Most people will notice that the density of scalp hair is reduced as they grow from childhood to adulthood. The reason: Our scalps expand as we grow. Hair has two distinct structures-first, the follicle itself, which resides in the skin, and second, the shaft, which is what is visible above the scalp. The hair follicle is a tunnel-like segment of the epidermis that extends down into the dermis. The structure contains several layers that all have separate functions. At the base of the follicle is the papilla, which contains capillaries, or tiny blood vessels that nourish the cells. The living part of the hair is the very bottom part surrounding the papilla, called the bulb. The cells of the bulb divide every 23 to 72 hours, remarkably faster than any other cell in the body.