The Biology of Hair
Why hair grows, rests and sheds in cycles. What hair is made of, how colour happens, and why it grows faster in some places than others.
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What Hair Is Made Of
Hair might look simple, but it has a surprisingly organised internal structure. Each strand is made almost entirely of a tough, fibrous protein called keratin. Keratin is also the main material of fingernails and toenails, and it forms the outermost protective layer of skin — making it one of the body's most versatile structural proteins.
A single hair shaft (the visible part above the skin) has three concentric layers:
- Medulla — the soft, loosely organised core, present in thick hairs but sometimes absent in fine hairs.
- Cortex — the thick middle layer that makes up most of the hair's bulk. Densely packed keratin fibres here give hair its strength, and pigment granules here determine hair colour.
- Cuticle — the outermost layer, made of overlapping, scale-like cells like roof tiles. A smooth cuticle makes hair appear glossy and feel silky; a roughened or damaged cuticle makes hair look dull and feel rough.
The hair shaft itself is dead tissue — there are no living cells in the part of hair that extends above the scalp. All the living activity happens deep in the hair follicle below the skin surface. For more on the skin structures that anchor hair, see the skin anatomy guide.
The Hair Follicle — Where Hair Is Made
The hair follicle is a small tube-shaped structure in the skin that produces hair. It extends from the skin's surface down through the dermis and sometimes into the subcutaneous fat layer below. Each follicle contains a cluster of actively dividing cells at its base called the hair matrix (or hair bulb).
Cells in the hair matrix divide rapidly, pushing older cells upward. As these cells are pushed further from the blood supply at the base, they fill with keratin, harden, and die — forming the hair shaft. The faster the cells divide, the faster the hair grows.
Connected to each follicle is a small muscle called the arrector pili. When this muscle contracts (triggered by cold or fright), it pulls the follicle upright and creates the small bump on the skin known as a "goosebump." Also attached to each follicle is a sebaceous (oil) gland that secretes an oily substance called sebum, which lubricates and waterproofs both the hair and the surrounding skin.
The Hair Growth Cycle
Hair does not grow continuously. Each follicle independently cycles through three distinct phases, meaning hairs across the scalp are at different stages at any given time. This is why hair falls out gradually rather than all at once.
- Anagen (growth phase): The active growing phase, during which the hair matrix cells divide rapidly and the hair shaft lengthens. For scalp hair, anagen typically lasts 2 to 7 years — which explains why head hair can grow very long, while eyebrow hair (anagen lasting only a few months) stays short. At any given moment, roughly 85 to 90% of scalp hairs are in anagen.
- Catagen (transition phase): A brief transitional period lasting about 2 to 3 weeks, during which growth stops and the lower follicle shrinks. Only about 1% of scalp hairs are in this phase at any moment.
- Telogen (resting phase): The follicle rests for about 3 months. The old hair remains in the follicle (a "club hair") until a new anagen hair growing beneath it pushes it out. About 10 to 15% of scalp hairs are in telogen at any time.
| Phase | Duration (scalp) | What happens | % of scalp hairs |
|---|---|---|---|
| Anagen (growth) | 2–7 years | Active cell division; hair shaft lengthens | ~85–90% |
| Catagen (transition) | 2–3 weeks | Growth stops; follicle shrinks | ~1% |
| Telogen (rest) | ~3 months | Follicle rests; old hair shed as new one grows | ~10–15% |
How Hair Colour Is Made
Hair colour is determined by melanin, a pigment produced by specialised cells called melanocytes located in the hair bulb at the base of the follicle. Melanocytes inject melanin granules into the hair cortex cells as they are formed, colouring the growing hair.
There are two main types of melanin involved in hair colour:
- Eumelanin — a dark pigment that produces brown and black shades. Higher concentrations produce darker hair.
- Pheomelanin — a reddish-yellow pigment that produces red and blonde shades.
The ratio and total quantity of these two pigments, controlled by genetics, determines your natural hair colour. Grey hair occurs when melanocytes in the follicle slow down and eventually stop producing melanin. The empty hair shaft appears white; a mixture of pigmented and white hairs looks grey to the eye. Genetics strongly influences when this process begins, though stress, illness, and nutritional deficiencies can also play a role.
Why Body Hair Differs From Scalp Hair
Hair is found across most of the body surface, but it varies enormously in thickness, length, colour, and function depending on its location. This variation is partly because different follicle types produce different hair types, and partly because the anagen phase differs across the body.
Eyebrows and eyelashes have short anagen phases, so they stay short. The fine, light-coloured hair that covers most of the body is called vellus hair. Coarser, usually pigmented hair found on the scalp, face (in adults), armpits, and pubic area is called terminal hair, and it develops in response to hormones (particularly androgens) during puberty.
The hair on the head has been widely theorised to provide insulation and some protection from ultraviolet radiation. Body hair may retain sensory function — hair follicles are richly supplied with sensory nerve endings, so hair can act as a sensitive touch detector, signalling movement or pressure on the skin.
Hair Health and Nutrition
Because hair grows from actively dividing cells, it is sensitive to nutritional status. Adequate protein intake is essential since hair is made of protein. Several minerals and vitamins are also associated with healthy hair growth in the scientific literature, including iron, zinc, and certain B vitamins like biotin — though it is important to note that supplements for hair growth are only likely to help if there is an underlying deficiency. Healthy, varied food choices remain the best foundation for hair and nail health.
Sudden or excessive hair shedding (telogen effluvium) can occur a few months after a significant physical or emotional stress — illness, surgery, rapid weight loss, or bereavement — because the stress can push a large number of follicles into the telogen phase simultaneously. Hair typically regrows normally once the underlying trigger has resolved, though this can take several months.
Hair loss that is patchy, persistent, or accompanied by other symptoms (scalp changes, itching, or scarring) has many possible causes — ranging from autoimmune conditions to hormonal changes — and deserves proper evaluation by a dermatologist or doctor rather than self-treatment. This guide is for educational purposes only and is not a substitute for professional medical assessment.
To learn about the closely related topic of nail biology, see the nail growth guide. You can also explore related terms in the anatomy glossary or use the biology revision tool to review what you have learned. More guides are available in the senses and skin category.