Hair morphology is the scientific study of the form and structure of hair, examining characteristics like its thickness, color, and shape. The insights from this field are applied in dermatology, anthropology, and forensic science, where a hair’s physical attributes can provide information about an individual’s health and ancestry.
The Structure of a Single Hair
A single strand of hair consists of two primary parts: the hair root and the hair shaft. The root is the portion located beneath the skin’s surface, anchored within a structure called the follicle. The shaft is the visible part of the hair that extends above the skin.
The hair shaft is composed of three distinct layers, often compared to the structure of a pencil. The outermost layer is the cuticle, which is formed by overlapping scales of keratin, similar to shingles on a roof. These scales point towards the tip of the hair and create a protective barrier for the inner layers. The pattern and condition of these scales can vary and are a focus of microscopic examination.
Beneath the cuticle lies the cortex, which constitutes the bulk of the hair shaft. This layer is made of long, tightly packed keratin protein fibers, giving hair its strength and elasticity. The cortex also contains melanin granules, the pigments that determine hair color.
At the very center of the hair shaft is the medulla, a core that can be continuous, fragmented, or even entirely absent. Its structure is highly variable among different individuals and even within the hairs of a single person. While its precise function is not fully understood, the characteristics of the medulla help differentiate between hair types.
The Hair Growth Cycle
Hair does not grow continuously but follows a cyclical process with distinct phases. Each hair follicle operates on its own timeline, which prevents all hair from shedding simultaneously and ensures a relatively consistent covering.
The anagen phase is the active growth stage. During this period, cells in the hair bulb at the base of the follicle divide rapidly, causing the hair shaft to lengthen. This phase is the longest in the cycle, typically lasting several years for scalp hairs. The duration of the anagen phase determines the maximum length a hair can achieve.
Following the growth phase is the catagen phase, a short transitional period. This stage lasts only a few weeks, during which hair growth stops. The hair follicle shrinks and begins to detach from its blood supply at the dermal papilla, ceasing nourishment to the hair.
The last stage of the cycle is the telogen phase, or resting phase. The hair, now known as a club hair, remains in the follicle but is no longer growing. This phase can last for several months before the hair is eventually shed. Shedding occurs when a new anagen hair forms beneath it, pushing the old hair out. Humans shed approximately 100 head hairs per day as a natural part of this process.
Variations in Human Hair
The appearance of hair varies significantly among individuals, largely due to differences in its physical and chemical composition. These variations in shape, color, and diameter are determined by genetic factors.
The texture of hair—whether it is straight, wavy, or curly—is determined by the cross-sectional shape of the hair shaft. A hair shaft with a circular cross-section tends to grow straight out of the follicle. An oval or elliptical cross-section results in wavy or curly hair, while a more flattened, kidney-bean shape produces tightly coiled hair.
Hair color is derived from pigment granules called melanin, which are embedded within the cortex. There are two main types of melanin: eumelanin and pheomelanin. Eumelanin is responsible for shades of brown and black, while pheomelanin produces red and blonde tones. The specific combination, concentration, and distribution of these pigments create the wide array of natural hair colors.
The diameter of the hair shaft dictates whether hair is considered fine, medium, or coarse. This thickness is influenced by the size of the hair follicle and is primarily determined by genetics. On average, a human hair has a diameter ranging from about 0.05 to 0.1 millimeters. Factors like age and nutrition can also influence hair thickness.
Forensic and Anthropological Significance
The morphological characteristics of hair provide valuable information in forensic investigations and anthropological studies. In forensics, hair analysis is used to distinguish between human and animal hairs. This is done by examining features like cuticle scale patterns and medullary structure, which differ between species.
Hair found at a crime scene can offer investigative clues. The morphology can suggest which part of the body the hair originated from; for instance, head hairs and pubic hairs have distinct structural differences. The condition of the hair root also indicates how it was shed. A hair with a follicular tag—tissue from the follicle—suggests it was forcibly removed rather than falling out naturally.
Morphological hair analysis has limitations. While it can provide class evidence, connecting a hair to a particular group of people, it cannot definitively identify a single individual. For individual identification, DNA analysis is required. In anthropology, hair morphology is used to study population genetics and ancient migration patterns, offering insights into human history.