How Is Hair Formed? The Science Behind Each Strand

Every strand of hair on your body is built inside a tiny, tunnel-shaped organ called a hair follicle, where a cluster of specialized cells at the base orchestrates the production of a protein fiber that is surprisingly strong and structurally complex. The process involves back-and-forth chemical signaling between skin layers, a dedicated pool of stem cells, pigment-producing melanocytes, and a growth cycle that repeats for years. What seems like dead material by the time it pokes through your scalp is actually the product of one of the most dynamic regenerative systems in the human body.

How a Hair Follicle Forms Before You Are Born

Hair follicles are not assembled after birth. They develop during embryonic life through a stepwise conversation between two layers of developing skin: the outer epithelial layer (which becomes the epidermis) and the underlying mesenchymal layer (which becomes the dermis). Signals pass back and forth between these layers, prompting small clusters of cells to thicken into structures called placodes on the surface and condensates in the dermis beneath them.¹ These paired structures then grow downward together into the skin, elongating through a series of stages until a fully formed follicle with all its compartments is in place.²

Interestingly, early human and mouse hair follicles are broadly comparable in their size, signaling activity, and gene expression, which is why mouse models have been useful for studying human hair development.³ By the time a baby is born, the total number of follicles is essentially fixed. Adults do not grow new follicles under normal circumstances, which is why understanding embryonic follicle formation matters so much for anyone trying to reverse hair loss later in life.

The Dermal Papilla Controls Almost Everything

At the very bottom of each hair follicle sits a small ball of specialized cells called the dermal papilla. If the follicle is the factory, the dermal papilla is the floor manager. It sends chemical signals to the surrounding epithelial cells, telling them when to divide, what type of hair to produce, how thick to make it, and even what color it should be.⁴ During embryonic development, the dermal papilla’s precursor cells are what initiate follicle formation in the first place. After birth, they continue regulating each new growth cycle for the life of the follicle.⁵

The size of the dermal papilla largely determines the size of the hair it produces. A study of 235 hair follicles from different body sites found a strong correlation between dermal papilla volume and the cross-sectional area of the resulting hair shaft. That volume depends on both the number of cells in the papilla and the amount of extracellular material around them.⁶ A thick beard hair has a large, cell-dense dermal papilla; a fine vellus hair on the cheek has a tiny one. This relationship becomes central to understanding pattern hair loss, where the papilla shrinks and the hairs it produces get progressively thinner.

What a Strand of Hair Is Actually Made Of

Once cells leave the dermal papilla region and begin moving upward through the follicle, they undergo a dramatic transformation. They fill up with a tough structural protein called keratin, lose their nuclei, and die. By the time hair emerges from the skin surface, it is technically dead tissue, but it retains an intricate architecture.

A single hair has three concentric layers. The outermost is the cuticle, made of flat, overlapping cells arranged like roof shingles. Each cuticle cell is roughly half a micrometer thick and stacked at intervals of about six to seven micrometers.⁷ Beneath the cuticle lies the cortex, which makes up the bulk of the hair fiber. The cortex consists of spindle-shaped cells packed with keratin filaments running parallel to the length of the hair. These cells are typically one to six micrometers wide and up to a hundred micrometers long. A glue-like substance called the cell membrane complex holds everything together.

The mechanical toughness of hair comes largely from disulfide bonds, which are chemical cross-links between sulfur-containing amino acids in the keratin chains. Computational modeling of keratin structures has shown that adding disulfide cross-links increases strength by about 20% and toughness by roughly 49% compared to keratin without them.⁸ These bonds also behave differently depending on whether hair is wet or dry. Under wet conditions, the bonds in different regions of the keratin structure become more accessible to disruption, which is why wet hair stretches more easily and why chemical treatments like perms work by breaking and reforming these bonds.⁹

The Growth Cycle That Repeats for Years

Hair does not grow continuously. Each follicle cycles through three main phases, and neighboring follicles operate on independent clocks. The active growth phase, called anagen, is when the cells around the dermal papilla divide rapidly and push new hair material upward. On the scalp, anagen lasts roughly two to six years, which is why scalp hair can grow so long. Elsewhere on the body, anagen is much shorter, explaining why arm hair or eyebrow hair stays relatively brief.

When anagen ends, the follicle enters catagen, a short transitional phase lasting a couple of weeks. During catagen, cell division stops, the lower portion of the follicle shrinks, and the hair shaft detaches from the dermal papilla. This phase involves programmed cell death in the lower follicle.¹⁰

The follicle then enters telogen, traditionally called the “resting” phase. But telogen is more active than its name suggests. Research has divided it into two sub-stages: an early “refractory” phase during which the follicle resists signals to start growing again, and a later “competent” phase during which it becomes responsive to those signals. This distinction matters because it helps explain why hair loss treatments that try to push follicles into growth don’t always work on every follicle at the same time. The old hair eventually falls out, and a new anagen cycle begins, regenerating the lower portion of the follicle from scratch.

Stem Cells Keep the Whole System Running

The reason a follicle can rebuild itself cycle after cycle lies in a reservoir of stem cells located in a region called the bulge, which sits partway up the outer root sheath of the follicle. These stem cells are slow-dividing under normal conditions, conserving their regenerative potential over a lifetime.¹¹ When a new growth cycle kicks off, signals from the dermal papilla activate bulge stem cells, which then migrate downward and differentiate into all the cell types needed to produce a new hair shaft and its surrounding structures.¹²

Bulge stem cells also interact with other specialized cells in their neighborhood, acting as a signaling hub that influences not just hair growth but aspects of skin maintenance as well.¹³ The health and quantity of these stem cells is a limiting factor for hair regeneration. When the stem cell pool is depleted or the connection between the bulge and surrounding structures is disrupted, the follicle can lose its ability to produce a proper hair, which is a key piece of the puzzle in permanent hair loss.

How Hair Gets Its Color

Hair color comes from pigment produced by melanocytes, which are specialized cells that sit near the dermal papilla during the growth phase. Melanocytes manufacture granules of a polymer called melanin and transfer them into the keratinocytes that are assembling the hair shaft. The amount, type, and packaging of melanin determine the final shade. Two broad forms of melanin exist: eumelanin, which produces brown and black tones, and pheomelanin, which produces red and yellow tones. Most people’s hair contains some combination of both.¹⁴

The genetics of hair color are complex. While dozens of genes contribute, one of the best-characterized is the melanocortin 1 receptor gene, MC1R, which has been linked to red hair, freckling, and sun sensitivity. Beyond the melanocytes themselves, the surrounding cells also play a role: research has shown that a transcription factor called Foxn1 helps mark which cortex cells are designated to receive pigment from melanocytes, adding another layer of control to the process.¹⁵

Why Hair Is Straight, Wavy, or Curly

Hair shape is one of the most visible differences between people, and the explanation is less straightforward than you might expect. The final shape of a hair strand is set during its formation inside the follicle, driven by a combination of genetic factors, growth signals, and the internal architecture of the hair shaft itself.¹⁶

The cortex of a hair strand contains two major types of cells, often called para-type and ortho-type, which differ in how their internal keratin filaments are arranged and how densely cross-linked their surrounding matrix proteins are. When these two cell types are unevenly distributed across the cross-section of the hair, the strand naturally bends as it hardens. The more asymmetric the distribution, the curlier the hair. An elliptical or flat cross-section amplifies this bending, though it is not the primary cause. The bending is triggered as the hair shaft solidifies in the upper follicle, in a zone called the transition region.

A separate but complementary model proposes that the shape of the dermal papilla itself, and whether it is partially divided within a single follicle, can drive differential growth rates on opposite sides of the hair, producing both bending and twisting.¹⁷ At the genetic level, genome-wide association studies have identified links between hair curliness and variants in genes including trichohyalin, a copper transporter called CUTC, and inner root sheath keratin 74.¹⁸ Comparisons between populations with very different hair textures have found marked differences in allele frequencies for genes in the keratin 71-74 gene cluster.¹⁹

What Sebaceous Glands Add to the Picture

Attached to each hair follicle is a sebaceous gland, a small sac-like structure that produces sebum, a complex mixture of lipids. Human sebum contains compounds not found elsewhere in the body, including squalene and wax esters.²⁰ Sebum travels up the follicle and coats the emerging hair shaft and surrounding skin, providing lubrication, some degree of waterproofing, and antimicrobial protection.²¹

This is why freshly washed hair feels different from hair that hasn’t been washed in a day or two. The sebum coating is stripped away by shampoo surfactants, then gradually replenishes. People with overactive sebaceous glands have oilier hair; those with underactive glands tend toward dry, brittle strands. The sebaceous gland is technically part of the follicle unit, and its activity is influenced by hormones, particularly androgens, which is why scalp oiliness often increases during puberty.

How Stress Gets Under the Skin

The connection between stress and hair loss is not just folk wisdom. Research in mice has shown that psychological stress prematurely terminates the active growth phase of hair follicles, pushing them into catagen ahead of schedule. The mechanism involves a neuropeptide called substance P, which is released by nerve endings around the follicle during stress. Substance P triggers inflammation around the follicle, increases programmed cell death in follicle cells, and suppresses cell division in the growing hair.²² When researchers administered a substance P blocker to stressed mice, most of these growth-inhibitory effects were counteracted. Nerve growth factor is also recruited as a key mediator, and together these molecules create a cascade that effectively tells the follicle to shut down early.²³

In humans, the clinical equivalent is telogen effluvium, a condition where large numbers of follicles are pushed into the resting phase simultaneously, causing diffuse shedding a few months after a major stressor. The shedding is usually temporary because the follicles themselves are not destroyed, just prematurely cycled. But chronic stress could compound this, keeping follicles in shortened growth phases over time.

Why Hair Thins and Turns Gray

Pattern hair loss, the most common form of balding, follows directly from changes in the dermal papilla. In androgenetic alopecia, the papilla progressively loses cells and shrinks, so the hairs it produces become finer and shorter with each successive growth cycle. Terminal hairs, the thick pigmented strands, gradually transform into vellus-like hairs, which are nearly invisible.²⁴ One reason this process is hard to reverse is that the attachment between the bulge stem cells and a small muscle called the arrector pili muscle is lost during miniaturization. Without that structural anchor, the stem cell niche is compromised in a way that other forms of hair loss, like alopecia areata, do not produce.²⁵

Recent research has added another layer to this picture. In miniaturized follicles from balding scalps, a cellular recycling process called autophagy appears to be severely impaired, and this impairment is accompanied by increased cell death and premature entry into catagen.²⁶ When researchers blocked autophagy in organ-cultured follicles, hair growth slowed and premature regression occurred, suggesting this could be a contributing mechanism worth targeting therapeutically.

Greying, meanwhile, is a separate process from thinning, though both accelerate with age. Hair turns gray because the melanocyte stem cells in the bulge region gradually deplete. Initially, individual follicles may produce hair with patchy or reduced pigment. Eventually, the melanocyte stem cell pool becomes so depleted that the follicle can no longer produce pigmented melanocytes at all, and the hair grows in white.²⁷ Studies of grey and white hairs have confirmed that this greying reflects a specific depletion of melanocytes in both the bulb, where pigment is actively produced, and the outer root sheath.²⁸ Once the stem cell pool is gone, greying at that follicle is essentially permanent.

How Chemical Treatments Damage the Structure

Understanding what hair is made of makes it easier to understand what dyeing and perming actually do to it. Both processes rely on chemicals that penetrate the cuticle layer and act on the cortex beneath. Hydrogen peroxide, a standard ingredient in both permanent dyes and bleaches, breaks disulfide bonds and alters protein conformation deep inside the fiber. The cuticle scales are forced open during treatment, exposing melanin granules to oxidation, which weakens the hair’s natural protection against light damage. Dyes also strip away a lipid layer called 18-MEA from the cell membrane complex, which is what makes chemically treated hair feel rougher and less shiny than untreated hair.²⁹

Because the visible portion of hair is dead, it cannot repair this damage the way living tissue does. Any recovery has to come from external coatings (conditioners, oils) that temporarily fill in the gaps, or simply from growing new, undamaged hair. This is why heavy chemical processing accumulates: each treatment strips more material from the cuticle and cortex, and there is no biological mechanism to rebuild what was lost.

Evolutionary Origins of Hair

How hair first evolved in mammals’ ancestors remains an open question, and two main hypotheses compete. One proposes that hair evolved from the scales of early synapsids, the lineage that eventually gave rise to mammals. This would require a dramatic transformation of the flat, lens-shaped skin structures that underlie scales into the small dermal papillae that drive hair growth. The other hypothesis suggests hair evolved from glandular structures in the skin, based on similarities in the signaling pathways that control hair and gland development.³⁰ Under this model, the onion-like layered organization of the hair follicle could have derived from glandular tubes whose central cells produced lipids and keratin.³¹

Neither hypothesis is fully proven, and fossil evidence for the earliest hair is sparse because soft tissues rarely preserve. What is clear is that keratinized hair is uniquely mammalian. Birds have feathers, reptiles have scales, but the particular combination of a cycling follicle, a dermal papilla, keratin intermediate filaments cross-linked by disulfide bonds, and melanocyte-based pigmentation is found only in mammals and their ancestors. How that remarkable system first assembled remains one of the more fascinating puzzles in evolutionary biology.