Hair texture, encompassing straight, wavy, curly, or coily, and fineness or coarseness, is a complex biological trait. Its diversity arises from biological and environmental factors, providing insight into hair types.
Genetic Blueprint
Genetic makeup determines hair texture. Inherited genes dictate hair strand characteristics, influencing straight, wavy, or curly hair. Hair texture is polygenic, with multiple contributing genes.
Specific genes play a role in inherited hair texture. Examples include EDAR (hair thickness/straightness, East Asian populations), TCHH (European hair texture variations, straight hair), and keratin genes (KRT74, KRT71, KRT25) influencing curl patterns via structural proteins. The CUTC gene, a copper transporter, is associated with hair curliness.
Hair Follicle Shape
Hair follicle shape directly influences its cross-section and texture. Hair from a round follicle is straight and circular.
Oval or elliptical follicles produce wavy or curly hair. Flatter, asymmetrical follicles are associated with curlier or coily hair, with flattening correlating to curl tightness.
Follicle emergence angle influences curl pattern. Straight hair follicles emerge almost vertically. Curly hair follicles are angled or perpendicular to the skin, contributing to spiraling.
Hair Strand Internal Structure
Hair strand’s microscopic composition contributes to its texture. Each hair shaft has three layers: outer cuticle, central cortex, and sometimes medulla. The cortex forms the bulk, determining strength, elasticity, and texture.
The cortex is rich in structural keratin proteins. Their arrangement and distribution, particularly orthocortical and paracortical cells, influence hair shape. Differences in cortical cell lengths (shorter paracortical cells on the inside of a curve, longer orthocortical cells on the outside) contribute to natural curl.
Disulfide bonds, strong chemical linkages in keratin proteins, help set hair’s shape. More disulfide bonds result in curlier hair. Even distribution creates straight hair; clustered or uneven arrangements create curlier hair.
Hormonal Fluctuations
Significant hormonal changes throughout life can influence hair texture. These shifts may alter existing texture (curlier, straighter, finer, coarser), but do not determine initial hair texture. Hair follicle response to hormonal signals can modify its shape and hair production.
During puberty, fluctuating hormone levels can lead to hair texture changes, with some experiencing wavier or curlier hair. Pregnancy often brings an estrogen surge, prolonging hair’s growth phase and sometimes changing texture, like straight hair developing waves. Hair typically reverts to original texture months after childbirth.
Menopause, with decreased estrogen/progesterone and increased androgen activity, can lead to hair texture changes, including dryness, frizz, or altered curl patterns. Thyroid hormone imbalances can cause dry, brittle, or thin hair. Androgens can affect hair texture, promoting thicker growth in some areas while contributing to scalp thinning in predisposed individuals.
External Factors
Environmental and lifestyle elements can temporarily or permanently alter hair texture. These factors modify existing texture, not its fundamental type.
Humidity causes hair to absorb moisture, leading to swelling, frizz, and curl pattern changes. Curly hair may become frizzier, straight hair wavier, due to weak hydrogen bond formation.
Heat styling tools (blow dryers, flat irons, curling irons) temporarily alter hair structure. Excessive heat damages hair, causing dryness, frizz, breakage, and texture changes by altering keratin proteins.
Chemical treatments (perms, relaxers, coloring) permanently alter hair texture by breaking and reforming disulfide bonds. Improper application can cause significant damage.
Environmental pollutants can bind to hair, infiltrate follicles, and cause damage, resulting in rough, dull hair. UV radiation can degrade hair proteins. Physical damage from aggressive brushing, combing, or tight hairstyles can lead to breakage and altered texture.