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As many of us age, the density of our scalp hair decreases. Some of us even become beautifully bald. What age-related changes contribute to hair loss? Contrary to common belief, stem cell numbers in skin do not decrease in density or number. Hair follicles maintain their stem cell compartment and normal molecular expression. However, signaling is altered, so human age-associated alopecia probably results from failure to activate stem cells in telogen hair follicles.
In a series of clever experiments, full-thickness skin with “telogen retention” (age-associated alopecia) from 24-month-old mice was transplanted onto immunodeficient 3- to 6-month-old mice. Hair cycling resumed in the transplant if the area was small. If the transplant area was …