Human hair is built almost entirely from a tough, fibrous protein called keratin, the same family of proteins that makes up your fingernails. Keratin’s strength comes from an abundance of the amino acid cysteine, whose sulfur atoms link neighboring protein chains together through chemical bridges that give each strand its resilience. But keratin alone does not explain everything about the feel, look, and behavior of your hair. A thin coat of specialized lipids on the surface, trace metals locked inside the shaft, pigment granules scattered through the core, and a surprising amount of absorbed water all contribute to making a single hair strand a remarkably complex biological fiber.
Keratin, Sulfur Bridges, and the Chemistry That Holds Hair Together
Keratin accounts for roughly 65 to 95 percent of hair’s dry weight, depending on how you measure it. What makes hair keratin special is its unusually high cysteine content. Cysteine residues on adjacent keratin chains react with each other to form disulfide bonds, essentially tiny sulfur bridges that cross-link the protein network into a rigid yet flexible mesh.1Advances in Redox Research. Human hair keratin responds to oxidative stress via reactive sulfur and supersulfides These disulfide bonds are the reason your hair can hold its shape after being twisted, braided, or curled, and they are also the bonds that chemical straighteners and perms deliberately break and reform.
Beyond protein, the outer surface of every hair fiber is coated in a thin layer of a specific fatty acid called 18-methyleicosanoic acid, usually abbreviated 18-MEA. This lipid is chemically bonded to the outermost cuticle cells and creates a hydrophobic barrier, essentially a waxy raincoat for each strand.2PubMed. The role of 18-methyleicosanoic acid in the structure and formation of mammalian hair fibres When 18-MEA is stripped away by harsh shampooing, bleaching, or weathering, the fiber becomes hydrophilic and rough. That shift explains the straw-like texture of heavily processed hair and is the target of many conditioning products that try to replicate 18-MEA’s protective function.3Cosmetics. Degradation of Hair Surface: Importance of 18-MEA and Epicuticle
Hair also holds a meaningful amount of water. Dry hair typically contains around 10 to 15 percent water by weight at normal indoor humidity. Research into how hair absorbs moisture has identified at least two transitions as humidity rises: one around 30 percent relative humidity, when the internal structure begins opening up to accommodate water molecules, and another around 60 to 70 percent, which corresponds to a glass-transition-like softening of the hair’s protein matrix.4PubMed. Learning from hair moisture sorption and hysteresis This is why your hair feels and behaves differently on a humid day: it has literally absorbed extra water into its internal framework, changing its stiffness and volume.
Layers of the Hair Shaft
If you could slice a single hair crosswise and look at it under a powerful microscope, you would see that it is not a uniform rod. It has distinct concentric layers, each with a different job.
The outermost layer, the cuticle, consists of flat, overlapping cells arranged like roof tiles or fish scales, all pointing from root to tip. This architecture makes the surface smooth in one direction and rougher in the other, which is why running your fingers from root to tip feels slicker than going the other way. The cuticle is itself a multilayered sandwich: an outer epicuticle, a dense A-layer rich in disulfide-bonded protein, an exocuticle, and a softer endocuticle. Nanoscale chemical mapping has confirmed that lipid, protein, and cystine concentrations vary distinctly across these sublayers.5PubMed Central. Nanoscale Molecular Characterization of Hair Cuticle Cells Using Integrated Atomic Force Microscopy-Infrared Laser Spectroscopy The cuticle is what gives hair its shine and its first line of defense against friction and environmental damage.
Beneath the cuticle sits the cortex, the thickest layer and the source of most of hair’s mechanical strength and color. The cortex is made up of elongated cortical cells packed with macrofibrils, which are bundles of keratin intermediate filaments embedded in a globular protein matrix. These filaments are arranged in a double-twist pattern, with a central filament surrounded by concentric rings of angled filaments, a design that distributes stress efficiently.6PubMed. Three-dimensional architecture of macrofibrils in the human scalp hair cortex The macrofibrils are held together by keratin-associated proteins that fill the gaps between filaments, and the matrix fraction is consistently about 40 percent of each macrofibril’s volume.6PubMed. Three-dimensional architecture of macrofibrils in the human scalp hair cortex
At the center of thicker hairs sits the medulla, a loosely organized channel that can be continuous, fragmented, or completely absent. Research on human scalp hair found that all terminal hairs were medullated for the majority of their length, but the medulla’s size fluctuated dramatically with the growth cycle. It was largest early in the active growth phase, occupying about 26 percent of the hair shaft’s minor axis and roughly 7 percent of the cross-sectional area, then shrank to virtually nothing by the end of growth.7British Journal of Dermatology. The size and form of the medulla of human scalp hair is regulated by the hair cycle and cross‐sectional size of the hair shaft Fine vellus hairs and many body hairs lack a medulla entirely. Its precise function remains unclear, though it may contribute to insulation in other mammals.
Inside the Follicle
Every hair strand originates from a follicle, a tiny organ embedded in the skin that functions as both factory and control center. At the base of each follicle sits the dermal papilla, a cluster of specialized cells that acts as the master regulator of hair size. The volume of the dermal papilla directly correlates with how thick the resulting hair fiber will be. That volume depends on two factors: the number of cells in the papilla and the amount of extracellular matrix each cell produces. In androgen-sensitive follicles, like those in the male beard, both cell number and matrix volume increase, which is why beard hairs are coarser than scalp hairs.8PubMed. Differences in hair follicle dermal papilla volume are due to extracellular matrix volume and cell number: implications for the control of hair follicle size and androgen responses
Higher up along the follicle is a region called the bulge, which houses a reservoir of slow-cycling stem cells. These cells can remain dormant for remarkably long periods. In mouse studies, bulge cells retained identifying labels for up to 14 months, essentially the entire lifespan of a mouse, highlighting just how quiescent they can be.9PubMed Central. Bulge Region as a Putative Hair Follicle Stem Cells Niche: A Brief Review When a new growth cycle begins, signals from the dermal papilla activate these stem cells, which divide and migrate downward to regenerate the lower follicle and produce a new hair shaft. This regenerative capacity is why follicles can produce hair for decades, even after the previous strand has fallen out.
The Growth Cycle
A single hair does not grow indefinitely. Instead, each follicle cycles independently through phases of growth, regression, rest, and shedding. On a healthy scalp, neighboring follicles are usually in different phases at any given time, which is why you shed individual hairs daily rather than losing them all at once.
The active growth phase, anagen, is the longest and most metabolically intense. During anagen, rapidly dividing cells in the hair bulb produce the keratin-rich shaft at a steady pace.10PubMed Central. Integrative and Mechanistic Approach to the Hair Growth Cycle and Hair Loss Scalp hair typically remains in anagen for two to six years, which is what determines its maximum length. Growth rates vary by body site and ethnicity; body hair growth rates range from roughly 180 to 485 micrometers per day, and the percentage of scalp hairs in anagen at any one time exceeds 85 percent in some populations.11PubMed. Exploring some characteristics (density, anagen ratio, growth rate) of human body hairs
When anagen ends, the follicle enters catagen, a brief regression phase lasting about two to three weeks. During catagen, the lower portion of the follicle undergoes controlled self-destruction through programmed cell death. Apoptotic cells appear not only in the regressing lower follicle but also in the inner root sheath and around the bulge region.12PubMed Central. Analysis of apoptosis during hair follicle regression (catagen) The follicle shrinks dramatically, pulling the base of the hair upward until only a small club-shaped root remains anchored near the bulge.
The resting phase, telogen, follows catagen and lasts roughly two to four months on the scalp. The hair is no longer growing but remains loosely anchored. For a long time, researchers assumed that hair shedding was simply part of this resting phase, but evidence from mouse models demonstrated that shedding is actually a separate, actively regulated phase called exogen. During exogen, specific proteolytic enzymes dissolve the attachments holding the old club hair in place, and the strand falls out with a characteristic scalloped base visible under electron microscopy.13PubMed. Exogen, shedding phase of the hair growth cycle: characterization of a mouse model The recognition that exogen is an independently controlled event has implications for understanding normal daily shedding as well as excessive hair loss conditions.14PubMed. From telogen to exogen: mechanisms underlying formation and subsequent loss of the hair club fiber
What Drives the Cycle
The transitions between growth phases are orchestrated by an interplay of signaling pathways rather than a single on-off switch. Among these, the Wnt/β-catenin pathway plays a central role in stimulating follicle development, maintaining growth, and enabling regeneration.15PubMed Central. The Molecular Mechanism of Natural Products Activating Wnt/β-Catenin Signaling Pathway for Improving Hair Loss Other pathways, including Bmp, Shh, Notch, and specific combinations of transcription factors, act at different stages, some promoting growth and others triggering regression.16PubMed Central. Hairy tale of signaling in hair follicle development and cycling Catagen itself involves the Edar signaling pathway: when Edar signaling is disrupted in mice, catagen is accelerated and apoptosis in the outer root sheath increases, while the anti-apoptosis protein XIAP drops.17PubMed Central. Involvement of the Edar signaling in the control of hair follicle involution (catagen) The practical upshot is that hair loss treatments, whether pharmaceutical or experimental, are almost always trying to nudge one or more of these signaling cascades in order to extend anagen or delay the onset of catagen.
Where Hair Color Comes From
Hair gets its color from melanin, specifically from two types produced by melanocyte cells nestled in the hair bulb. Black and dark brown shades come predominantly from eumelanin, while reddish and yellowish tones arise from pheomelanin. Most hair contains at least a trace of both types, but the ratio differs dramatically. Black hair is loaded with eumelanin and has minimal pheomelanin. Blond hair has much less eumelanin but a similar baseline of pheomelanin. Red hair is the exception: it contains roughly equal levels of the two pigments, which is chemically unusual and explains its distinctive hue.18PubMed. Diversity of human hair pigmentation as studied by chemical analysis of eumelanin and pheomelanin
The melanocytes that supply pigment are themselves replenished from a stem cell pool housed in the follicle’s bulge region. Research using transgenic mice and human follicle samples showed that graying is caused by the gradual depletion of these melanocyte stem cells.19PubMed. Mechanisms of hair graying: incomplete melanocyte stem cell maintenance in the niche As the stem cells fail to self-renew with each cycle, fewer melanocytes populate the bulb, and the new hair grows in without pigment. Oxidative stress and reactive oxygen species accelerate this depletion, and impaired antioxidant defenses in the follicle make the process worse.20PubMed Central. Aging of the hair follicle pigmentation system The timeline is heavily genetic, but the underlying mechanism is the same regardless of ethnicity: fewer melanocyte stem cells means less pigment means gray or white hair.
What Determines Straight, Wavy, or Curly
Hair shape is not controlled by a single gene. A growing list of genetic variants is being linked to differences in curliness, with researchers focusing on how these genes affect the internal asymmetry of the growing fiber.21PubMed Central. Changing human hair fibre colour and shape from the follicle A genome-wide association study comparing people with very straight versus very curly hair in South Africa found strong associations with variation in trichohyalin (a structural protein in the inner root sheath), a copper transporter protein called CUTC, and the inner root sheath component keratin 74.22PubMed. The biology and genetics of curly hair
The emerging picture is that curl forms inside the follicle, not after the hair exits the skin. If one side of the growing fiber hardens faster than the other, or if the follicle itself is curved rather than straight, the strand exits in a coiled shape. The cross-sectional shape of curly hair is typically more elliptical than that of straight hair, which is more circular. These differences are set during development and maintained by the protein composition of the inner root sheath, which acts as a kind of mold for the growing shaft.
Mechanical Strength and Elasticity
Despite being dead tissue from the moment it exits the scalp, hair is remarkably strong for its diameter. Tensile strength measurements place it in the range of about 150 to 270 megapascals, which means a single strand can support a significant load before snapping.23PubMed. Structure and mechanical behavior of human hair That strength depends on humidity and how fast you pull: hair is stiffer when dry and more pliable when wet.
The elasticity of hair is particularly interesting. A dry strand can stretch 20 to 30 percent of its original length before breaking, and wet hair can stretch up to about 50 percent.24PubMed Central. Effect of Oil Application, Age, Diet, and Pigmentation on the Tensile Strength and Breaking Point of Hair This stretchiness comes from the alpha-helical structure of keratin inside the cortex. When you pull on a strand, the coiled alpha-helices begin to unwind and can transform into a flatter beta-sheet configuration, absorbing energy before the fiber fails.23PubMed. Structure and mechanical behavior of human hair When the tension is released before the strand breaks, some of the uncoiling reverses and the hair springs partway back. This is the same mechanism that makes wool stretchy, since wool is also an alpha-keratin fiber.
How Chemical and Heat Treatments Cause Damage
Understanding hair’s structure makes it clear why chemical and heat treatments can cause irreversible harm. Bleaching, which uses alkaline hydrogen peroxide to oxidize melanin, is among the most destructive common treatments. Under electron microscopy, bleached hair shows cuticle scales that have separated, peeled off, and in severe cases been stripped away entirely, exposing the cortex underneath. Inside the cortex, melanin granules dissolve and leave behind visible holes and pores.25PubMed Central. Effects of excessive bleaching on hair: comparative analysis of external morphology and internal microstructure The disulfide bonds that hold keratin chains together are also oxidized and broken during bleaching, weakening the fiber’s structural integrity.26PubMed. The physical and chemical disruption of human hair after bleaching – studies by transmission electron microscopy and redox proteomics
Combining treatments compounds the damage. Hair that has been both bleached and acid-straightened suffers more deterioration from heat styling than hair that has undergone only one treatment, because the protective surface components, including 18-MEA and the epicuticle, have already been compromised.27PubMed Central. Thermal Induced Changes in Cuticle and Cortex to Chemically Treated Hair Once the cuticle is gone, every subsequent insult acts directly on the cortex, which is why heavily processed hair breaks more easily and feels progressively rougher over time. Since hair above the scalp is dead and cannot repair itself, the only real fix for severe damage is growing new hair and cutting the damaged portions away.
An Evolutionary Perspective on Keratin Diversity
Hair is a defining mammalian trait, and the keratin-associated proteins that fill the matrix between intermediate filaments have evolved rapidly across species. An analysis of the keratin-associated protein gene family across mammals found that most subfamilies appeared early in mammalian evolution, meaning the ancestral mammal likely already had a diverse toolkit for building hair. Within each species, however, these gene subfamilies have been homogenized through a process of concerted evolution, while between species they have diverged quickly.28PubMed Central. Molecular evolution of the keratin associated protein gene family in mammals, role in the evolution of mammalian hair This pattern helps explain why mammalian hair varies so enormously in texture, thickness, and density across species: the matrix proteins that fine-tune hair properties have been under strong selective pressure and have diverged in step with each lineage’s ecological needs.
Hair as a Biological Record
Because hair is metabolically inert once it leaves the follicle, it preserves a chemical snapshot of what was circulating in your blood at the time that segment was produced. As the hair grows at a relatively steady rate, different segments correspond to different time windows, creating a timeline that can stretch back months or even years depending on the strand’s length. This makes hair a valuable specimen in forensic toxicology, where it is used to detect drugs, environmental toxicants, and heavy metals over extended periods, something a blood or urine test, which captures only recent exposure, cannot do.29PubMed. Hair as a biological indicator of drug use, drug abuse or chronic exposure to environmental toxicants
Beyond forensics, hair analysis is increasingly being explored in clinical medicine. Its stability, non-invasive collection, and ability to provide what researchers describe as a “fossilized” record of a person’s health make it appealing for monitoring chronic nutritional deficiencies, hormone levels, and environmental exposures over time.30PubMed Central. Human hair as a diagnostic tool in medicine The field has limitations: external contamination from hair products, water, and environmental pollutants can complicate results, and standardized reference ranges are still lacking for many analytes. But the basic principle, that your hair quietly logs what passes through your bloodstream, gives it a diagnostic potential that no other readily accessible tissue quite matches.