- Category: Integumentary System
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Hair is present on all skin surfaces except the palms, soles, lips, nipples, and various parts of the external reproductive organs; however, it is not always well developed. For example, it is very fine on the forehead and the inside surface of the arm. Each hair develops from a group of epidermal cells at the base of a tube-like depression called a hair follicle. This follicle extends from the surface into the dermis and may pass into the subcutaneous layer. The cells at its base receive nourishment from dermal blood vessels that occur in a projection of connective tissue, called the derma papilla, at the base of the follicle. As these epidermal cells divide and grow, older cells are pushed toward the surface. The cells that move upward and away from the nutrient supply then die. Their remains constitute the shaft of a developing hair. In other words, a hair is composed of dead epidermal cells.
A bundle of smooth muscle cells, forming the arrector pili muscle, is attached to each hair follicle. This muscle is positioned so that the hair within the follicle stands on end when the muscle contracts. If a person is emotionally upset or very cold, nerve impulses may stimulate the arrector pili muscles to contract, causing gooseflesh (or goose bumps). Each hair follicle is also associated with one or more sebaceous glands.