Dr. Cotsarelis first localized presumptive hair follicle stem cells to the hair follicle bulge. He then developed methods for isolating these cells and studying their lineage during normal hair follicle cycling. His laboratory demonstrated that bulge cells were normally responsible and necessary for the continued growth and cycling of the hair follicle, and that, after isolation, these cells could regenerate all major epithelial cell types within the skin. Thus, bulge cells possessed stem cell properties and potentially could be useful for tissue engineering purposes. Dr. Cotsarelis is also studying the role of hair follicle stem cells in wound healing and in skin regeneration following wounding. Hair follicle stem cells contribute to repair of the epidermis after wounding, but do not contribute long-lived epidermal stem cells to the healing wound. He is currently working on defining the cellular and molecular differences between skin repair and skin regeneration. Dr. Cotsarelis is also studying the role of hair follicle stem cells in human disease, including different forms of alopecia. He showed that hair follicle stem cells also localize to the human hair follicle bulge. Destruction of the stem cell area may explain why some types or alopecia are scarring and permanent. Dr. Cotsarelis is studying differences in stem cell number and activation in different cutaneous disorders.