Discovery of HAP Stem Cells.

Cells expressing the stem cell marker, nestin, were selectively labeled in transgenic mice by placing green fluorescent protein (GFP) under the control of the nestin promoter in transgenic mice. In these transgenic mice, neural and other stem cells brightly expressed GFP. The mice were termed nestin-driven GFP (ND-GFP) mice. During early anagen or growth phase of the hair follicle, ND-GFP appeared in the permanent upper hair follicle immediately below the sebaceous glands in the follicle bulge. The relatively small, oval-shaped, nestin-expressing cells in the bulge area surrounded the hair shaft and were interconnected by short dendrites. The location of the nestin-expressing cells in the hair follicle varied with the hair cycle. During telogen or resting phase and in early anagen, the GFP-positive cells are mainly in the bulge area. However, in mid- and late-anagen, the GFP-expressing cells were located in the upper outer-root sheath as well as in the bulge area. The expression of the unique protein, nestin, in both neural stem cells and hair follicle stem cells, which suggested their relationship. The ND-GFP hair follicle stem cells were later termed hair-follicle-associated pluripotent (HAP) stem cells.