Last revised by Daniel J Bell on 4 Jun 2024

Hair (TA: pilus/pili) remains important physiologically and psychologically for humans. The hair shaft develops from a structure known as the hair follicle. Each hair has an arrector pili muscle and both sensory and sympathetic neural connections.

The hair shaft (TA: stipes pili) arises within hair follicles (TA: folliculus pili), which are tubular epidermal structures that penetrate deep into the dermis. At their deepest point, the follicle expands to form the hair bulb. The hair root is the portion of the hair shaft deep to its exit point from the skin surface.

The base of the bulb is turned in on itself, forming a depression in which the vascular dermal papilla sits. The papilla contains multiple supplying and draining capillaries and specialized mesenchymal cells that regulate the growth of the hair shaft. 

Encasing the papilla within the bulb of the follicle is a matrix comprising the germination cells, which undergo rapid division (fastest mitosis of any tissue in the body) to form the new hair cells. These developing cells ascend to form the hair shaft and internal root sheath. Melanocytes are interspersed throughout the matrix releasing melanin to be incorporated into the growing hair cells.

An arrector pili muscle originates from the dermis and inserts into the hair follicle at the bulge. Contraction of this smooth muscle, under sympathetic control, causes hair to become erect and also compresses an associated sebaceous gland which secretes its sebum onto the shaft. Sebum, a fat-rich substance has a protective function for the hair.

The hair follicle is conventionally divided into three main segments

  • inferior segment: from base of follicle, where bulb and papilla are located, up to the "bulge" - insertion site of arrector pili muscle

    • subdivided into bulbar and suprabular regions

  • isthmus: from bulge to sebaceous gland meatus

  • infundibulum: from opening of sebaceous gland to the skin surface

Of course, the largest concentration of hair follicles is on the scalp, with hair follicles over most of the rest of the body, with the exception of:

  • vermilion of the lips

  • palms of hands and soles of feet

  • thick skin, e.g. heels of the feet

  • urogenital openings

Distribution of hair is further modified by age, sex and ethnicity 3.

In most mammals, hair remains important for thermoregulation and camouflage. Clearly, these roles are diminished in people. However, hair still has an important physiological role with regards to sweating and touch. It helps protect the skin from ultraviolet radiation and other external elements. Most importantly, in humans, is the importance of hair as part of one's personal identity 4.

Arterial supply and venous drainage of each follicle arise from the dermal plexus of the hair bulb 4,5

The dermal lymphatics drain the hair follicles 4,5.

Hair follicles are innervated by two separate neural systems, with both systems having both sensory and sympathetic fibers 4:

  • sensory C fibers and sympathetic fibers: encompass neck of follicle

  • longitudinal A-delta and circular C fibers envelop mid follicle

During its active growth phase (anagen), each hair shaft is formed of three concentric layers, from inner to outer these are:

  • medulla: centermost

    • transparent cells admixed with air spaces, varying with hair type

    • maybe absent 

  • cortex: most important region

    • contributes most of the mass and provides strength to hair

    • keratin is the protein composed of multiple filaments with a helical substructure underlying hair's strength and flexibility

    • as the cells ascend from the hair matrix they gradually keratinize

    • one hair shaft has a strain resistance of 100 grams

    • color of the hair comes from the melanin content of this region

  • cuticle: outer protective layer

    • interdigitating strata of 8-10 flat cells

    • it is a reflective region supplying the hair's shine

Enveloping the thin cuticle of the growing shaft are the inner and outer root sheaths of the follicle whose role is to keratinize the growing hair shaft.

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