Curly hair, with its diverse patterns ranging from gentle waves to tight coils, is a common human trait. This unique hair texture results from an intricate biological process involving microscopic structures and cellular activities beneath the scalp. Understanding the science behind these natural spirals reveals how hair achieves its shape and texture from its roots. This exploration delves into the underlying biological mechanisms that contribute to the formation of curly hair.
The Hair Follicle’s Influence
Hair growth begins beneath the scalp within tiny organs called hair follicles. The shape of these follicles determines whether a hair strand will be straight or curly. Straight hair typically grows from follicles that are round in cross-section. Conversely, curly hair originates from follicles that are oval or flattened in shape; the flatter the oval, the curlier the hair often becomes.
This distinct follicle shape influences how the hair shaft grows as it emerges from the skin. A round follicle allows hair to grow straight upwards, resulting in a cylindrical strand. However, an oval or asymmetrical follicle causes the hair to grow in a curved or angled path. This curved path forces the hair shaft to bend and twist as it forms, contributing to its spiral or wave pattern.
The angle at which the hair follicle tunnels into the scalp also contributes to the curl. Slanted or curved follicles produce wavy or curly hair, while vertically oriented follicles tend to produce straight hair. For those with curly hair, the hair bulb, a pear-shaped structure at the base of the follicle, is often slightly hooked. This consistent curvature of the follicle itself contributes significantly to the eventual coil of the hair fiber.
The Hair’s Internal Architecture
Beyond the external shape of the follicle, the internal structure and chemical composition of each hair strand also dictate its curl pattern. Hair is primarily composed of a fibrous protein called keratin. In straight hair, keratin proteins are distributed evenly throughout the hair shaft, leading to a uniform structure. In curly hair, these keratin proteins are distributed asymmetrically within the hair fiber. The hair cortex, which forms the bulk of the hair, is composed of two different types of keratin cells, orthocortex and paracortex, that are unevenly distributed in curly hair strands.
This uneven distribution means one side of the hair strand grows differently or at a different rate than the other. This creates tension within the fiber, causing it to twist and coil as it lengthens. The elasticity and strength of textured hair are associated with the presence of disulfide bonds, which are strong cross-linking structures. These permanent chemical bonds form between sulfur atoms found in the amino acid cysteine, a key building block of keratin.
Curly hair typically possesses a higher proportion of these cysteine amino acids, leading to more numerous disulfide bonds. The uneven placement and greater abundance of these bonds along the hair shaft contribute significantly to the hair’s coiled structure. These bonds “lock” the hair into its curved shape, differentiating them from temporary hydrogen bonds that can be broken by water or heat. This intricate internal architecture allows the hair fiber to act as a shape memory material, consistently returning to its curled form.
The Role of Genetics
The specific characteristics of hair follicles and internal hair structure are largely determined by an individual’s genetic makeup. Curly hair is an inherited trait, meaning genes passed down from parents play a significant role in its development. While often considered a dominant trait, curly hair inheritance is complex and influenced by multiple genes, making it a polygenic trait. This means there isn’t one single “curl gene,” but rather an interplay of several genes.
These genes provide instructions for the hair follicle’s shape, ensuring it forms with the characteristic oval or asymmetrical structure that produces curly hair. They also influence the production and asymmetrical distribution of keratin proteins and the formation of disulfide bonds within the hair shaft. For instance, variations in genes like TCHH (trichohyalin) and KRT74 (keratin 74) have been linked to hair curliness, affecting the proteins that contribute to the hair’s shape.
The amount of curl an individual has depends on how many curly hair gene variants they inherit. Even if both parents have curly hair, their offspring might exhibit a range of hair textures due to the complex interaction of these genes. This genetic blueprint ultimately dictates the entire biological process that results in hair growing with a distinct curl pattern.