Iris Kulbatski in The Scientist:
People have complicated relationships with hairy skin moles. For some, they symbolize individuality and good fortune. For others, a bothersome blemish. Maksim Plikus, a professor of developmental and cell biology at the University of California, Irvine regards these mounds of wild-growing hair as islands of knowledge. His curiosity and astuteness compelled him to look closer at a biological anomaly that others overlooked. “Nothing exists for no reason. There is some really interesting biology hidden in everything,” Plikus said. Skin moles are common growths that contain clusters of melanocytes, the skin’s pigment producing cells. These melanocytes are senescent1—they are old and no longer reproduce.
…Plikus and his team used genetically engineered mouse models of skin moles and single cell RNA-sequencing to study hair follicle stem cell activity. They discovered that senescent melanocytes in hairy moles secrete a signaling molecule called osteopontin, which stimulates dormant hair follicle stem cells and triggers hair growth. When they examined human samples of hairy skin moles, Plikus’ team found higher levels of osteopontin compared to nearby non-mole skin, and when they injected osteopontin into hair follicles of human skin grafts, new hair grew.