MICROANATOMY
A hair is a complicated, composite material with many intricately organized structures—only some of which are visible under the microscope. A single hair on a macroscale has a root, a shaft, and a tip, as depicted in Figure 1. The root is that portion that formerly was in the follicle, the proximal (the direction toward the body) most portion of the hair. The shaft is the main portion of the hair. The tip is the distal (the direction away from the body) most portion of the hair. Internally, hairs have a variable and complex microanatomy. The three main structural elements in a hair are the cuticle, the cortex, and the medulla. The cuticle of a hair is a series of overlapping layers of scales that form a protective covering. Animal hairs have scale patterns that vary by species, and these patterns are a useful diagnostic tool for identifying animal hairs, shown in Figure 2. Humans have a scale pattern called imbricate, but it is fairly common among animals and, despite attempts to use scales as an individualizing tool for human hairs, is not generally useful in forensic examinations. The next structure is the cortex and makes up the bulk of the hair. The cortex consists of spindle-shaped cells (sometimes called fusiform) that contain or con strain numerous other structures. Pigment granules are found in the cortex and are dispersed variably throughout the cortex. The granules vary in size, shape, aggregation, and distribution—all excellent characteristics for forensic comparisons. Small bubbles, called cortical fusi, may appear in the cortex; when they do appear, they may be sparse, aggregated or evenly distributed throughout the cortex. Cortical fusi also vary in size and shape. Many telogen root hairs will have an aggregate of cortical fusi near the root bulb; it is thought that this is related to the shutdown of the growth activity as the follicle transitions from catagen to telogen phase. This “burst” of fusi, then, is most likely related to physiology, so it is not necessarily useful for forensic comparisons, as pictured in Figure 3.
FIGURE 1 Macroscopically, a single hair has a root, a shaft, and a tip (a). The root is that portion that formerly was in the follicle, the proximal (the direction toward the body) most portion of the hair. The shaft is the main portion of the hair. The tip is the distal (the direction away from the body) most portion of the hair. Microscopically, a hair consists of three main portions (b). The cuticle is an outer covering of overlapping scales, like shingles on a roof. The cortex is the main mass of the hair and contains numerous microanatomical features. The medulla is the central portion of the hair and consists of air- or fluid-filled cells.
FIGURE 2 The cuticle of a hair is a series of overlapping layers of scales that form a protective covering. Animal hairs have scale patterns that vary by species, and these patterns are a useful diagnostic tool for identifying animal hairs. Humans have a scale pattern called imbricate (outlined in red), but it occurs among animals as well. Despite attempts to use scales as an individualizing tool for human hairs, they are not generally useful in forensic examinations.
FIGURE 3 Numerous microanatomical features are useful in the examination of hairs, including pigment granules, cortical fusi, and ovoid bodies (odd structures that look like very large pigment granules).
Odd structures that look like very large pigment granules, called ovoid bodies, may appear irregularly in the cortex. They may, in fact, be large, aggregated, or aberrant pigment granules, but no one knows; little if any research has been con ducted on what ovoid bodies are. Another phenomenon that can be found in hairs is cortical or medullary disruptions. These appear as if a small explosion occurred in the middle of the hair and may be found singly or in multiples.
