When my mother was 60, she burned the inside of her forearm on a baking dish. The rectangular wound turned red and oozed for weeks. When it did heal, the rectangle had turned brown but remained visible for the rest of her life.
When my daughter was 4, she accidentally rested the inside of her forearm on a hot grill. Her skin blistered and looked like a strip of cooked bacon. But in a week, the burned skin had fallen off leaving just a red patch. Within a month, the skin had healed so well that I could not detect any redness at the burn site.
Why did my daughter's skin heal with such speed while my mother's skin remained discolored?
The most obvious difference is age. But what is it about aging that slows healing?
As we age, our skin becomes thinner. We tend to lose some of the fat beneath the skin, and we produce less collagen (those stretchy protein fibers that make skin supple and pliable). The real reason for slower wound healing, however, lies deeper - literally - in the blood that supplies oxygen and white blood cells to the wound site.
"We tried to isolate factors that may impede healing," said Dr. Marty Robson, professor emeritus at the University of South Florida, who conducted research on wound healing for 35 years.
"Age itself did not inhibit healing, but the elderly tend to have a lot of problems that do inhibit healing, such as decreased blood supply, especially to the legs. A decrease in oxygen will inhibit healing."
Aging also affects the plaque buildup in our arteries. As plaque increases, blood flow becomes constricted and the amount of oxygen that reaches our tissues is reduced. Respiratory problems, such as emphysema or lung cancer, can limit the amount of oxygen that gets into the blood. Blood also delivers white blood cells and proteins known as growth factors. The white blood cells that attack bacteria at the wound site become less vigorous, and the platelets deliver fewer growth factors. If that isn't enough, our cells become less responsive to the growth factor they do receive. Finally, our bone marrow produces fewer adult stem cells that help remodel damaged skin and organs.
"In older people there's a delay in the appearance of these adult stem cells at the wound site," said Dr. Jeffrey Davidson, a professor of pathology at Vanderbilt University in Nashville.
"The rate of closure is slower; the rate at which the skin gains strength is slower. It's not that the wounds won't heal; it's just that they heal more slowly."
Sometimes, however, wounds don't heal and the cause can be found in the blood - or the lack of it.
"Most wounds that fail to heal involve an insufficient blood supply," said Dr. Thomas Mustoe, chief of plastic surgery at Northwestern University Medical School in Chicago and the author of "Mechanisms of Chronic Wounds" in the December issue of the American Journal of Surgery. This problem is compounded because in older people, the genes that trigger a response to the stress of low oxygen become sluggish.
"If you look at 5,000 genes, you'll find that maybe 80 percent of them will be up-regulated in response to a period of low oxygen," Mustoe said. "In older people, only about 10 percent will be turned on."
The most notorious cause of nonhealing wounds is diabetes.
Diabetes, which causes the level of sugar in the blood to rise to destructive levels, accelerates heart disease and damages the delicate capillaries that supply blood to the skin. The disease also inhibits white blood cells from engulfing bacteria, and it slows the production of collagen, which is essential for remodeling the skin after an injury. A diabetic's white blood cells respond less robustly to growth factors, so they do not divide and multiply quickly.
"In addition to all this vascular pathology, there's nerve pathology, so the diabetic doesn't feel an injury," Davidson said. "They may not feel a wound, say, on the bottom of the toe until the toe starts to turn black, and by then you have a serious problem because the infection may have reached the bone, and the toe may have to be removed."
Only one drug, Regranex, has been approved for the treatment of diabetic foot ulcers. It contains a growth factor known as PDGF, which stimulates surrounding cells to divide and fill in the wound.
"We have seen some very nice results with Regranex," said Dr. Wyatt Payne, chief of plastic surgery at Bay Pines VA Medical Center. "It's not a cure-all, and it's expensive - about $300 for a small tube - but it's a very good adjunct to the standard care of chronic wounds."
Babies and young children have an astonishing ability to heal quickly. If my daughter had been a teenager, her burn almost certainly would have generated a keloid - a form of heavy scar tissue - produced by growth factors that cause skin cells to proliferate too much.
But if a wound remains clean and receives a generous supply of oxygen from the blood, it will heal well, while producing a minimum of scar tissue.
Tom Valeo is a freelancer who writes about medical and health issues. Write to Tom Valeo, c/o Seniority, the St. Petersburg Times, P.O. Box 1121, St. Petersburg, Fl 33731 or e-mail firstname.lastname@example.org