Human Hair Distribution and Function

Hair Distribution and Function

• 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.

Hair Structure

• 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.

Hair Follicle

• 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.

Hair Growth

• 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.

Associated Structures

• 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.