Hair Anatomy

The entire skin surface is covered by hair follicles, except for the palms, soles, genital orifices, and lips. Approximately 100,000 of the body’s 5 million hair follicles are located on the scalp. Most hairs on the scalp are large terminal hairs, while the majority of the body’s hairs are small vellus hairs. The scalp has the highest density of hair follicles, with 300 to 500 hairs per square centimeter, which play crucial roles in thermoregulation and UV radiation protection.

The hair follicle is divided into three segments:

• Infundibulum: Extends from the skin opening to the sebaceous gland.
• Isthmus: Extends from the sebaceous gland opening to the arrector pili muscle insertion.
• Inferior segment: Encompasses the deep follicle, ending with the bulb, which contains the hair matrix and dermal papilla.

The lower portion of the hair follicle comprises five main components:

• Dermal papilla
• Matrix
• Hair shaft: Consists of the medulla, cortex, and cuticle layers (from inside out).
• Inner root sheath (IRS): Includes the inner root sheath cuticle, Huxley’s layer, and Henle’s layer.
• Outer root sheath (ORS)

The dermal papilla is made up of dermal connective tissue that invaginates into the hair follicle base and features a capillary loop supplying blood to the follicle. The hair matrix cells, surrounding the dermal papillae within the hair bulb, grow into the hair follicle and shaft as they proliferate. These matrix cells also house melanocytes, which provide pigment to the hair shaft.

Hair shafts typically have 2-3 layers. The medulla, the innermost layer, is found only in thicker terminal hairs and is surrounded by the cortex and cuticle, which fill with keratin as the hair grows. Keratin is a strong structural protein that composes hair. The hair shaft is fixed within the inner root sheath, also containing three layers: the inner root sheath cuticle, Huxley’s layer, and Henle’s layer. The inner root sheath contributes keratin and trichohyalin granules to the growing hair shaft. Trichohyalin is another protein that provides mechanical strength. The inner root sheath is integral to the lower hair shaft but less important to the upper hair shaft; therefore, it is only present until the isthmus before disintegrating.

The outer root sheath, situated outside the inner root sheath, forms the hair follicle’s external portion, extending from the lower follicle to the skin surface and opening at the infundibulum. It also serves as the attachment site for the pilosebaceous unit (oil gland) and arrector pili muscle (a tiny muscle connecting each hair follicle to the skin).

Located near the sebaceous duct opening and arrector pili muscle insertion is the follicular bulge, containing undifferentiated epithelial stem cells. These cells contribute to hair follicle growth and play a crucial role in wound healing. It is postulated that the destruction of these cells leads to permanent follicle loss in scarring alopecia.