Human hair is a biological fiber that serves various protective and sensory functions. While every strand shares a common underlying structure, hair exhibits a remarkable spectrum of differences in length, thickness, color, and texture across the human body and between individuals. These diversities are determined by a complex interplay of genetic coding, hormonal regulation, and environmental factors.
The Common Components of Human Hair
Every hair strand is composed of two main parts: the hair follicle, the living structure beneath the skin, and the hair shaft, the non-living, visible portion above the surface. The hair shaft is primarily made up of keratin, a durable protein that accounts for approximately 90% of its dry weight. This keratin is organized into three distinct layers that give the hair its structure and resilience.
The outermost layer, the cuticle, consists of transparent, overlapping, scale-like cells that protect the hair’s inner core. Beneath this protective layer is the cortex, the thickest part of the shaft, which contains the majority of the keratin and provides the hair with its strength and elasticity. The cortex also houses the melanin pigments that give hair its natural color. The innermost layer, the medulla, is a core of cells and air pockets often only present in thicker, coarser hair types.
Hair Variation Based on Body Location
Hair differences are apparent even on a single person’s body, where hair is categorized into two main types: vellus and terminal. Vellus hair is fine, short, and lightly colored, often described as “peach fuzz,” and covers most of the body’s surface. Its primary role is to assist in temperature regulation by providing insulation.
Terminal hair is thicker, longer, and more deeply pigmented. This type is found on the scalp, eyebrows, and eyelashes, and develops in areas like the armpits and pubic region after puberty. Terminal hair grows from follicles that extend deeper into the skin and often possess a medulla, contributing to its coarser texture. The transition from vellus to terminal hair is largely driven by an increase in androgen hormones during puberty, which causes some follicles to produce the thicker, darker strands.
Genetic Factors Driving Hair Shape and Color
The visible characteristics of hair, such as its shape and color, are fundamentally determined by genetic instructions. The specific shape of the hair follicle—round, oval, or flat-oval—dictates the cross-sectional shape of the hair fiber as it emerges from the scalp. A perfectly round follicle produces straight hair, while an increasingly oval or flat-oval shape results in wavy, curly, or coily hair, respectively.
The curl pattern is further stabilized by the chemical architecture within the hair’s cortex, specifically the arrangement of disulfide bonds. These permanent bonds form between sulfur atoms in the keratin protein, acting as an internal architectural framework. Curly hair contains a greater number and more asymmetrical distribution of these bonds than straight hair, resulting in the hair strand bending and twisting into its characteristic spiral shape.
Hair color is controlled by the type and amount of melanin pigment produced by specialized cells called melanocytes within the hair follicle. Two main types of melanin exist: eumelanin, responsible for black and brown shades, and pheomelanin, which contributes to red and yellow tones. The concentration and ratio of these two pigments determine the full spectrum of human hair color.
High concentrations of eumelanin result in black hair, while lower amounts of brown eumelanin lead to blonde hair. Red hair occurs when there is an abundance of pheomelanin, often due to variations in the MC1R gene that reduce the production of eumelanin.
How Age and Environment Change Hair
Even hair with a fixed genetic code will change over a person’s lifespan due to internal and external influences. A common age-related change is the gradual graying of hair, a process known as achromotrichia. This occurs because the melanocytes in the hair follicle progressively lose their ability to produce pigment, causing the new hair growth to lack color and appear white or gray.
Another significant change with age is the thinning of hair, often linked to follicle miniaturization, which can be influenced by hormonal shifts. Some follicles may stop producing hair altogether, leading to a decrease in overall hair density. The diameter of individual hair strands may also lessen, making the hair feel finer and more fragile.
External factors, referred to as environmental stressors, can also alter the hair shaft and affect its quality. Exposure to ultraviolet (UV) radiation from the sun, for instance, can damage the cuticle layer and break down the melanin pigments, leading to a loss of natural color and increased fragility. Other external forces, such as heat styling, chemical treatments, and air pollution, can physically disrupt the hair’s protein structure, making it more prone to breakage.