Hair morphology is the scientific study of the structure, shape, and form of the human hair shaft. This field provides insight into human adaptation and genetic variation. In health and dermatology, hair morphology helps diagnose inherited or acquired hair disorders. Hair strand properties are also utilized in forensic science, where characteristics like diameter and pigment distribution help associate samples with individuals or determine ancestral origin.
Internal Structure and Composition
A single hair strand, which is technically the hair shaft, is a complex, non-living cylinder composed of three distinct concentric layers. The outermost layer is the cuticle, which functions as a protective shield made of overlapping, flattened cells that resemble shingles on a roof. A healthy cuticle lies flat, reflecting light to create a smooth, shiny appearance, and helps to regulate the movement of moisture into and out of the strand.
Beneath this outer barrier is the cortex, which makes up the majority of the hair’s mass, often constituting up to 90% of its total weight. This layer is packed with elongated cells containing hard keratin intermediate filaments, which are the primary source of the hair’s mechanical strength and elasticity. The cortex also houses melanin, the pigment granules that determine the hair’s natural color, with the ratio of eumelanin (dark pigment) to pheomelanin (light/red pigment) dictating the resulting hue.
The innermost layer is the medulla, a central core that is not present in all hair types, particularly finer or thinner strands. When present, the medulla is an unstructured, open, and sometimes porous region composed of transparent cells and air spaces. The structural integrity of all three layers is largely maintained by keratin, which accounts for approximately 95% of the hair’s dry weight.
Architectural Shape and Texture
The visible texture of hair—whether it is straight, wavy, or tightly coiled—is a direct result of the shape of the hair follicle embedded in the scalp. The follicle acts as a mold for the growing hair shaft, determining the cross-sectional geometry of the strand as it emerges. A perfectly round hair follicle produces a hair shaft with a round cross-section, which results in straight hair.
A hair follicle that is oval or elliptical in shape causes the hair shaft to have a corresponding oval cross-section, leading to wavy hair. The flatter or more asymmetrical the follicle becomes, the more pronounced the curvature of the hair strand, resulting in curly or coily hair. This geometric difference influences how the hair bends and twists as it grows, which is why curlier hair often appears to have more volume.
Hair diameter, which dictates whether hair is classified as fine, medium, or coarse, is also determined by the follicle size. Coarse hair has a wider diameter and often includes a medulla, while fine hair has a narrower diameter and typically lacks this central core. The distribution of keratin fibers within the cortex affects the hair’s mechanical properties. The bonds between these keratin molecules, particularly disulfide bonds, influence the hair’s tensile strength and resistance to breakage.
Factors Determining Morphological Variation
The primary force dictating the fundamental morphology of hair, including follicle shape, diameter, and pigmentation, is genetics. Specific genes inherited from parents serve as the blueprint, controlling the entire process of hair formation. For instance, variations in genes like EDAR and TCHH have been linked to differences in hair thickness and texture in various populations.
The traits are complex, often determined by multiple genes, which explains why hair texture can sometimes change between generations or even siblings. The genes determine not only the curl pattern but also the type and amount of melanin produced.
Non-genetic factors also influence hair morphology, though they often relate to acquired changes rather than inherent structure. Age is a significant factor, as the hair follicle’s production of melanin slows down over time, leading to a loss of color and the appearance of gray hair. The diameter of hair strands may also decrease with age, resulting in overall thinning.
External environmental stressors can physically damage the existing hair shaft, altering its integrity and appearance. Exposure to intense UV radiation, harsh chemical processing, and excessive heat styling can lift and erode the protective cuticle layer. This damage exposes the underlying cortex, making the hair more porous, brittle, and susceptible to physical breakage.