Hair is a protein filament grown by follicles embedded in the skin, and its apparent simplicity hides a surprisingly dynamic organ system. Each strand is built from dead cells packed with a tough structural protein called keratin, but the follicle that produces it is one of the few structures in the human body that completely regenerates itself in repeating cycles throughout life. Color comes from pigment cells tucked inside the follicle, shape is determined by the follicle’s geometry and internal chemistry, and the whole process is influenced by hormones, immune signaling, circadian rhythms, and even the microbial community living on your scalp.
What a Hair Strand Is Made Of
A single strand of hair is essentially a tube of hardened keratin protein. Keratin belongs to a family of fibrous structural proteins that also show up in fingernails, hooves, and the outer layer of skin, but hair keratin has its own molecular arrangement. At the smallest scale, the protein chains coil into helical structures and bundle together into fine filaments roughly 60 angstroms in diameter. These filaments run parallel to each other along the length of the strand and are cemented together by a sulfur-rich amorphous material. The filaments provide tensile strength; the cement holds them in formation.
1Journal of Cell Biology. THE ELECTRON MICROSCOPY OF THE HUMAN HAIR FOLLICLE : PART 1. INTRODUCTION AND THE HAIR CORTEXZooming out slightly, a hair strand has three concentric layers. The cuticle is the outermost shell, made of overlapping flat cells that look like roof shingles under a microscope. Beneath that sits the cortex, which makes up the bulk of the strand and contains most of the keratin filaments along with pigment granules. Some thicker hairs also have a medulla, a loosely packed core in the center. The cuticle’s job is protective: when its overlapping scales lie flat, the strand looks glossy and resists moisture loss. When those scales get roughed up by chemical treatments or UV exposure, the cortex becomes vulnerable to damage.
The chemical bonds that hold this structure together matter for how hair behaves day to day. Disulfide bonds, which are covalent links between sulfur atoms in neighboring protein chains, give keratin much of its rigidity and resilience. Computational modeling of keratin proteins has shown that introducing disulfide cross-links increases the material’s strength by about 20% and its toughness by roughly half compared to the same protein without those bonds.
2PubMed. Structure and mechanical properties of human trichocyte keratin intermediate filament proteinThe Follicle and Its Command Center
The living part of a hair is not the strand you see but the follicle beneath the skin that manufactures it. At the base of each follicle sits a cluster of specialized cells called the dermal papilla. Think of the dermal papilla as the follicle’s project manager: it sends chemical signals to the surrounding stem and progenitor cells, telling them when to divide, how fast to grow, and what kind of hair to produce. The size of the dermal papilla directly influences hair thickness. Research has shown that a reduction in the number of cells in this structure is enough, on its own, to cause hair thinning and loss.
3PubMed Central. The dermal papilla: an instructive niche for epithelial stem and progenitor cells in development and regeneration of the hair follicleThe dermal papilla does not work alone. The follicle also houses dermal stem cells that replenish both the papilla itself and the surrounding sheath with each growth cycle. When researchers ablated these stem cells in mice, hair regrowth slowed and the type of hair produced changed, shifting from thick terminal hairs toward thinner ones.
4Developmental Cell. Hair Follicle Dermal Stem Cells Regenerate the Dermal Sheath, Repopulate the Dermal Papilla, and Specify Hair TypeHumans carry roughly two to three million hair follicles on the body, but the follicles on different body regions produce dramatically different hair. The fine, barely visible fuzz on your forearms is vellus hair, while the thick strands on your scalp are terminal hair. Both emerge from the same basic follicle architecture, and the dermal papilla’s signaling is a major factor in whether a follicle produces one type or the other. This distinction has evolutionary roots: humans still have the same follicle density as many other primates, but most of those follicles were dialed down to produce vellus hair as our ancestors adapted to rely on sweating for thermoregulation rather than a thick fur coat.
5British Journal of Dermatology. Not quite naked: the bare necessities of human body hair evolutionThe Growth Cycle
Hair does not grow continuously. Each follicle cycles through four phases independently of its neighbors, which is why you do not shed all your hair at once. The active growth phase, called anagen, is when the follicle is rapidly dividing and pushing the hair shaft outward. On the scalp, anagen typically lasts two to six years, which is why scalp hair can grow so long. When anagen ends, the follicle enters catagen, a brief transition period lasting a few weeks during which the lower part of the follicle shrinks and detaches from its blood supply. Then comes telogen, a resting phase that lasts a few months. Finally, exogen is the shedding phase, when the old strand falls out and the follicle gears up for a new anagen cycle.
6PubMed Central. Integrative and Mechanistic Approach to the Hair Growth Cycle and Hair LossAt any given time, about 9% of your scalp follicles are in the resting telogen phase, while the vast majority are actively growing. The balance between these phases determines how thick your hair looks overall. Many everyday insults, from hormonal shifts and nutritional deficiencies to chronic stress and poor sleep, can push follicles out of anagen and into telogen prematurely, resulting in increased shedding.
6PubMed Central. Integrative and Mechanistic Approach to the Hair Growth Cycle and Hair LossTelogen used to be thought of as a truly dormant phase, but more recent work has recast it as a period of active preparation. During telogen, the follicle suppresses molecular brakes on growth while ramping up pro-regenerative signals, setting the stage for the next round of anagen.
7PubMed Central. Resting no more: re-defining telogen, the maintenance stage of the hair growth cycleWhat Controls the Cycle
Among the many signaling pathways involved in hair growth, one stands out as especially central. Wnt/β-catenin signaling plays a key role in hair follicle regeneration and is required for hair follicle stem cells to proliferate and kick off a new growth cycle.
8PubMed Central. Targeting Wnt/β-Catenin Pathway for Developing Therapies for Hair Loss When researchers blocked this pathway in mouse skin, hair follicle stem cells stopped dividing, but the stem cells themselves were not destroyed. Once the block was removed, the follicles resumed proliferating, which suggests the pathway controls activation rather than survival of the stem cell population.
9PubMed Central. Distinct functions for Wnt/β-catenin in hair follicle stem cell proliferation and survival and interfollicular epidermal homeostasisThe growth cycle also has timing inputs that go beyond individual molecular pathways. Hair follicles contain their own circadian clock, oscillating on a roughly 24-hour cycle, and clock genes have been found to influence the much longer weeks-to-months cycle of hair growth. This is an unexpected connection: a daily timer helping to regulate a process that plays out over months.
10PubMed Central. Clock genes, hair growth and agingThere is also a seasonal rhythm layered on top. A study analyzing trichograms from healthy women found that the proportion of hairs in the resting phase peaked during summer, with a smaller secondary peak in spring and the lowest resting rates in late winter. In practical terms, you may notice slightly more hair shedding in late summer and early fall, not because something is wrong, but because your follicles are responding to an annual cycle.
11Dermatology. Seasonality of Hair Shedding in Healthy Women Complaining of Hair LossHow Hair Gets Its Color
Hair color is determined by pigment-producing cells called melanocytes that sit near the dermal papilla at the base of the follicle. These melanocytes manufacture melanin and transfer it into the keratin-producing cells as the hair shaft forms. The process involves a coordinated handoff between melanocytes, the surrounding keratinocytes, and the dermal papilla fibroblasts, all working together in what researchers call the hair follicle pigmentary unit.
12PubMed Central. Hair follicle pigmentationTwo types of melanin do most of the work. Eumelanin is a brown-to-black pigment, while pheomelanin is a reddish-yellow one. The ratio between them, along with the size and distribution of the melanin granules, produces the full range of natural hair colors. Very dark hair has a high concentration of eumelanin. Blond hair has much less of both types. Red hair has a relatively high proportion of pheomelanin. The biosynthesis of these pigments and their transfer into growing hair cells depend on a complex web of chemical precursors and signaling pathways.
13PubMed. Human hair pigmentation–biological aspectsOne thing worth noting: pigment is only deposited during anagen, the active growth phase. This means that each individual hair strand is colored as it is being built. The follicle does not retroactively add or remove pigment from hair that already exists above the skin.
Why Hair Goes Gray
Graying happens because the melanocyte stem cells that replenish the pigment-producing cells in each follicle gradually become depleted. With each growth cycle, the follicle needs a fresh supply of active melanocytes. When the reservoir of stem cells runs low, fewer melanocytes are available to inject pigment into the growing shaft, and the hair emerges lighter. Once the stem cell pool in a follicle is fully exhausted, graying in that follicle becomes largely irreversible.
14PubMed. The biology of human hair greyingOxidative damage appears to be a major driver of this process. It disrupts both the survival of melanocytes and the enzymatic machinery of melanin production itself. This is one reason why graying can be accelerated by factors that increase oxidative stress in the body, including chronic psychological stress, smoking, and certain nutritional deficiencies. The widespread belief that stress can turn your hair gray has a genuine biological basis, though the effect plays out over growth cycles rather than overnight.
14PubMed. The biology of human hair greyingWhat Makes Hair Curly, Straight, or Wavy
Hair shape is primarily a product of follicle geometry. A round follicle opening tends to produce straight hair; an oval or asymmetric one produces curly hair. The curvature is set while the strand is still being formed inside the follicle, as one side of the growing hair may keratinize (harden) slightly before the other, creating an internal asymmetry that causes the strand to bend as it exits. Researchers have organized the factors involved into tiers, examining the fiber’s curvature from the level of the follicle down to its internal chemistry.
15PubMed. Variation in human hair ultrastructure among three biogeographic populationsThe structural differences between hair types go deeper than just shape. A study comparing hair from European, African, and East Asian populations found statistically significant ancestry-related patterns in cross-sectional shape, cuticle dimensions, and the distribution of pigment granules within the strand.
15PubMed. Variation in human hair ultrastructure among three biogeographic populations Separate spectroscopic analysis has shown differences in the lipid composition of the outermost cuticle layer: African-type hair tends to have more lipids in a less ordered arrangement, while East Asian hair has very low levels of cuticle lipids overall.
16PubMed. Ethnic hair: Thermoanalytical and spectroscopic differencesThese differences have real practical consequences. Tightly coiled hair is more prone to mechanical breakage because the curl creates points of structural stress along the strand. It is also generally drier, partly because sebum from the scalp has a harder time traveling down a tightly coiled shaft and partly because of those cuticle lipid differences. Hair care practices that work well for straight hair can be damaging for coily hair, and vice versa.
Hair’s Surprising Mechanical Strength
Hair is tougher than it looks. A single strand has a tensile strength in the range of 150 to 270 megapascals, which puts it in the same neighborhood as some metals like copper wire of similar diameter.
17PubMed. Structure and mechanical behavior of human hair That strength is significantly affected by moisture: wet hair stretches more easily but is also weaker under tension. It also varies with how fast you pull. Under high strain rates, hair resists deformation more strongly, with a strain-rate sensitivity comparable to common synthetic polymers.
17PubMed. Structure and mechanical behavior of human hairWhen you stretch a hair past its normal elastic range, something interesting happens at the molecular level: the helical keratin structures begin to unwind and may transform into a flatter, sheet-like configuration. This structural change is what gives hair its remarkable ductility; you can stretch a healthy strand roughly 30% beyond its resting length before it breaks. The disulfide bonds in keratin act as molecular anchors that hold the overall architecture together during this deformation.
What Damages Hair
The two biggest environmental threats to hair are ultraviolet radiation and chemical treatments. UV radiation degrades both the proteins and the pigments in hair. UVB wavelengths are particularly damaging to the protein structure, causing measurable protein loss, while UVA radiation is more responsible for color changes like the lightening you notice after a summer spent outdoors.
18PubMed. Hair color changes and protein damage caused by ultraviolet radiation The protein-damaging effect of UVB has been measured at roughly two to five times greater than that of UVA plus visible light, depending on the hair type.
18PubMed. Hair color changes and protein damage caused by ultraviolet radiationUV damage happens at the surface first: the cuticle’s protective lipids and proteins undergo oxidation, which roughens the cuticle and exposes the cortex beneath to further harm.
19PubMed. Impact of ultraviolet radiation-induced damage to hair fiber integrity: A multi-technique physicochemical characterization of surface and cortex propertiesChemical bleaching works by a different mechanism but attacks the same vulnerable points. Bleach uses strong oxidizing agents to strip melanin from the cortex, but in the process it also breaks the disulfide bonds that give keratin its strength. Proteomic analysis of bleached hair has shown that the oxidation is concentrated in the cortical intermediate filaments, the most abundant structural proteins, and primarily targets the sulfur-containing amino acids that form those critical disulfide cross-links.
20PubMed. The physical and chemical disruption of human hair after bleaching – studies by transmission electron microscopy and redox proteomics This is why heavily bleached hair feels straw-like and breaks easily: the very bonds that gave it tensile strength have been chemically destroyed.
Hair as a Touch Sensor
Hair follicles are not just factories for producing fiber. They are also sensory organs. Each follicle on hairy skin is surrounded by a network of nerve endings that detect the slightest deflection of the hair shaft, which is why you can feel a light breeze across your arm even though the hair itself has no nerve supply. Different types of nerve fibers wrap around follicles, each tuned to different kinds of touch: some respond to fast, fleeting contact, while others detect sustained pressure. The result is that each hair follicle acts as its own mechanosensory unit.
21Neuron. The cellular and molecular basis of touchRecent work has added an unexpected layer to this picture. Researchers found that the outer root sheath cells of the hair follicle are not passive bystanders but actively release signaling molecules, including serotonin, histamine, and ATP, when the follicle is mechanically stimulated. These molecules then activate the surrounding sensory neurons. In other words, the follicle itself is acting as a signal amplifier, converting mechanical force into chemical signals before the nerve endings even fire.
22PubMed Central. Mechanical stimulation of human hair follicle outer root sheath cultures activates adjacent sensory neuronsHow Hair Loss Happens
The most common form of hair loss, androgenetic alopecia, affects both men and women and is driven by androgens acting on genetically susceptible follicles. The process is one of progressive miniaturization: over successive growth cycles, affected follicles produce thinner and shorter hairs until the visible strand is barely there.
23PubMed. Androgenetic alopecia: a review The dermal papilla shrinks, the anagen phase shortens, and the follicle essentially shifts from producing terminal hair to producing vellus-like hair. Genetics determine which follicles are vulnerable and how early the process begins, but hormonal levels modulate the speed.
Stress-related hair loss operates through a different path. In a mouse model, psychoemotional stress was shown to prematurely terminate anagen, pushing follicles into catagen and then telogen well ahead of schedule. The mechanism involves neuropeptide signaling and inflammation around the follicle.
24PubMed Central. Stress inhibits hair growth in mice by induction of premature catagen development and deleterious perifollicular inflammatory events via neuropeptide substance P-dependent pathways In humans, this manifests as telogen effluvium: a noticeable increase in shedding a few months after a stressful event, surgery, illness, or nutritional shock. Because the shedding is delayed, people often do not connect it to the original trigger.
Alopecia areata is yet another mechanism. Here, the follicle’s immune privilege collapses, meaning the immune system, which normally ignores the hair follicle, suddenly attacks it. The follicle is one of a handful of body sites that are normally shielded from immune surveillance, and when that shield fails, the result is patchy or total hair loss.
25PubMed Central. Immune Privilege Collapse and Alopecia Development: Is Stress a Factor26PubMed Central. Immunology of alopecia areata The encouraging news is that because the follicle itself often remains intact, hair regrowth is possible if the immune attack can be suppressed. JAK inhibitors, a class of drugs originally developed for other autoimmune conditions, have shown the ability to reverse hair loss in moderate to severe alopecia areata.
27PubMed Central. JAK Inhibitors for Treatment of Alopecia AreataThe Scalp Microbiome
Your scalp hosts a thriving community of bacteria, fungi, and other microorganisms that interact with hair follicles in ways researchers are only starting to map. These microbes influence sebum production and local pH, creating a feedback loop: the microbial community shapes the follicular environment, and that environment, in turn, determines which microbes thrive.
28The Microbe. Unlocking the secrets of the hair microbiome: From scalp health to therapeutic advances The scalp microbiome also plays roles in maintaining the skin barrier and modulating local immune responses.
29PubMed Central. The Scalp Microbiome-Hair Axis: Mechanisms and Therapeutic TranslationDisruptions to this microbial balance are associated with common scalp conditions like dandruff and seborrheic dermatitis, and there is growing interest in whether microbial shifts might influence the hair growth cycle itself. The field is still young, but the direction of research suggests that future hair-care and hair-loss treatments may involve managing the microbiome alongside traditional approaches.
Hair as a Forensic and Diagnostic Record
Because hair grows at a relatively steady rate and locks substances into its keratin matrix as it forms, a strand of hair works like a biological timeline. Forensic toxicologists routinely analyze hair to detect psychoactive substances, environmental contaminants, doping agents, and toxins, with different segments of the strand corresponding to different windows of exposure.
30WIREs Forensic Science. Hair Testing in Forensic Toxicology: Recent Insights From Root to Tip Unlike blood or urine, which capture a snapshot of the last hours or days, a long hair strand can offer a record stretching back months.
31PubMed. Hair as a biological indicator of drug use, drug abuse or chronic exposure to environmental toxicantsHair drug testing is now used widely in workplace screening, child custody proceedings, and criminal investigations. The basic principle is straightforward: as blood circulates through the follicle during active growth, trace amounts of whatever is in the bloodstream get incorporated into the forming strand. Sectioning the hair at intervals and analyzing each segment can reconstruct a rough chronological profile of drug use or toxic exposure. The method is not without complications, since external contamination, cosmetic treatments, and differences in melanin content can all influence results, but it remains one of the most useful long-range biological indicators available.