Hair follicle

Hair is present on all skin surfaces except the palms, soles, nipples, and glans penis, however it is not always well developed. 

For example, it is very fine on the forehead and the underside of the arm. 

Each hair develops from a group of epidermal cells at the base of a tubelike depression called a hair follicle. This follicle extends from the surface into the dermis and may pass into the subcutaneous layer. 

The epidermal cells at its base receive nourishment from nearby dermal blood vessels, and as these cells divide and grow, older cells are pushed toward the surface. The cells that move upward and away from their nutrient supply become keratinized and die. 

Their remains constitute the shaft of a developing hair. In other words, a hair is composed of dead epidermal cells.

Hair Follicle

Usually a hair grows for a time and then undergoes a resting period during which it remains anchored in its follicle. Later a new hair begins to grow from the base of the follicle, and the old hair is pushed outward and drops off. Sometimes, however, the hairs of the scalp are not replaced, and the result is baldness (alopecia). 

This condition is commonly due to heredity and is most likely to occur in males. 

Hair color is determined by the type and amount of pigment produced by the cells at the base of the hair follicles. For example, dark hair contains an abundance of melanin, while blond hair contains an intermediate quantity; white hair lacks this pigment. Bright red hair contains an iron pigment that does not occur in hair of any other color. A mixture of pigmented and un-pigmented hair usually appears gray.

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.