Hair curl, ranging from straight to tightly coiled, is a complex biological characteristic. It arises from an interplay of anatomical structures, cellular processes, and molecular components within the hair. Understanding these mechanisms reveals the intricate biology shaping each strand.
The Hair Follicle’s Shape
The primary determinant of hair curl originates within the hair follicle beneath the skin. Its shape dictates the cross-sectional form of the hair strand as it grows. A round follicle typically produces straight hair with a circular cross-section. Conversely, an oval or elliptical follicle gives rise to wavy or curly hair with a flattened cross-section; the more elliptical, the tighter the curl.
Curly hair follicles are often curved, exhibiting an S-shaped structure. This curvature imposes a preset path on the hair shaft, causing it to coil as it grows. The angle of the follicle within the skin also contributes, with follicles at a near 90-degree angle tending to produce straighter hair. Asymmetric cell division within the follicle, where cells multiply unevenly, further contributes to the non-uniform shape and tension leading to curl. This follicular shape is established during embryonic development and re-established after each hair growth cycle.
The Role of Keratin and Disulfide Bonds
Beyond the follicle’s shape, the hair shaft’s internal chemical structure defines its curl. Hair is primarily composed of keratin proteins, the structural building blocks of each strand. In curly hair, the distribution of keratin types, specifically ortho- and para-cortical cells, is often uneven within the cortex. Orthocortical cells are typically longer and positioned on the outer, convex side of the curl, while shorter paracortical cells are on the inner, concave side. This difference in cell length creates tension, causing the hair shaft to curve and twist.
Strong chemical links called disulfide bonds also contribute to hair’s shape. These bonds form between sulfur atoms in the cysteine amino acids within keratin protein chains. In straight hair, these bonds are evenly distributed across the hair strand; in curly hair, they are often unevenly distributed or tend to cluster on one side. This asymmetric arrangement creates differential tension, pulling the hair into its characteristic bends, waves, and coils. These bonds maintain the hair’s form and can only be altered by chemical treatments like perming or relaxing.
Genetic Influence on Hair Curl
Hair curl is largely an inherited characteristic. It is a polygenic trait, meaning multiple genes interact to determine the final hair type. This complex genetic inheritance explains the wide spectrum of curl patterns observed, rather than a simple dominant or recessive expression.
Specific genes influence the developmental shape of hair follicles and the distribution of keratin proteins and disulfide bonds. Research has identified several genes, such as TCHH (trichohyalin), KRT74 (keratin 74), EDAR, and CUTC, linked to variations in hair texture. The collective action and variations within these genes contribute to whether hair grows straight, wavy, or tightly curled.