The Crown Nobody Sees Being Built: The Anatomy and Physiology of Scalp Hair

You wash it, cut it, cover it, style it, and sometimes worry over it. Yet most of us know almost nothing about what scalp hair actually is. A single strand is the visible end of one of the most active miniature organs in the human body. It builds itself, retires, rests, and rebuilds itself again for decades.

1. The ground it grows from: the scalp itself

The scalp is more than skin. Classical anatomy teaches it as five layers, remembered by the mnemonic S-C-A-L-P:

1. Skin, the epidermis and dermis, which contains the hair follicles and sebaceous glands
2. Connective tissue, a dense fibrous layer that is richly vascular
3. Aponeurosis, the galea aponeurotica, a tough sheet linking the frontalis and occipitalis muscles
4. Loose areolar tissue, the "danger layer" that lets the scalp slide over the skull
5. Pericranium, the membrane covering the skull bone

The first three layers are bound tightly together and move as one unit. That is why a scalp wound gapes and bleeds heavily. The vessels in the connective tissue layer are held open by the surrounding fibrous tissue and cannot constrict easily (Gray's Anatomy, scalp chapter).

Blood arrives from the supratrochlear, supraorbital, superficial temporal, posterior auricular, and occipital arteries. Sensation comes from branches of the trigeminal nerve at the front and the cervical spinal nerves at the back. This rich supply matters for hair. Follicles in the growth phase are among the most metabolically demanding structures in the skin.

2. How many hairs, and where do they come from?

An adult scalp carries roughly 100,000 hairs. Estimates run from about 80,000 to 150,000, and the number tends to be higher in fair-haired people and lower in redheads. Typical density in young adults is on the order of 200–300 follicles per cm².

Here is a fact that surprises many people: you are born with nearly all the follicles you will ever have. Follicle formation (morphogenesis) begins in the embryo and is largely complete by around the middle of gestation, with the scalp among the first regions to develop. After birth, the body does not make new follicles in any meaningful number. Hair density thins as the body grows, because the same follicles are spread over a larger surface (Paus & Cotsarelis, NEJM, 1999).

Follicles form through a conversation between two tissues. The epidermis and a cluster of dermal cells beneath it signal back and forth, using pathways such as Wnt/β-catenin, Sonic hedgehog, and BMP, until the epidermis folds downward into the dermis and the follicle is formed (Schneider, Schmidt-Ullrich & Paus, Current Biology, 2009).

3. The follicle: a living organ

The follicle is a tube of epithelium sunk into the skin at an angle, with a bulb at its base. It is conventionally divided into regions from top to bottom:

• Infundibulum: the funnel-shaped opening at the surface, extending down to the sebaceous gland duct
• Isthmus: the short middle segment, between the sebaceous duct and the attachment of the arrector pili muscle. This is where the bulge sits, a key stem-cell niche
• Inferior segment: the lower part that rebuilds and regresses with each cycle, ending in the hair bulb

The hair bulb

The bulb is the factory. At its centre sits the dermal papilla, a small cluster of specialized mesenchymal cells, richly supplied with capillaries. It wraps around the papilla like a cap over a thumb. Around the papilla lies the matrix, a population of epithelial cells that are among the fastest-dividing cells in the human body.

The dermal papilla is the conductor. Classic experiments showed that cultured dermal papilla cells, implanted into skin, can induce new follicle formation (Jahoda, Horne & Oliver, Nature, 1984). The papilla instructs and the matrix obeys.

Matrix cells divide, move upward, and differentiate into the concentric layers of the hair shaft and its supporting sheath. Melanocytes interspersed in the bulb donate pigment to the cells that become the shaft.

The sheaths

Around the growing hair lie protective and guiding layers:

• Inner root sheath: three layers (Henle's, Huxley's, and the sheath cuticle) that mould and guide the hair upward, then break down near the isthmus
• Outer root sheath: continuous with the surface epidermis, containing stem cells
• Connective tissue sheath: the surrounding dermal layer

The companions

Each follicle sits within a pilosebaceous unit:

• Sebaceous gland: secretes sebum, an oily mix that lubricates hair and skin. It is notably large on the scalp, which is why scalp hair can look greasy within a day
• Arrector pili muscle: a small smooth muscle that tilts the follicle upright in cold or fear (goosebumps), anchored to the bulge region

4. The strand itself

What we call "hair" is really the hair shaft, the dead, keratinized product of the follicle. It has three concentric layers:

Cuticle. The outer armour, made of overlapping flat cells stacked like roof tiles, typically five to ten layers deep. Each cell points toward the tip. The outer surface carries a thin, covalently bound lipid layer, notably 18-methyleicosanoic acid (18-MEA), which gives healthy hair its water-repelling slip. Chemical treatments and heat damage this layer first (Robbins, Chemical and Physical Behavior of Human Hair).

Cortex. The bulk of the strand and the source of its strength, colour, and elasticity. It is packed with keratin intermediate filaments embedded in a matrix of keratin-associated proteins, rich in the amino acid cystine. Cystine forms disulfide bonds, strong cross-links that give hair its toughness. Perming and relaxing work by breaking and re-forming these bonds. The cortex also holds melanin granules.

Medulla. A central core, sometimes continuous, sometimes fragmentary, sometimes absent, especially in fine hairs. Its function in humans is not well established.

Chemically, hair is about 65–95% protein by weight, with water, lipids, and trace minerals making up the rest. The proportions shift with humidity and with treatment.

Why hair is straight, wavy, or curly

The shape of the cross-section and the curvature of the follicle both matter. Straight hair tends toward a round cross-section from a relatively straight follicle. Wavy and curly hair tends toward oval or flattened sections from a curved follicle, with asymmetric growth in the bulb. The curl is largely programmed at the level of the bulb and not applied afterward to a straight shaft. Genetics governs a great deal of this. Textures vary across human populations, and so do growth rates, diameters, and densities, though individual variation within any group is large and broad generalizations deserve caution.

Colour

Hair colour comes from two melanin types made by follicular melanocytes: eumelanin (brown to black) and pheomelanin (red to yellow). The ratio and quantity determine shade. Pigment production is tied to the growth phase, and the melanocytes are renewed from a stem-cell reservoir.

Greying is now understood to involve the gradual loss of these melanocyte stem cells. In a landmark mouse study, Nishimura and colleagues showed that incomplete maintenance of melanocyte stem cells in the niche leads to loss of pigment-making cells (Science, 2005). The timing in humans varies widely with genetics and age.

5. The hair cycle: grow, regress, rest, shed

Hair does not grow continuously. Each follicle cycles through phases, independently of its neighbours. Humans have what researchers call a mosaic pattern, which is why we do not shed in seasonal waves as many animals do.

Phase	What happens	Approximate duration (scalp)
Anagen	Active growth. The matrix divides rapidly and the follicle is at its deepest and largest	Typically, 2–7 years
Catagen	Controlled regression. Matrix cells die by programmed cell death and the lower follicle shrinks	About 2–3 weeks
Telogen	Rest. A "club hair" is anchored while the follicle is dormant	About 3 months
Exogen	The old club hair is released and shed, often as new growth begins beneath	Variable

Roughly 85–90% of scalp hairs are in anagen at any time, around 10–15% in telogen, and only a small fraction in catagen. The long anagen phase is what allows scalp hair to reach great lengths. Growth averages about 0.3–0.4 mm per day, or about a centimetre a month, though this varies between individuals (Loussouarn, British Journal of Dermatology, 2001).

A person normally loses on the order of 50–100 hairs per day. This is not hair loss in the pathological sense. It is the cycle doing its job.

The cycle is not a simple clock. It is regulated by a balance of activating and inhibiting signals, including growth factors, Wnt, BMP, and others, interacting between the dermal papilla, the stem cells, and the surrounding tissue (Stenn & Paus, Physiological Reviews, 2001).

When the cycle is disrupted

• Telogen effluvium: a shock such as high fever, major surgery, severe stress, or childbirth pushes a large proportion of follicles into telogen at once. Heavy shedding follows a few months later. The classic description is Kligman's (Archives of Dermatology, 1961). It is typically reversible.
• Anagen effluvium: rapid loss from the interruption of active growth, as with chemotherapy.
• Pregnancy: many women notice fuller hair, as anagen is prolonged, followed by shedding after delivery.

6. Stem cells: the follicle's renewal system

How does a follicle rebuild itself every cycle, for decades? The answer lies in its stem cells.

In 1990, Cotsarelis, Sun and Lavker identified slow-cycling, label-retaining cells in the bulge region of the follicle, a hallmark of stem cells (Cell, 1990). Later work showed that bulge stem cells can regenerate the follicle and also contribute to repair of the surrounding epidermis after injury, though not to normal day-to-day maintenance of the epidermis (Ito et al., Nature Medicine, 2005).

At the start of each new anagen, the dermal papilla sends activating signals, and stem cells are recruited to form the new matrix. It is a beautifully self-contained system of renewal.

The follicle is also an immune-privileged site in its growth phase. It dampens certain immune signals, which protects its delicate cycling tissue. When that privilege collapses, as is believed to occur in alopecia areata, the immune system can attack the follicle (Paus, Nickoloff & Ito, Trends in Immunology, 2005).

7. Hormones and the scalp paradox

Hormones have a profound influence on hair, and the effects differ dramatically by body region.

Androgens drive the transition from fine vellus hair to coarse terminal hair at puberty in the beard, underarms, and pubic area. Yet in genetically predisposed scalp follicles, the same androgens, particularly dihydrotestosterone (DHT), shorten anagen and progressively miniaturize the follicle. This is the basis of androgenetic alopecia, or pattern hair loss. This opposite response in different body regions is sometimes called the "androgen paradox."

Two pieces of the historical evidence are worth knowing. Hamilton documented the patterns of male baldness in the mid-20th century, and his classification is the foundation of later grading scales (Annals of the New York Academy of Sciences, 1951). Imperato-McGinley and colleagues described men with an inherited deficiency of the enzyme 5α-reductase type 2, which converts testosterone to DHT, who did not develop male-pattern baldness (Science, 1974).

Thyroid hormones also matter. Both underactive and overactive thyroid states can change hair texture and cause diffuse shedding.

8. What hair is for

Human scalp hair has several plausible roles:

• Protection from ultraviolet radiation and mechanical injury
• Thermal regulation, insulating against both cold and sun
• Sensation, since follicles are wrapped in nerve endings and the movement of a single hair can be felt
• Social and psychological significance, which, across cultures and history, is enormous

Why humans retained dense scalp hair while losing most body hair remains debated. Hypotheses include heat protection of the head and a role in social signalling, but there is no consensus.

9. Takeaways

• The scalp has about 100,000 follicles, mostly formed before birth.
• Each follicle is an independent miniature organ with its own stem cells, blood supply, and glands.
• The visible strand is dead keratin, made in the bulb by some of the fastest-dividing cells in the body.
• Hair cycles through growth, regression, rest, and shedding, so daily shedding is normal.
• Hormones act differently on scalp follicles than on beard or body follicles.
• Hair thinning and shedding have many different causes, and the right response depends on which one it is. A dermatologist can tell them apart.

Sources and further reading

• Standring S (ed.). Gray's Anatomy: The Anatomical Basis of Clinical Practice. Elsevier. Scalp and integument chapters.
• Robbins CR. Chemical and Physical Behavior of Human Hair, 5th ed. Springer, 2012.
• Paus R, Cotsarelis G. "The Biology of Hair Follicles." New England Journal of Medicine 341:491–497, 1999.
• Schneider MR, Schmidt-Ullrich R, Paus R. "The Hair Follicle as a Dynamic Miniorgan." Current Biology 19:R132–R142, 2009.
• Stenn KS, Paus R. "Controls of Hair Follicle Cycling." Physiological Reviews 81:449–494, 2001.
• Cotsarelis G, Sun TT, Lavker RM. "Label-retaining cells reside in the bulge area of pilosebaceous unit." Cell 61:1329–1337, 1990.
• Ito M, et al. "Stem cells in the hair follicle bulge contribute to wound repair but not to homeostasis of the epidermis." Nature Medicine 11:1351–1354, 2005.
• Jahoda CAB, Horne KA, Oliver RF. "Induction of hair growth by implantation of cultured dermal papilla cells." Nature 311:560–562, 1984.
• Nishimura EK, Granter SR, Fisher DE. "Mechanisms of hair graying: incomplete melanocyte stem cell maintenance in the niche." Science 307:720–724, 2005.
• Paus R, Nickoloff BJ, Ito T. "A 'hairy' privilege." Trends in Immunology 26:32–40, 2005.
• Kligman AM. "Pathologic dynamics of human hair loss: I. Telogen effluvium." Archives of Dermatology 83:175–198, 1961.
• Hamilton JB. "Patterned loss of hair in man: types and incidence." Annals of the New York Academy of Sciences 53:708–728, 1951.
• Imperato-McGinley J, et al. "Steroid 5α-reductase deficiency in man." Science 186:1213–1215, 1974.
• Loussouarn G. "African hair growth parameters." British Journal of Dermatology 145:294–297, 2001.