It has happened to tons of people: strands of hair that were once brown, black, or other shades now appear gray. Why does that happen? Hair color is determined by melanocyte stem cells that multiply inside hair follicles, previous research has shown. These stem cells get signals to mature and make proteins that produce hair color. But researchers at the New York University Grossman School of Medicine might have a reason for why it turns gray. According to the team of scientists, melanocyte stem cells become stuck inside the hair follicle and are unable to produce pigment. Read more in the link in bio.