The face scars less than the body — a Stanford Medicine study unravels why

Tweaking a pattern of wound healing established millions of years ago may enable scar-free injury repair after surgery or trauma, Stanford Medicine researchers have found. If results from their study, which was conducted in mice, translate to humans, it may be possible to avoid or even treat the formation of scars anywhere on or within the body.

Scarring is more than a cosmetic problem. Scars can interfere with normal tissue function and cause chronic pain, disease and even death. It’s estimated that about 45% of deaths in the United States are due to some type of scarring (also known as fibrosis) — usually of vital organs like the lungs, liver or heart.

Scars on the skin’s surface, while rarely fatal, are stiffer and weaker than normal skin and they lack sweat glands or hair follicles, making it difficult to compensate for temperature changes.

Surgeons have known for decades that facial wounds heal with less scarring than injuries on other parts of the body. This phenomenon makes evolutionary sense: Rapid healing of body wounds prevents death from blood loss, infection or impaired mobility, but healing of the face requires that the skin maintain its ability to function well.

“The face is the prime real estate of the body,” said professor of surgery Michael Longaker, MD. “We need to see and hear and breathe and eat. In contrast, injuries on the body must heal quickly. The resulting scar may not look or function like normal tissue, but you will likely still survive to procreate.”

Exactly how this discrepancy happens has remained a mystery, although there were some clues.

“The face and scalp are developmentally unique,” said professor of surgery Derrick Wan, MD. “Tissue from the neck up is derived from a type of cell in the early embryo called a neural crest cell. In this study we identified specific healing pathways in scar-forming cells called fibroblasts that originate from the neural crest and found that they drive a more regenerative type of healing.”

Activating this pathway in even a subset of fibroblasts around small wounds on the abdomen or backs of mice caused them to heal with much less scarring — similar to untreated facial or scalp wounds.

Longaker, the Deane P. and Louise Mitchell Professor in the School of Medicine, and Wan, the Johnson & Johnson Distinguished Professor in Surgery II, are the senior authors of the study, which was published Jan. 22 in Cell. Plastic surgery resident Michelle Griffin, MD, PhD, and clinical and postdoctoral scholar Dayan Li, MD, PhD, are the lead authors of the research.

“Many of the authors on this paper are fellow physician scientists,” said Li, who is board certified in dermatology. “This project was inspired by what we’ve observed in our patients — facial wounds in general heal with less scarring. We wanted to understand, mechanistically, why this is.”

Proteins determine scarring

Li and his colleagues used laboratory mice to investigate differences in wound healing at various sites on the animals’ bodies. They anesthetized the mice before creating small skin wounds on the face, scalp, back and abdomen. The wounds were stabilized by suturing small plastic rings around them to prevent differences in mechanical forces as the animals moved. Mice were given pain relief during the healing process.

After 14 days, the wounds on the face and scalp expressed lower levels of proteins known to be involved in scar formation as compared with those on the abdomen or back of the animals. The sizes of the scars were also smaller.

The researchers then transplanted skin from the face, scalp, back and abdomen of mice onto the backs of control mice. After the transplants had engrafted, they repeated the experiment on the transplanted skin. As before, wounds in the skin transplanted from the faces of the donor mice expressed lower levels of scarring-associated proteins.

Additionally, Li and his colleagues isolated fibroblasts from skin samples from the four body sites in the donor mice and injected them into the backs of control mice. They observed reduced levels of scarring-associated proteins on the recipient animals’ backs injected with fibroblasts from the donor animals’ faces as compared with fibroblasts from the scalp, back or abdomen.

“We found you don’t need to change or manipulate all fibroblasts within the tissue to have a positive outcome,” Li said. “When we injected fibroblasts that we had genetically altered to more closely resemble facial fibroblasts, we saw that the back incisions healed very much like facial incisions, with reduced scarring, even when the transplanted fibroblasts made up only 10% to 15% of the total number of surrounding fibroblasts. Changing just a few cells can trigger a cascade of events that can cause big changes in healing.”

A less-fibrotic wound healing

Digging deeper, the researchers identified changes in gene expression between facial fibroblasts and those from other parts of the body and followed these clues to identify a signaling pathway involving a protein called ROBO2 that maintains facial fibroblasts in a less-fibrotic state. They also saw something interesting in the genomes of fibroblasts making ROBO2.

“In general, the DNA of the ROBO2-positive cells is less transcriptionally active, or less available for binding by proteins required for gene expression,” Li said. “These fibroblasts more closely resemble their progenitors, the neural crest cells, and they might be more able to become the many cell types required for skin regeneration.”

In contrast, the DNA in fibroblasts from other sites of the body allows free access to genes like collagen that are involved in the creation of scar tissue.

“It seems that, in order to scar, the cells must be able to express these pro-fibrotic genes,” Longaker said. “And this is the default pathway for much of the body.”

ROBO2 doesn’t act alone. It triggers a signaling pathway that results in the inhibition of another protein called EP300 that facilitates gene expression. EP300 plays an important role in some cancers, and clinical trials of a small drug molecule that can inhibit its activity are underway. Li and his colleagues found that using this pre-existing small molecule to block EP300 activity in fibroblasts prone to scarring caused back wounds to heal like facial wounds.

“Now that we understand this pathway and the implications of the differences among fibroblasts that arise from different types of stem cells, we may be able to improve wound healing after surgeries or trauma,” Wan said.

The findings are likely to extend to internal scarring as well, Longaker said. “There’s not a million ways to form a scar,” he said. “This and previous other findings in my lab suggest there are common mechanisms and culprits regardless of the tissue type, and they strongly suggest there is a unifying way to treat or prevent scarring.”

Researchers from the University of Arizona contributed to the work.

The study was funded by the National Institutes of Health (grants R01-GM136659, U24DE029463, R01-DE032677, R01-AR081343, RM1-HG007735 and 5T32AR007422-43), The Hagey Laboratory for Pediatric Regenerative Medicine, the Wu Tsai Human Performance Alliance, the Scleroderma Research Foundation, the A.P. Giannini Foundation and the Howard Hughes Medical Institute.

Longaker is an inventor on a patent application that covers a machine-learning algorithm for analysis of connective tissue networks in scarring and chronic fibroses.

Longaker is a member of Stanford’s Bio-X, the Stanford Cardiovascular Institute, the Wu Tsai Human Performance Alliance, the Institute for Stem Cell Biology and Regenerative Medicine, the Maternal and Child Health Research Institute, and the Stanford Cancer Institute.

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