Overview

Introduction

Forensic hair examination is a unique field which was developed specifically for criminal investigations and which is not practiced in industrial or academic fields. It is, however, based on concepts and information developed in medicine, physical anthropology, wildlife studies, and the cosmetics industry. Knowledge of background information from these fields is essential to a forensic hair examiner. Of particular importance are the structure of hair, hair growth, the chemistry of hair, and other miscellaneous information on hair.

Hair Structure

Hair is a fibrous outgrowth from the skin of mammals which grows from papillae embedded in the bases of follicles situated in the dermis or true skin. Structurally, hair consists of three main parts: the cuticle, cortex, and medulla.

The cuticle

The cuticle, the outer scaly layer of the hair, is composed of flat overlapping cells or scales. Maintaining the hair’s structural integrity, binding (preventing hair from fraying and breaking away in small fragments), and prevention of transfer of soluble substances from outside to inside the hair are the major functions of the cuticle.

The cortex

The cortex, which constitutes the major part of most hairs, consists of elongated cells fused together with intercellular binding material. Cortical cells consist of spindle-shaped fibrous structures called macro-filaments (macro fibrils), nuclear remnants, and pigment granules. Nuclear remnants are small, elongated cavities near the center of the cells.
Pigment granules are small, spherical particles 0.20.8 um in diameter. They are composed of melanin, of which there are two types: eumelanin (brown-black) and pheomelanin (yellow-red).
Macro fibrils are mainly composed of micro fibrils, which in turn are made up of proto fibrils consisting of precise arrays of low sulphur proteins containing short sections of alpha helical proteins in a coiled formation.
Trapped within the cortex may be small air spaces called cortical fusi or vacuoles. As the hair shaft is pushed upward from the papilla, passage through the follicle transforms the cortical cells from an ovoid to an elongated spindle shape. The irregularly shaped tissue fluid-filled cavities that are carried with the cells later form the fusi. As the fusi are borne upward, they become longer and thinner and, as the cortex dries out, they lose their tissue fluid and become air filled. Although in some hairs fusi can be found throughout the shaft, they are most common near the root. In general, the lighter the hair, the more fusi will be observable.
Because they are air filled, cortical fusi appear opaque under transmitted light. Though they are generally larger than pigment granules, they can sometimes be close in size, making distinction difficult. An examiner in doubt as to whether dark spots are pigment granules or cortical fusi can differentiate them by focusing up and down on the microscope. Fusi disappear from view, whereas pigment granules remain as out-of-focus dark spots. Alternatively, top lighting can be used. With this illumination, true fusi will appear bright.

The medulla

Most hairs usually have a central core, termed the medulla, which is composed of shrunken cells which may or may not contain pigment. The spaces between the medullary cells are usually filled with air, giving an opaque appearance under transmitted light. When cells are damaged or for some reason very shrunken, the mounting medium will fill the air spaces, leading to a translucent appearance. Depending on the type of hair and the individual, the size of the medulla is highly variable – constituting anything from 0 to 95% of the hair. In addition, the general appearance of the medulla varies enormously depending on the species from which the hair originated, the type of hair, body area of origin, and the portion of the hair shaft viewed.

Hair Growth

Growth of hair is achieved by the proliferation of cells in the matrix of the follicle and by their increase in volume as they move into the upper bulb. Since the cells are under pressure as they pass through the neck of the bulb, there must be a constraining mechanism to funnel them upward and to keep them from expanding laterally. Henle’s layer of the outer root sheath is the first to become firmly keratinized and forms the first part of the funnel. The outer sheath, the vitreous membrane, and the connective tissue sheath give the funnel resiliency and firmness. Throughout its life, each hair follicle undergoes recurring cycles of active growth, regression, and rest. This is known as the hair cycle.
In most mammals, the hair cycle in each region of the body is synchronized, producing waves of growth activity which manifest themselves as seasonal variations. In humans (and guinea pigs), follicular activity is not synchronized, leading to what is termed a mosaic cycle, whereby neighboring follicles operate independently and are normally at different stages of the cycle.
There are three main stages in the hair cycle. Anagen is the phase of active growth; catagen the phase of regression; and telogen the resting stage. At the end of the active growth of the anagen phase, activity of the matrix ceases, and the hair moves into the catagen stage. There is an upward movement of the hair root which becomes bulb-shaped and surrounded by a capsule of partially keratinized cells, around which is an epithelial sac of epidermal cells. Once catagen is complete the hair simply rests in the telogen stage with no noticeable changes until anagen commences in the follicle. In the next phase, anagen, the follicle is reconstituted and a new hair grows up alongside the bulb hair. The bulb-shaped telogen hair is usually retained until anagen is well-advanced in the follicle, at which time it is shed. The human scalp contains compound follicles in which two, three, or even more hairs, each produced by a separate follicle, share a common orifice to the outside.
In alopecia (common baldness), normal follicular activity ceases. In telogen effluvium, psychological factors, disease, or pregnancy cause the cycle of many hairs to transform prematurely to the telogen stage and be shed. Side effects of cancer chemotherapy are the only known instance in which hair is shed from follicles which continue in the anagen phase.
In a healthy human scalp approximately 85-95% of the follicles are in anagen, 4-14% in telogen, and less than 1% in catagen. The average daily loss of human scalp hairs is about 75 to 100.
As seen in Table 1, the rate of growth of human hairs varies with location on the body. In addition, age, sex, race, hormones, and individual factors all influence the rate of hair growth. Although they appear to play little part in human hair growth, environmental influences have a considerable effect on animal hair growth.
The duration of the anagen phase ranges from 2 years and 2 months to 6 years and 6 months; catagen lasts about 2 weeks, and telogen approximately 1 year. These figures can, however, be quite variable. After 3-7 years an individual will not have any of the same hairs on his or her scalp as he/she has today. This has important implications for hair comparisons in that if a period of more than about 5 years elapses between collection of the questioned and known samples, a meaningful comparison may not be possible. In other parts of the body, the hair cycle is considerably shorter than that for scalp hairs.

Table 1 Rate of growth of hairs on various parts of the human body

Chemistry of Hair

Human hair is composed of 65-95% protein, up to 32% water, 1-9% lipids, and less than 1% pigment and trace elements. The protein is composed of amino acids, 21 of which have been reported in human hair. The amino acid content of hair varies with the structural components of the hair and is influenced by genetics, weathering, cosmetic treatment, and diet. Water content of hair is dependent on relative humidity. The lipids in hair come principally from sebum and consist of free fatty acids and neutral fat (esters, glyceryl, wax, hydrocarbons, and alcohols). The chemical structure of hair creates unique physical characteristics. Strong disulfide bonds linking adjacent keratin chains produce a structure that is extremely resistant to chemical and biological degradation.
The chemical content of hair is influenced by genetic, environmental, and dietary factors. Chemical content can in turn influence structural and morphological characteristics. For example, it has been noted that the hair of persons suffering from extreme protein malnutrition is mechanically weaker, finer in diameter, sparser, and less pigmented than the hair of healthy individuals.
A large number of different trace elements have been reported in human hair. They can arise from metabolism, cosmetic preparations, or the external environment. Trace element content of hair is affected by personal factors (age, sex, diet, etc.), external factors (shampoos, air-borne contaminants, etc.), frequency of hair washing, the structure of the hair itself, and by sample treatment prior to analysis. Furthermore, hair trace element content varies with location on the body, from hair to hair on the same body area, from root to tip along the hair shaft, across the diameter of the hair, and according to the stage in the hair growth cycle. Because of such variation and sample treatment problems, trace element analysis of hair is not considered to be a reliable method of forensic hair comparison.

Miscellaneous Information on Hair

Animals have four types of hairs. These are the coarse outer guard hairs, the fine under fur hairs, intermediate hairs with characteristics in between the preceding two types, and vibrissae or tactile hairs such as whiskers. Similarly, there are four types of human hairs. The most obvious type is the terminal hairs such as scalp, pubic, beard, axillary, eyebrow, eyelash, and nasal hairs. Vellus hairs are fine, short, unmedullated hairs which cover the entire body. Again, there are intermediate hairs between these two types. Lanugo hairs are fine, soft, unpigmented, unmedullated hairs which cover the body in the prenatal period. Three classes of human terminal hairs have been identified: hair that is the same in both sexes (such as scalp hair); hair that is under hormonal influence and behaves as an ambosexual character (e.g. pubic and axillary hairs); and hair (such as beard hair) that serves as a true secondary sexual character.
The presumed functions of hair include decoration, protection against injury, increasing the perception of touch, insulation, acting as a filtering mechanism, and drawing away perspiration.
The adult human body has about 5 million hair follicles, the distribution of which over the body being somewhat sex and age related. Hair grows on all parts of the human body except the following: eyes, lips, areolae of nipples, umbilicus, anus, urogenital openings, soles, palms, finger tips and tips of toes.
The traditionally accepted average number of hairs on the scalp is 100 000-120 000, although a more recent estimate placed the number at 150 000200 000. All estimates have been based on extrapolations of counts over small areas. No one has actually counted the number of hairs on a complete scalp. Further, there are no accurate data available on the average number of pubic hairs on the human body. In view of the implications to forensic science (such as collecting known hair samples), this lack of basic research is distressing.
Gross hair color is influenced by the microscopic color of individual hair shafts, the degree of medullation, cortical fusi, density of hair growth, hair diameter, cleanliness, and artificial treatment, as well as the angle and type of illumination. Microscopically, hair color is principally influenced by the type, density, and distribution of hair pigment.
Animals can show seasonal changes in hair color. The only comparable effect on human hair is the sun bleaching of the distal ends of scalp hairs often noticed in summer. Human hair color can change with age, generally becoming darker after childhood and then often gray to white in later life. The most noticeable change is graying, which is usually a manifestation of the aging process. The graying process results in a gradual dilution of pigment in affected hairs. In the early stages of this process, a full range of color from normal to white can be seen, both along individual hairs and from one hair to the next. Loss of color is associated with the decrease and eventual cessation of tyrosine activity in the lower bulb. Graying occurs at different rates in different areas of the body. Beard hair is usually the first to turn gray and body hair the last. The average age for the onset of graying in scalp hair is 34 in Caucasians, and by age 50, 50% of the population have at least 50% gray hairs. This grayness usually starts at the temples and gradually extends to the top of the head. Although graying is more obvious in dark-haired individuals, the rate at which hair pigmentary content is diminished is independent of the initial pigment concentration. Some diseases can temporarily affect hair color, and certain environmental influences can lead to detectable color changes in people with light colored hair. The hair of heavy smokers can be stained yellow. Exposure to high concentrations of copper can induce a greenish cast. Cobalt workers sometimes develop blue hair, and indigo handlers can have a deep blue tint in their hair.
The most important hair defects and diseases are monilithrix (beaded hair), pili torti (twisted hair), trichorexis nodosa (frayed nodes), trichorexis invaginata (bamboo hair), trichonodosis (knotted hair), and pili annulati (ringed hair). A detailed discussion of hair diseases and abnormalities is beyond the scope of this article. However, since they are occasionally encountered in forensic hair examinations, the reader is referred to the further reading list.
The disappearance of pigments from the hair shaft due to graying or disease creates tiny cavities in the shaft. The eventual compression of these cavities can cause a decrease in hair shaft diameter with age.