The Biology of How Hair Gets Its Shape
Every single hair on your head grows from a tiny living factory buried beneath your skin: the hair follicle. The follicle is not just a tube. It is a complex, dynamic organ with multiple layers of specialized cells, all working together to produce a single strand of hair.
At first glance, a strand of hair looks simple. But the shape of that strand—whether it falls straight, bends into a wave, spirals into a curl, or compresses into a tight coil—is determined by a series of microscopic decisions made inside the follicle. These decisions involve asymmetry, timing, protein distribution, and even the shape of the follicle itself.
This article explores the biology of hair shape. We will go beneath the skin to understand how follicles are built, how they differ between straight and curly hair, and why a single head can sometimes produce multiple curl patterns.
The Follicle: A Tiny Living Factory
Before we can understand why hair curls, we need to understand how hair grows at all.
The hair follicle is a small, tunnel-shaped organ in the skin. It has seven layers of specialized epithelial cells arranged in a concentric pattern, like an onion, with the hair fiber at the center. Each layer has a unique differentiation pathway and set of properties.
At the very bottom of the follicle sits the dermal papilla (DP)—a small cluster of mesenchymal cells that directs the regulation of hair growth and the hair cycle. Think of the DP as the control center. It sends signals to the surrounding cells, telling them when to divide, when to stop dividing, and what kind of hair to produce.
Above the DP is the hair bulb, the actively growing region of the follicle. Here, cells are rapidly dividing in a process called mitosis. These dividing cells generate the “force” behind hair formation—pushing older cells upward, where they will eventually harden into the hair shaft.
As cells move up the follicle, they go through a process of differentiation. They stop dividing and begin to produce keratin—the tough, fibrous protein that makes up hair. They also flatten, lengthen, and fuse together, forming the three main layers of the hair fiber: the cuticle (outer protective layer), the cortex (middle layer that gives hair its strength and color), and sometimes the medulla (a soft central core).
This is the basic blueprint for all hair. But the blueprint alone does not explain curl.
The Shape of the Follicle Itself
One of the most important discoveries in hair biology is that a curly follicle makes a curly hair. You cannot get a curly hair out of a straight follicle, and you cannot get a straight hair out of a curly follicle. The shape is set before the hair even emerges from the skin.
In straight hair, the follicle is straight and symmetrical. It grows vertically from the skin, like a blade of grass. The dermal papilla at the base is centered, and the cells divide evenly all around it.
In curly hair, the follicle is curved. But not just curved in one direction. The follicles of curly hairs exhibit retrocurvature—a bend in the lower part of the follicle that causes the bulb to hook sideways. This retrocurvature has been observed in curly hair follicles independent of ethnic origin. In other words, whether a person is of African, European, or Asian descent, if they have curly hair, their follicles show the same characteristic bend.
Researchers have observed this phenomenon using transmission electron microscopy and have confirmed that the curved shape of the follicle is strongly related to hair curliness. As one study concluded, “The curly shape of the hair shaft seems to result from the asymmetric differentiation of the precortex.”
Asymmetry: The Engine of Curls
If the follicle is curved, what causes that curvature? The answer appears to be asymmetry—cells dividing and differentiating unevenly around the follicle.
In a straight hair follicle, the proliferative compartment (the region where cells are actively dividing) is symmetric. Cells divide at roughly the same rate on all sides of the dermal papilla. As they move upward, they differentiate evenly, producing a hair fiber that is round or nearly round in cross-section.
In a curly hair follicle, the proliferative compartment is asymmetric. The dividing zone extends higher up on the convex side of the curvature—the side that bulges outward. This asymmetry causes delayed differentiation of the inner and outer root sheaths on that side. The cells on the convex side keep dividing longer and differentiate later than the cells on the concave side.
The result is a hair fiber that grows unevenly. One side of the fiber is pushed upward faster than the other side. This differential growth rate creates a bending moment, forcing the fiber to curve as it emerges from the skin.
Think of it like a bimetallic strip. Two different materials bonded together will bend when heated because they expand at different rates. Similarly, the two sides of a curly hair fiber are being “built” at different rates, so the fiber naturally curves.
The Cortex: Where the Curl Is Locked In
The cortex of the hair fiber is most likely to structurally support curly hair characteristics. The cortex is the middle layer of the hair, composed of long, spindle-shaped cells that are aligned with the long axis of the hair. These cells are packed with keratin filaments—dense, almost crystalline arrays of protein that give hair its strength and flexibility.
In straight hair, the cortex is symmetric. Cortical cells are evenly distributed throughout the fiber, and the hair fiber itself is circular or nearly circular in cross-section.
In curly hair, the cortex is elliptical and asymmetric. The hair fiber is flattened on one side, giving it an oval or kidney-bean shape in cross-section. Studies have shown that the degree of ellipticity correlates with the degree of curliness: straighter fibers are more circular, while curlier fibers are more elliptical.
This shape matters because an elliptical fiber bends more easily in one direction than another. It has a preferred bending plane. This is why curly hair tends to curl in a consistent direction, rather than bending randomly.
But the asymmetry goes deeper than just shape. Researchers have discovered that a specific keratin protein called hHa8 (also known as K38, encoded by the gene KRT38) is distributed unevenly in curly hair follicles. In curly hair, this keratin accumulates on the concave side of the curvature (the inside of the curl). In straight hair, positive cortical cells are evenly distributed throughout the circular fiber.
This asymmetric distribution of structural proteins suggests that the curl is not just a physical bending of the fiber—it is programmed into the molecular structure of the hair itself.
What About Wavy Hair?
Not all hair is either perfectly straight or tightly curled. Most people fall somewhere in between.
Wavy hair represents an intermediate state. The follicles of wavy hair are less curved than those of curly hair, and the asymmetry in cell division is less pronounced. The cross-section of wavy hair is neither perfectly round nor highly elliptical—it is somewhere in between.
This is why hair forms a spectrum, not discrete categories. The terms “straight,” “wavy,” “curly,” and “coily” are useful for everyday conversation, but biology does not respect neat boxes. A single head can even contain different curl patterns in different places. A person might have tighter curls at the nape of the neck and looser waves on top.
The Role of the Inner Root Sheath
The follicle does not just make the hair fiber. It also produces the inner root sheath (IRS)—a rigid, tube-like structure that surrounds the growing hair fiber and molds it into shape. The IRS is like a cookie cutter. It hardens around the developing hair, giving it its final form before the hair emerges from the skin.
The IRS is composed of three layers, and its shape mirrors the shape of the follicle. If the follicle is curved, the IRS is curved. If the follicle is straight, the IRS is straight.
Researchers have observed that in curly hair, the IRS shows the same retrocurvature as the follicle itself. Some scientists have proposed that the IRS may actually play an active role in determining curl, rather than just passively reflecting the follicle’s shape.
If the IRS is asymmetric—stiffer on one side than the other, or growing faster on one side—it could physically bend the developing hair fiber. This is an active area of research, and the exact role of the IRS in curl formation is still being debated.
A Note on Labels and Categories
The categories we use to describe hair—straight, wavy, curly, coily—are useful for talking about hair. But biology does not always respect neat labels.
A single head can contain different curl patterns in different places. The same person’s hair can look different depending on humidity, length, damage, oils, or how it is cared for. And the genetic basis of hair texture is polygenic, meaning many genes contribute, each with small effects.
This is why you cannot look at one person’s hair and explain their entire ancestry or environment. Evolution is never that clean.
What This Means for Understanding Your Hair
The next time you look at a strand of your hair, consider what it took to make it.
Somewhere beneath your scalp, a curved follicle with an asymmetric dividing zone produced cells at different rates on different sides. A specific keratin protein accumulated on the inside of that curve, perhaps helping to lock the curl into place. An elliptical cortex and an asymmetrical inner root sheath further molded the fiber, giving it its final shape.
Then, over days and weeks, that strand grew upward, pushed by the force of thousands of dividing cells, until it finally emerged from your skin.
Your hair is not random. It is not just a texture. It is the product of a complex biological system that has been fine-tuned over thousands of generations.
Straight hair, wavy hair, curly hair, coily hair—they are not better or worse. They are just different solutions to the same problem: how to build a fiber that protects the head, manages heat, and maybe also catches the eye.
And that solution is written, strand by strand, in the shape of your follicles and the asymmetry of your cells.
References
Cloete, E., Khumalo, N. P., & Ngoepe, M. N. (2019). The what, why and how of curly hair: a review. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 475(2231), 20190516. https://doi.org/10.1098/rspa.2019.0516
Daniels, G., Fraser, A., & Westgate, G. E. (2023). How different is human hair? A critical appraisal of the reported differences in global hair fibre characteristics and properties towards defining a more relevant framework for hair type classification. International Journal of Cosmetic Science, 45(1), 50-61. https://doi.org/10.1111/ics.12819
Thibaut, S., Barbarat, P., Leroy, F., & Bernard, B. A. (2007). Human hair keratin network and curvature. International Journal of Dermatology, 46(Suppl 1), 7-10. https://doi.org/10.1111/j.1365-4632.2007.03454.x
Westgate, G. E., Ginger, R. S., & Green, M. R. (2017). The biology and genetics of curly hair. Experimental Dermatology, 26(6), 483-490. https://doi.org/10.1111/exd.13347
Disclaimer: This article was researched and drafted with the assistance of AI. All sources are real and verifiable. Readers are encouraged to check the references themselves and draw their own conclusions.
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