And that all they did was cause scarring. But scientists like he and Atit set out to study them and discovered that not only were there lots of different kinds, but that they sent out signals to tell the stem cells where to make sweat glands and hair follicles. Longaker’s lab took that work and ran with it.

In 2015, his team at Stanford amassed an inventory of the different types of fibroblasts living in the skin of a mouse’s back. They found that only one subset of fibroblasts — dubbed EPFs, because they expressed a protein called engrailed-1 — was responsible for the formation of most scar tissue. When they knocked this cell line out, those mice recovered more slowly, with less scarring. The next thing to do was to figure out how those EPFs worked. If they could turn them off with a drug, they might be able to stop scarring in humans, too. For the last three and a half years, that’s what Longaker and his colleagues have been doing.

Mice scarring
Stanford scientists genetically tweaked mice to not express the YAP protein in fibroblasts. Those mice (middle and bottom rows) regenerated skin, including hair follicles and sweat glands.MASCHARAK ET AL., SCIENCE 2021

First, the researchers used fluorescent markers to trace where EPFs come from. They learned that the scar-producing cells actually arise from another population of fibroblasts that don’t produce engrailed-1, and instead regenerate healthy skin. It’s only when the animal gets wounded that the gene flips on. Longaker, a pediatric plastic surgeon who directs the program in regenerative medicine at Stanford, said the group hypothesized that the trigger might be mechanical — the force of the skin splitting apart.

“If I make an incision in a bowl of jello, it doesn’t gape open,” he said. Humans, on the other hand, are tight-skinned animals. Gashes and incisions leave flesh flapping — at least, once we leave the womb. In utero, our skin is gelatinous. It can secrete things and absorb things; it’s not yet watertight. “That fact pointed to mechanics,” said Longaker.

So the group studied how fibroblasts responded to a variety of different mechanical cues. When they were grown in soft substrates they didn’t flip on engrailed-1. The researchers also messed with the tension of wounds in mice and found the same thing. And they noticed that as they applied more tension, the fibroblasts produced more of a protein called YAP. They wondered if maybe it was the key chemical signal for kicking off scarring.

To test that, they blocked YAP a few different ways: first, by genetically modifying mice to not express it in their fibroblasts, and then, with a YAP-disrupting chemical called verteporfin. In both cases, the cells that flooded into each mouse’s wound weren’t the scar-producing EPFs, but the other kind of fibroblast, the one that tells the skin to regenerate, not just repair. “Discovering that YAP starts the fibrotic response, that was the last piece of the puzzle,” said Longaker.

Mice treated with the YAP-blocker not only grew back hair follicles and glands within 30 days, their new skin also recovered normal collagen structure. And when tested for mechanical breaking strength, it was comparable to normal skin.

Washington State’s Driskell sees the discovery as more of a beginning than the end. “If we want to get to full regeneration we have to understand how all these sub-populations of fibroblasts work together to rebuild the tissues properly,” he said. While Longaker’s group catalogued the return of some skin structures, it wasn’t a complete list. More work will need to be done to see if YAP-blockers can turn on all the signals needed to regrow everything healthy skin needs to function, such as temperature- and pressure-sensing nerves. “There’s always more to it,” said Driskell. “But I think it’s definitely worthwhile to move to the next step and try some clinical trials.”

Over the last decade, several companies have sought to commercialize wound-healing therapies — spray-on skins and living sheets of stem cells. But none has yet achieved scar-free healing. Verteporfin, if it works in humans, would be the first. It’s already on the market, sold under its brand name Visudyne. The U.S. Food and Drug Administration approved it in 2000 for treating age-related macular degeneration. That should make it easier to move from testing it in pigs to human trials.

Longaker envisions a time when doctors will be able to inject a bit of verteporfin around a laceration or incision as they stitch it up, encouraging the wound to repair itself slowly, carefully, and completely. That’s scar prevention. But Longaker said the drug might be able to erase old scars too. It would require some minor surgery to cut the damaged tissue out and inject the verteporfin in. But for particularly disfiguring or painful scars that limit people’s mobility, the procedure might be an attractive option.

It’s this future that got Atit so excited she began bombarding her children with fibroblast chat over breakfast this week. “I told them, ‘Guys, this means you will not have scars in your lifetime,’” she said.

Scars are more than just disfiguring. You lose sweat glands and nerves and other critical ways of sensing and responding to the world around you. “Right now the best we can offer people is little bits of skin, but there’s nothing in it — no hair follicles, no blood vessels, no nerve endings,” said Atit. “To be able to give people the ability to regenerate their own skin, which is where this is heading, it’s really just beyond exciting.”

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