Douglas W. Deedrick  Sandra L Koch

Supervisory Special Agent Physical Scientist, Forensic Examiner
Scientific Analysis Section Trace Evidence Unit
  Federal Bureau of Investigation
  Quantico, Virginia
Introduction

During the course of a criminal investigation, many types of physical evidence are encountered. One of the most common is hair evidence. The identification and comparison of human and animal hairs can be helpful in demonstrating physical contact with a suspect, victim, and crime scene. Hairs can provide investigators with valuable information for potential leads.

Until recently, the comparison microscope was considered the only reliable tool for the identification and comparison of the microscopic characteristics found in hair. Today, nuclear and mitochondrial DNA (mtDNA) testing can provide additional information that can influence the value of microscopic examinations. When the microscope is coupled with DNA technologies, the combination of these technologies profoundly affects the way forensic scientists, investigators, and prosecutors view hair evidence.

Although DNA technologies may add significant information to hair evidence recovered at a crime scene, the first step necessary in the analytical process is the identification and comparison of human and animal hairs. This revision of the 1977 Microscopy of Hairs: A Practical Guide and Manual by John W. Hicks is intended to introduce hair evidence to the forensic examiner and to provide a foundation for its proper identification and comparison.

Basic Structure of Hair

A hair can be defined as a slender, thread-like outgrowth from a follicle in the skin of mammals. Composed mainly of keratin, it has three morphological regions—the cuticle, medulla, and cortex. These regions are illustrated in Figure 1 with some of the basic structures found in them. The illustration is a diagram used to emphasize structural features discussed in this guide. Certain structures may be omitted, and others enhanced for illustrative purposes.

Figure 1.
Hair Diagram

A hair grows from the papilla and with the exception of that point of generation is made up of dead, cornified cells. It consists of a shaft that projects above the skin, and a root that is imbedded in the skin. Figure 2 diagrams how the lower end of the root expands to form the root bulb. Its basic components are keratin (a protein), melanin (a pigment), and trace quantities of metallic elements. These elements are deposited in the hair during its growth and/or absorbed by the hair from an external environment. After a period of growth, the hair remains in the follicle in a resting stage to eventually be sloughed from the body.

Figure 2.
Diagram of
Hair in Skin

Cuticle

The cuticle is a translucent outer layer of the hair shaft consisting of scales that cover the shaft. Figure 3 illustrates how the cuticular scales always point from the proximal or root end of the hair to the distal or tip end of the hair.

Figure 3.
Scanning Electron
Photomicrograph
of Hair

There are three basic scale structures that make up the cuticle—coronal (crown-like), spinous (petal-like), and imbricate (flattened). Combinations and variations of these types are possible. Figures 4-9 illustrate scale structures.

The coronal, or crown-like scale pattern is found in hairs of very fine diameter and resemble a stack of paper cups. Coronal scales are commonly found in the hairs of small rodents and bats but rarely in human hairs. Figure 4 is a diagram depicting a longitudinal view of coronal scales, and Figure 5 is a photomicrograph of a free-tailed bat hair.

Figure 4.
Diagram of
Coronal Scales

Figure 5.
Photomicrograph
of Bat Hair

Spinous or petal-like scales 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. Figure 6 is a diagram of spinous scales, and Figure 7 is a photomicrograph of the proximal scale pattern in mink hairs.

Figure 6.
Diagram of
Spinous Scales

Figure 7. Photomicrograph of Proximal Scale Pattern (Mink)

The imbricate or flattened scales type consists of overlapping scales with narrow margins. They are commonly found in human hairs and many animal hairs.

Figure 8 is a diagram of imbricate scales, and Figure 9 is a photomicrograph of the scale pattern in human hairs.

Figure 8.
Diagram of
Imbricate Scales

Figure 9.
Photomicrograph
of Scale Pattern
(Human)

Medulla

The medulla is a central core of cells that may be present in the hair. If it is filled with air, it appears as a black or opaque structure under transmitted light, or as a white structure under reflected light. If it is filled with mounting medium or some other clear substance, the structure appears clear or translucent in transmitted light, or nearly invisible in reflected light. In human hairs, the medulla is generally amorphous in appearance, whereas in animal hairs, its structure is frequently very regular and well defined. Figures 10 through 13 are photomicrographs of medullary types found in animal hairs. Figure 10 exhibits a uniserial ladder, and Figure 11 exhibits a multiserial ladder, both found in rabbit hairs. Figure 12 exhibits the cellular or vacuolated type common in many animal hairs. Figure 13 exhibits a lattice found in deer family hairs.

Figure 10.
Photomicrograph
of Uniserial
Ladder Medulla

Figure 11.
Photomicrograph
of Multiserial
Ladder Medulla

Figure 12.
Photomicrograph
of Animal Hair

Figure 13.
Photomicrograph
of Deer Medulla

When the medulla is present in human hairs, its structure can be described as—fragmentary or trace, discontinuous or broken, or continuous. Figure 14 is a diagram depicting the three basic medullary types.

Figure 14.
Diagram of
Medullas
(Trace, top;
Discontinuous,
middle;
Continuous,
bottom)

Cortex

The cortex is the main body of the hair composed of elongated and fusiform (spindle-shaped) cells. It may contain cortical fusi, pigment granules, and/or large oval-to-round-shaped structures called ovoid bodies.

Cortical fusi in Figure 15 are irregular-shaped airspaces of varying sizes. They are commonly found near the root of a mature human hair, although they may be present throughout the length of the hair.

Figure 15.
Photomicrograph
of Cortical Fusi
in Human Hair

Pigment granules are small, dark, and solid structures that are granular in appearance and considerably smaller than cortical fusi. They vary in color, size, and distribution in a single hair. In humans, pigment granules are commonly distributed toward the cuticle as shown in Figure 16, except in red-haired individuals as in Figure 17. Animal hairs have the pigment granules commonly distributed toward the medulla, as shown in Figure 18.

Figure 16.
Photomicrograph
of Pigment
Distribution in
Human Hair