Hair anatomy: the hair shaft, the hair follicle and how hair grows

Hair is a keratinous filament that grows out of the epidermis and is made mostly of dead, keratinised cells. Hair follicles, like sweat glands, sebaceous glands and nails, originate from the epidermis during development and can extend down through the dermis into the hypodermis. (OpenStax Anatomy and Physiology 2e, 5.2: Accessory Structures of the Skin.)

Keratin is an intracellular fibrous protein that gives hair, nails and skin their hardness and water-resistant properties. (OpenStax Anatomy and Physiology 2e, 5.1: Layers of the Skin.)

Hair follicles are present everywhere on the skin except the lips, the palms of the hands and the soles of the feet. They lie in the dermis, the layer between the epidermis and the fat layer, and new hair is made in the hair matrix at the base of each follicle. (Merck Manual Consumer Version: Overview of Hair Growth.)

The epidermis, dermis and hypodermis in 3D, the layers a hair follicle passes through. Layers of the skin.

The hair shaft is the part of a hair that projects above the skin, and the hair root is the part anchored in the follicle below the surface. The root ends deep in the dermis at the hair bulb, which contains the hair matrix, a layer of dividing basal cells, and surrounds the hair papilla, a peg of connective tissue containing blood capillaries and nerve endings. (OpenStax Anatomy and Physiology 2e, 5.2: Accessory Structures of the Skin.)

The visible hair is completely dead, so cutting or shaving it causes no sensation and does not damage the root. Electrolysis and plucking aim to destroy or remove the bulb so that the hair cannot grow back, while most chemical hair removers act only on the surface. (OpenStax Anatomy and Physiology 2e, 5.2: Accessory Structures of the Skin.)

A hair shaft has three layers. The medulla forms the central core and is surrounded by the cortex, a layer of compressed keratinised cells, which is covered by an outer cuticle of very hard keratinised cells. Not all hairs have a medulla. Hair texture, straight or curly, depends on the shape and structure of the cortex and, where present, the medulla, which in turn follow the shape of the follicle. (OpenStax Anatomy and Physiology 2e, 5.2: Accessory Structures of the Skin.)

In Kenhub's histology account the cuticle is a thin, hard layer of overlapping keratin plates, the cortex is the main body of the hair and the medulla is its core. Cut across, straight hair is round, wavy hair oval and curly hair kidney-shaped, but the basic structure is the same in all of them. (Kenhub: Scalp and hair histology.)

The shaft is produced by dividing matrix cells in the hair bulb. As these cells differentiate and move upwards they build the growing shaft, whose cortex is made of intermediate filaments and proteins. (Natarelli N, Gahoonia N, Sivamani RK. Integrative and Mechanistic Approach to the Hair Growth Cycle and Hair Loss. J Clin Med 2023.)

The wall of the hair follicle is made of three concentric layers of cells. The inner (internal) root sheath, formed from the basal cells of the hair matrix, surrounds the root of the growing hair. The outer (external) root sheath is an extension of the epidermis that encloses the root. The glassy membrane is the basement membrane between this epithelium and the surrounding connective tissue. (OpenStax Anatomy and Physiology 2e, 5.2: Accessory Structures of the Skin.)

Kenhub describes the same layers from the inside out. The medulla, cortex and cuticle keratinise to form the hair. The inner root sheath is mildly keratinised and breaks down at the level of the sebaceous gland ducts, leaving space for sebum around the hair. The outer root sheath does not form hair and is separated from the surrounding connective tissue by the glassy membrane, a specialised basement membrane. (Kenhub: Scalp and hair histology.)

Outside the epithelial sheaths is a fibrous root sheath of dermal connective tissue that supplies blood to the follicle. Each follicle has an arrector pili muscle, several sebaceous glands that release sebum into it, and free nerve endings that sense movement and bending of the hair. (Kenhub: Hair follicle.)

Hair grows from the bulb. The basal cells of the hair matrix divide to produce keratinocytes, which push the hair up through the follicle and become keratinised on the way. Matrix cells near the centre of the bulb differentiate into the cells of the inner root sheath. (OpenStax Anatomy and Physiology 2e, 5.2: Accessory Structures of the Skin.)

The dermal papilla, a group of mesenchymal cells at the base of the follicle, controls the number of matrix cells and so the size of the hair. Melanocytes scattered among the matrix cells of the bulb pass pigment into the growing hair. (Natarelli N, Gahoonia N, Sivamani RK. Integrative and Mechanistic Approach to the Hair Growth Cycle and Hair Loss. J Clin Med 2023.)

Kenhub describes the papilla as containing small blood vessels and nerve projections, with the mass of dividing cells around it forming the hair root and the hair matrix that produces the shaft. (Kenhub: Scalp and hair histology.)

Each hair root is connected to a smooth muscle, the arrector pili, which contracts in response to signals from the sympathetic nervous system. Contraction makes the hair stand up, trapping a layer of air that adds insulation; this produces goose bumps. (OpenStax Anatomy and Physiology 2e, 5.2: Accessory Structures of the Skin.)

The arrector pili muscles lie at an angle to the follicles and are attached to the papillary layer of the dermis, so when they contract they pull the hair upright. (Kenhub: Scalp and hair histology.)

A sebaceous gland is an oil gland. These glands are found all over the body, most are associated with hair follicles, and they secrete sebum, a mixture of lipids, onto the skin surface. (OpenStax Anatomy and Physiology 2e, 5.2: Accessory Structures of the Skin.)

Sebum released into the follicle keeps the hair soft and has a bactericidal (bacteria-killing) action. (Kenhub: Hair follicle.)

Hair colour comes from melanin made by melanocytes in the hair papilla region of the bulb. The type of melanin, and so the colour, is genetically determined. Melanin production falls with age, so hair turns grey or white. (OpenStax Anatomy and Physiology 2e, 5.2: Accessory Structures of the Skin.)

Melanocytes in the follicle incorporate melanin into the hair shaft. Melanin comes in black, brown and yellow forms, and their combinations give the range of natural hair colours. (Kenhub: Scalp and hair histology.)

Greying happens when hair follicles make less melanin. It often begins in the 30s, typically at the temples, and its timing is largely determined by genes. With age, strands also become finer and many follicles stop producing new hairs. (MedlinePlus Medical Encyclopedia: Aging changes in hair and nails.)

Lanugo is unpigmented, fine, soft and silky hair that covers the fetus. It is usually shed at around 8 months of gestation and replaced by fine vellus hair and terminal scalp hair in preparation for birth. (DermNet: Hypertrichosis.)

OpenStax's timeline of fetal development agrees: the scalp begins to grow hair in weeks 13 to 16, a silky hair called lanugo covers the skin during weeks 17 to 20, and the lanugo is shed in the last weeks before birth. (OpenStax Anatomy and Physiology 2e, 28.3: Fetal Development.)

Vellus follicles produce tiny, virtually invisible hairs. Under the influence of androgens, vellus follicles in androgen-sensitive sites, such as the face, chest, lower abdomen, upper back and inner thighs, become larger intermediate and terminal follicles that make bigger, pigmented hairs. Androgens also change how long a hair grows and the size and activity of the dermal papilla. (DermNet: Hirsutism.)

Hair growth is regulated by androgens (male hormones). Testosterone stimulates hair growth in the pubic area and armpits, and dihydrotestosterone stimulates hair growth in the beard area and hair loss on the scalp. (Merck Manual Consumer Version: Overview of Hair Growth.)

Hair grows in a continuous cycle of four phases: anagen (growth), catagen (regression), telogen (rest) and exogen (shedding). Each follicle cycles independently of its neighbours and goes through ten to thirty cycles in a lifetime. (Natarelli N, Gahoonia N, Sivamani RK. Integrative and Mechanistic Approach to the Hair Growth Cycle and Hair Loss. J Clin Med 2023.)

In anagen the cells at the root divide rapidly and push the hair out; OpenStax gives this phase 2 to 7 years. Catagen, the transition, lasts about 2 to 3 weeks and growth stops. Telogen, the rest, lasts about 2 to 4 months, after which a new anagen phase begins and pushes the old hair out. (OpenStax Anatomy and Physiology 2e, 5.2: Accessory Structures of the Skin.)

Natarelli and colleagues give scalp anagen as two to eight years and note that hair length matches the length of anagen, which is why eyebrow hair, with an anagen of only two to three months, stays short. In catagen, which lasts about two weeks, the follicle regresses and detaches from the dermal papilla, cells of the bulb die by apoptosis, and the papilla moves up towards the hair-follicle bulge; if it cannot reach the bulge, cycling stops and the hair is lost. Telogen lasts about two to three months. (Natarelli N, Gahoonia N, Sivamani RK. Integrative and Mechanistic Approach to the Hair Growth Cycle and Hair Loss. J Clin Med 2023.)

The Merck Manual gives a growing phase of 2 to 6 years for scalp hair, a transitional phase of about 3 weeks and a resting phase of 2 to 3 months, at the end of which the hair falls out and a new hair starts growing in the same follicle. (Merck Manual Consumer Version: Alopecia (Hair Loss).)

For eyebrows and eyelashes the Merck Manual gives a growing phase of 1 to 6 months, much shorter than on the scalp. (Merck Manual Consumer Version: Overview of Hair Growth.)

DermNet estimates that about 85% of scalp follicles are growing (anagen) and about 15% resting (telogen) at any one time, with a follicle usually growing for about 4 years and resting for about 4 months. Natarelli and colleagues put the resting fraction of scalp hair at about 9%, compared with 40 to 50% of hair on the trunk. (DermNet: Telogen effluvium.)

The five layers of the scalp, where about 100,000 hair follicles sit. Scalp anatomy.

During anagen hair typically grows about 0.3 mm a day, and on average about 50 hairs are lost and replaced each day. (OpenStax Anatomy and Physiology 2e, 5.2: Accessory Structures of the Skin.)

Natarelli and colleagues give an average growth rate of about 1 cm a month, which matches 0.3 mm a day. They estimate that most people have about 100,000 scalp hairs and that normal shedding is 100 to 150 telogen hairs a day. (Natarelli N, Gahoonia N, Sivamani RK. Integrative and Mechanistic Approach to the Hair Growth Cycle and Hair Loss. J Clin Med 2023.)

The NHS says we can lose between 50 and 100 hairs a day, often without noticing. (NHS: Hair loss.)

The Merck Manual also gives about 50 to 100 scalp hairs a day as normal. It counts losing more than 100 a day as abnormal, except after shampooing, when up to 250 may come out. (Merck Manual Consumer Version: Alopecia (Hair Loss).)

Hair protects: scalp hair shields the skull from the sun, and hairs in the nose and ears and the eyelashes trap dust and particles that may carry allergens and microbes; eyebrows keep sweat and particles out of the eyes. A hair root plexus of sensory nerves around each follicle makes hair sensitive to air movement and to anything touching the skin, and the arrector pili help keep in heat by raising the hairs. (OpenStax Anatomy and Physiology 2e, 5.2: Accessory Structures of the Skin.)

Kenhub adds that hair cushions the skin, protects against ultraviolet light, filters air, takes part in temperature regulation and sensation, and is part of the secondary sexual characteristics. (Kenhub: Hair follicle.)

The 3D viewer highlights the three hair surfaces in the BodyParts3D model, an adult male built from MRI: the hair of the head, the eyebrows and the pubic hair. The hair of the head and the pubic hair belong to the skin system, which starts hidden, so raise the Skin slider to see them; the eyebrows show at once. The model shows where hair grows but not the follicles, which are microscopic, so the diagrams on this page show those. (Mitsuhashi N et al. BodyParts3D: 3D structure database for anatomical concepts. Nucleic Acids Res 2009.)

The other keratin appendage of the skin: nail plate, matrix, lunula and nail bed. Nail anatomy.

Eyebrow

The eyebrow is a band of hairs on the ridge above the orbit.

Hair of head

The hair of the head is made of keratinised hair shafts growing from follicles in the scalp.

Pubic hair

The pubic hair is terminal hair growing over the pubic region after puberty.