GROWTH OF HAIRS
Hairs are a particular structure common only to mammals; they are the fibrous growths that originate from their skin. Other animals have structures that may appear to be or are even called hairs but they are not: Only mammals have hairs. Humans use hairs as signs of culture, status, and gender, as well as for personal or artistic expression. Hairs grow from the skin or, more precisely, epidermis, of the body, as shown in Figure 1. The follicle is the structure within which hairs grow; it is a roughly cylindrical tube with a larger pit at the bottom. Hairs grow from the base of the follicle upward. In the base of the follicle, the hair is still very soft; as the hair proceeds up the follicle, it slowly begins to harden and dry out. Hair is made of keratin, a tough protein-based material from which hair, nails, and horns are made in animals. The hardening process of hair growth is therefore called keratinization. Hair is one of the most durable materials produced by nature; hairs from mummies, both natural and cultural in origin, have been found thousands of years after the person’s death. Keratinization also explains why it doesn’t hurt when hair is cut: Hair is “dead” from the moment it peeks above the skin. The only place hair is “alive” is in the base of the follicle, which is why it does hurt when a hair is pulled out.
FIGURE 1The epidermis is a complicated structure, containing many different structures. The hair, composed of the tip, shaft, and root, develops within the follicle. As the hair grows, it slowly hardens and is fully keratinized by the time it reaches the surface of the skin. Sebaceous glands open into the follicle to secrete oils onto the hair. The pili arrector muscle controls the position of the hair, creating the “goosebump” effect when a person gets chilled by contracting and pulling the hair upright.
FIGURE 2 Hairs grow in three phases. In the anagen, or actively growing, phase, the follicle produces new cells and pushes them up the hair shaft. After 2–7 years, the follicle transitions into the catagen phase. The follicle begins to shut down and quits producing hair in about 2–3 weeks. The final phase, the telogen or resting phase, which lasts about 100 days, the follicle is shut down completely and the root is dried to a bulb. At this point, the hair is only attached mechanically and will be shed naturally.
The follicle contains other structures, such as blood vessels, nerves, and sebaceous glands, the latter producing oils that coat hairs, helping to keep them soft and pliable. Hairs even have muscles, called pili arrector muscles (pilius is the Latin word for “hair”) that raise hairs when a person gets chilled (so-called goose bumps). Hairs go through three phases of growth, as depicted in Figure 2. In the anagen, or actively growing, phase, the follicle produces new cells and pushes them up the hair shaft as they become incorporated into the structure of the hair. The hair is moved up the shaft by a mechanical method. As the cells are produced, they “ratchet” up the shaft by opposing scales—much like gears in a machine! Between this mechanical method and the upward pressure from the growth of the cells in the follicle, the hairs grow outward from the skin. Specialized cells in the follicle produce small colored granules, called melanin or pigment, that give hairs their particular color; these cells are called melanocytes. Only two types of melanin are found in hairs: a dark brown pigment called eumelanin and a lighter pigment called pheomelanin. The combination, density, and distribution of these granules produce the range of hair colors seen in humans and animals. After the active growth phase, the hair transitions into a resting phase; this transitional phase is called the catagen phase. During the catagen phase, the follicle begins to shut down production of cells, the cells begin to shrink, and the root condenses into a bulb-shaped structure called, understandably, a root bulb or a club root. The telogen phase is the resting phase for the follicle. Cell production has ceased completely; the root has condensed into a bulb and is held in place only by a mechanical connection at the base of the root/follicle. When this mechanical connection breaks (through combing, brushing, or normal wear), the follicle is triggered into the anagen phase again and the cycle renews. On a healthy human head of hair, about 80–90% of the hairs would be in the anagen phase, about 2% in the catagen phase, and about 10–18% in the telogen phase. When the telogen hairs are removed, new hairs begin to grow at once; clipping and shaving have no effect on growth. The time required for human follicles to regrow hairs varies from 147 days for scalp hairs to 61 days for eyebrow hairs. Humans, on average, lose about 100 scalp hairs a day; this provides for an adequate and constant source of potential evidence for transfer and collection. Forensic hair examiners are sometimes asked whether they can determine if a hair was removed forcibly, during a struggle or assault, for example, to document the severity of the assault. This is a difficult question. Obviously, if the hair has a bulb root (meaning it was removed during the telogen phase), then the question can’t be answered. If tissue from the follicle is attached to the root, then the hair was removed during the anagen or possibly catagen phase, that is, while the hair and the follicle were attached through active cellular growth. Because the actively growing hair is still soft and unkeratinized, the root may stretch before it is torn out of the follicle. Therefore, if the root is stretched and has follicular tissue attached, the examiner may state that the hair was forcibly removed, as shown in Figure 3. That does not, however, tell the examiner what kind of force was used—a violent assault, hair being caught in something or a friendly wrestling match—and the examiner must be cautious about making unsupportable statements. Plucking hairs does not guarantee follicle tissue on the root. King, Wigmore, and Twibell showed in 1981 that only 65% of forcibly removed hairs yielded sheaths. Moreover, of the hairs removed by fast plucking, 53% had sheaths, while of those that were pulled slowly, only 11% had sheath tissue. Sheath cells were always associated with anagen and catagen hairs in this study. This study also suggests that in a bulk sample submitted as evidence, the anagen/telogen ratio may be more significant to the investigator than the presence or absence of sheath material.
FIGURE 4 Roots that are stretched and have follicular tissue attached were probably removed by force; otherwise, this anagen phase hair would have stayed in the follicle. This does not, however, tell the examiner what kind of force was used—a violent assault, hair being caught in something or a friendly wrestling match—and the examiner must be cautious about making unsupportable statements.
