Few things about growing older are as inevitable and obvious as "going gray," yet scientists have been unable to explain the precise cause of this usually unwelcome transformation.
In a report posted on the Web site of the journal Science, researchers from Dana-Farber Cancer Institute and Children's Hospital Boston say they have found the cellular cause of graying hair while investigating the origins of malignant melanoma, the potentially deadly skin cancer.
The scientists traced the loss of hair color to the gradual dying off of adult stem cells that form a reservoir that spawns a continuous supply of new pigment-manufacturing cells, called melanocytes, that give hair its youthful hues. Not only do the non-specialized stem cells become depleted: They also progressively make errors, turning into fully committed pigment cells in the wrong place within the hair follicle, where they are useless for coloring hair.
The new findings won't lead to a scientific alternative to hair dyes any time soon, if ever, even if they do solve a longstanding puzzle about the underlying mechanism of graying. Of more interest to the researchers is the pattern of cellular signals that triggers the death of pigment stem cells, since melanoma is dangerous for the opposite reason –melanocytes proliferate uncontrollably to form tumors and are hard to kill with treatment.
"Preventing the graying of hair is not our goal," emphasizes David E. Fisher, MD, PhD, director of the Dana-Farber Program in Melanoma, and senior author of the Science paper. "Our goal is to prevent or treat melanoma, and to the extent this research is revealing the life cycles of melanocytes, which are the cells that become cancerous in melanoma, we would love to identify a signal that would make a melanoma cell stop growing."
Fisher and the report's lead author, Emi K. Nishimura, MD, PhD, also of the melanoma program, are in the Department of Pediatric Oncology at Children's Hospital Boston as well as at Dana-Farber. The second author, Scott R. Granter, MD, is a pathologist at Brigham and Women's Hospital.
The American Cancer Society expects about 55,100 people to be diagnosed with melanoma, the most serious form of skin cancer, in 2004, with an estimated 7.910 deaths. Melanoma can be cured when it is detected and treated early, but if the lesion penetrates deeply into the skin it is often fatal. Sun exposure is a major risk factor in the disease, which has been increasing in the past several decades.
Melanocytes, which manufacture and store the pigment that combines with hair-making cells called keratinocytes to color the hair, are specialized cells spawned by colorless melanocyte stem cells. These cells were discovered by Nishimura in 2002.
A pool of undifferentiated melanocyte stem cells resides in the hair follicle, and during the hair's grow-and-rest cycle, the stem cells give rise to color-making melanocytes that journey to the bottom of the hair follicle: That is where they tint the keratinocytes with the person's characteristic hue.