Hair is a fibrous protein structure, primarily composed of keratin, which grows from follicles embedded in the skin. The difference between straight hair and a tightly coiled curl is rooted in biology and structure. To understand why some hair grows straight and other hair spirals, one must look at the shape of the hair’s origin and the internal components of the strand itself. This structural variation dictates the hair’s mechanical strength and its behavior in different environments.
The Determining Factor of Follicle Shape
The primary factor determining the shape of a hair strand is the form of the hair follicle, the tiny organ from which the hair grows beneath the scalp’s surface, molding the hair as it is generated. Straight hair emerges from a follicle that is perfectly round in cross-section, allowing the hair shaft to grow symmetrically.
Curly hair, in contrast, grows from a follicle that is oval or elliptical in shape, which can sometimes be curved or hooked. This asymmetrical tunnel forces the hair shaft to adopt a flattened, oval, or kidney-bean cross-section as it is formed. The more flattened or asymmetrical the follicle’s shape, the tighter the resulting curl will be.
The angle at which the follicle is implanted into the scalp also plays a role in the direction of the curl. For curly hair, the follicle often enters the skin at a noticeable angle and may even curve beneath the surface. This curvature contributes to the hair’s tendency to bend and coil as it exits the skin.
Internal Architecture of the Hair Shaft
The hair shaft, primarily the cortex layer, is not uniformly structured in curly hair, which is a significant factor in the coiling mechanism. Straight hair possesses a cortex with a symmetrical arrangement of its components, resulting in even growth tension across the strand. Curly hair exhibits a distinct asymmetry in its internal cellular structure.
The cortex is often divided into two distinct regions: the orthocortex and the paracortex. The orthocortex is typically found on the outer, convex side of the curl, while the paracortex is located on the inner, concave side. These two cortical cell types have different properties, including variations in the arrangement of keratin intermediate filaments and their resistance to swelling.
This difference in composition creates a differential growth tension within the hair shaft. As the hair is formed, the two sides grow at different rates or with differing mechanical properties, forcing the strand to bend or twist upon itself, causing the hair fiber to naturally coil into a spiral. The greater the difference in properties between the orthocortical and paracortical cells, the tighter the resulting curl radius becomes.
The Chemistry Behind the Curve
Molecular forces stabilize the curl. The primary chemical stabilizers are disulfide bonds, which are strong covalent bonds between sulfur atoms in the amino acid cysteine found within the keratin protein chains. These bonds are permanent, locking the asymmetrical, coiled shape of the hair shaft into place.
Curly hair naturally has a higher concentration of these disulfide bonds, aligned to reinforce the fiber’s spiral structure. This arrangement prevents the hair from naturally relaxing into a straight form. Only chemical treatments like perms or relaxers can alter them by breaking the bonds, reshaping the hair, and then reforming the bonds in the new configuration.
Weaker hydrogen bonds also exist within the hair structure, responsible for temporary changes in shape. Hydrogen bonds are easily broken by water and are the reason hair loses its shape when wet or exposed to high humidity. When water molecules enter the hair, they disrupt the existing hydrogen bonds, allowing the hair to swell and temporarily lose its definition. When the hair dries, new hydrogen bonds form, and the hair returns to its permanent, disulfide-bond-reinforced coiled shape.
Genetics and Inheritance
The curved follicle and asymmetrical hair shaft are fundamentally determined by an individual’s genetic makeup. Curliness is a highly heritable trait. This characteristic is not controlled by a single gene but is considered a polygenic trait, involving the complex interaction of multiple genes.
Specific genes have been identified that influence hair texture and follicle shape. For example, variants in the KRT74 gene, which codes for a type of keratin protein, have been strongly linked to hair curliness. The EDAR gene, which provides instructions for a protein involved in the development of ectodermal structures like hair follicles, is associated with hair straightness, particularly in East Asian populations.
The combination of inherited genetic variants dictates the developmental process of the hair follicle during the embryonic stage, programming the follicle to be either round (straight hair) or oval and curved (coiled strand). While a few genes account for a portion of the variation, the complete genetic picture for the wide spectrum of human hair curl remains under study.