Identification and Comparison of Hair

Many objects can connect a suspect to a victim or a crime scene; the key is to compare these items to trace evidence found in or on the suspect's person, home, car, or other belongings. Hair and fibers are examples of trace evidence that can be compared to make these connections. The processes involved in examining hair and fiber are very similar, yet each can yield unique and significant information.

The study of the scalp and hair is called trichology. Hair evidence can be transferred during the commission of a crime, so it can be used to establish an association of a suspect with a victim or a crime scene. Before proceeding, it is important to note that it is NOT possible to link hair to a specific individual based on morphology, or structural characteristics of the hair, alone. The value of hair is that it can provide corroborative evidence for placing an individual at a crime site. There has been increasing controversy over the use and interpretation of hair comparison evidence because on occasion, it has been misused. However, under specific conditions, hair may provide DNA evidence which is a valuable forensic resource. Another important contribution from hair evidence is that of toxic substances that may be present in the body. Hair can be tested for toxic substances such as illegal drugs or poisons even after they have been metabolized and excreted by the body. Some substances remain in the hair for months after the last use or exposure! Hair can thus be a very important part of Toxicology testing.

Morphology and Structure of Hair

Hair is made up of a protein known as Keratin; it is the same protein that fingernails are made of. Hair grows from the papilla which is a large structure at the base of the hair follicle. The papilla is made up mainly of connective tissue and a capillary loop. It is responsible for connecting the hair to the body's blood supply and nutrients via the capillary loop. Surrounding the base of the hairs are tiny muscles known as Arrector Pili muscles. These tiny muscle fibers are attached to each hair follicle, which contract to make the hairs stand on end, causing "goose bumps". Arrector pili muscles are found in most mammals including humans. Another important structure involving hair is the Sebaceous Gland. The Sebaceous glands are small glands in the skin which secrete a lubricating oily matter (sebum) into the hair follicles to lubricate the skin and hair.

The basic structure of hair is composed of three regions: the cuticle, cortex, and medulla. Each of these regions has unique structures that can assist investigators in identifying which species the hair came from as well as other characteristics. When thinking of the structure of hair, consider that a hair is like a pencil. A pencil has a thin outer covering of paint followed by a thicker middle layer of wood that surrounds a central layer of "lead" or graphite. A hair is structured in much the same way. Hair has a thin outer covering known as the cuticle, followed by a thicker middle layer known as the cortex and an innermost layer known as the medulla.

Cuticle

The cuticle is the protective coating made of overlapping scales. The scales always point toward the tip of hair and produce a characteristic pattern that can be used for species identification. While there are some variations, there are three basic scale patterns: coronal, spinous, and imbricate.

Hair Scale Patterns image: coronal, spinous, and imbricate scales Coronal scales are described as a crown-like scale pattern found in hairs of very fine diameter which resemble a stack of paper cups. Coronal scales are commonly found in the hairs of small rodents and bats, but rarely in human hairs.

Spinous scales are petal-like scales which are triangular in shape and protrude from the hair shaft. They are found at the proximal region of mink hairs and on the fur hairs of seals, cats, and some other animals. They are never found in human hairs.

Imbricate scales are flattened scales; consists of overlapping scales with narrow margins. They are commonly found in human hairs and many animal hairs, and resemble puzzle pieces under magnification.

Preservation of the scale pattern can be accomplished by laying a piece of hair in a thin layer of clear nail polish on a slide, allowing it to nearly dry, and then removing the hair. The scale pattern can then be view by viewing the slide using a microscope.

Medulla

mouse hair (ladder medulla) under microscope The center of the hair, known as the medulla, is surrounded by the cortex. Not all hairs have a medulla, but when the it is present in humans, it is one of three different types: fragmented, interrupted, or continuous. Animal hair can have special types of medulla patterns not found in humans such as Ladder, or Lattice. These distinct medulla patterns help to differentiate between human and animal hairs if there is question about the origin of the hair. It is important to note that Medullae are only found in animal hair (including humans), and are never found in plant fibers!

Medulla: ABSENT, CONTINUOUS, LATTICE, INTERRUPTED, FRAGMENTAL, LADDER

Scientists are still not sure of the purpose of the medulla, but some believe that it may be simply an air space within the central portion of the hair.

The medullary index is a measure of the diameter of the medulla relative to the diameter of the hair shaft. In humans, the medullary index is that the width of the medulla is less than ⅓ of the total width of the hair. In other animals, however, the medullary index is greater than ½ the total width of the hair. The presence of a medulla can vary, even from hair to hair in the same source. People of European and African descent may have fragmented medullae or they may not have any medullae at all. People of Asian descent usually have continuous medullae. Most other animals have continuous or interrupted medullae and the shape of the medulla can help identify different species.

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