Human Hair Distribution and Function

Study Notes

Hair is present on most skin surfaces, except palms, palmar surfaces of fingers, soles, and plantar surfaces of feet.
In adults, hair is most heavily distributed on the scalp, eyebrows, axillae (armpits), and around external genitalia.
Genetic and hormonal influences determine the thickness and pattern of hair distribution.
Eyebrows and eyelashes protect the eyes from foreign particles, similar to hair in the nostrils and external ear canal.

Each hair is composed of dead, keratinized epidermal cells bonded together by extracellular proteins.
The hair shaft is the superficial portion that projects above the skin surface.
The hair root is the portion below the shaft, penetrating the dermis and sometimes the subcutaneous layer.
The shaft and root consist of three concentric layers: medulla, cortex, and cuticle.
The medulla contains pigment granules, which vary in amount depending on hair color.
The cortex forms the major part of the shaft and consists of elongated cells.
The cuticle is the outermost layer, comprising a single layer of thin, flat cells.

Surrounding the root is the hair follicle, consisting of an external root sheath and an internal root sheath.
The external root sheath is a downward continuation of the epidermis.
The internal root sheath is produced by the matrix and forms a cellular tubular sheath.
The epithelial root sheath comprises the external and internal root sheaths.
The dermal root sheath is the dense dermis surrounding the hair follicle.

The hair bulb is an onion-shaped structure at the base of each hair follicle and surrounding dermal root sheath.
The hair bulb contains the papilla of the hair, which has areolar connective tissue and many blood vessels.
The papilla nourishes the growing hair follicle.
The hair matrix is a germinal layer of cells responsible for hair growth and replacement.

Sebum-producing sebaceous glands are associated with hairs.
A bundle of smooth muscle cells, the arrector pili, extends from the superficial dermis to the dermal root sheath.
The arrector pili causes "goose bumps" when stimulated by autonomic nerve endings.
Dendrites of neurons surrounding each hair follicle form a hair root plexus, sensitive to touch.