Why does hair turn grey with age? In work reported last December in Science Express , scientists studying mutant mice which turn prematurely grey give a possible answer . Greying happens because of disturbances in special stem cells, called melanoblasts, which give rise to the cells which color hair. When these melanoblasts are disturbed, the pigmenting cells (called melanocytes) don't get replaced, and subsequent hair lacks color.
Many, and possibly all, adult tissues have a population of special reserve cells, called stem cells , which contribute new cells to the tissue following normal wear-and-tear or injury. For example, stem cells in hair follicles mobilize after a hair is lost to generate melanocytes and other cells. These daughter cells begin building a new hair, and the stem cells become dormant again. This cycle repeats over and over throughout life.
Mice with mutations in the Bcl-2 gene start going grey after just a few weeks of age, and in fact lack all their melanocytes by 6 weeks after birth. The scientists in this paper wondered whether defects in the the stem cell population might be to blame. They used a genetic trick to mark the melanoblasts in Bcl-2 deficient mice, and found that the melanoblasts were dying right at the step in the hair follicle cycle when they should have been returning to dormancy. Mice with a second sort of mutation showed a different type of melanoblast disturbance, but the result was the same: grey hair.
The scientists then looked at old normal mice, which show some greying, and found disrupted melanoblasts in hair follicles from those animals as well. To consider if these changes also occur in humans, they looked at cells which express MITF, a gene they knew from the mouse work to be expressed by melanoblasts. In hair follicles from 70 to 90 year old humans, the MITF-expressing population was absent. Thus, in two different mouse mutations, and in normal greying of two different species, the melanoblast population seems to be the key to color.
Stem cell disturbances might contribute to other changes which come with age, including heart problems and, of course, cancer . One feature of greying hair which interests me is the apparent "all-or-none" effect of greying. Individual hairs seem to be either fully colored, or fully grey. Does this mean that all the melanoblasts in a given hair follicle stop functioning at once? A second point worth remembering is that grey hair does continue to grow. That is, the other stem cells involved in hair growth (which come from a different developmental source) seem to keep on doing their job after the melanoblasts have given up the ghost.