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Basic Structure of Hair
Hair can be defined as slender, thread-like outgrowth from follicles (opening in the skin through which a hair grows) in the skin of humans and animals. Composed mainly of keratin, it has three morphological regions - the cuticle, medulla, and cortex.
Hair Diagram
(Certain structures may be omitted in the photos, and others enhanced for illustrative purposes)
Cuticle
The cuticle is a translucent outer layer of the hair shaft consisting of scales that cover the shaft. Cuticular scales always point from the proximal or root end of the hair to the distal or tip end of the hair.
There are three basic scale structures that make up the cuticle:
- Coronal (crown-like) - 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.
- Spinous (petal-like) - 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.
- Imbricate (flattened) - Imbricate or flattened scales type consists of overlapping scales with narrow margins. They are commonly found in human hairs and many animal hairs.
Combinations and variations of these types are possible.
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.
When the medulla is present in human hairs, its structure can be described as fragmentary or trace, discontinuous or broken, or continuous.
Cortex
The cortex is the main body of the hair composed of elongated and fusiform (spindle-shaped) cells. It may contain:
- Cortical Fusi - Air spaces located in the cortex of hairs. They are commonly found near the root of a mature human hair, although they may be present throughout the length of the hair.
- Pigment Granules - 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, except in red-haired individuals. Animal hairs have the pigment granules commonly distributed toward the medulla.
And/Or
- Ovoid Bodies - Large oval-to-round-shaped structures - Large (larger than pigment granules), solid structures that are spherical to oval in shape, with very regular margins. They are abundant in some cattle and dog hairs as well as in other animal hairs. To varying degrees, they are also found in human hairs.
A hair grows from the papilla and with the exception of that point of generation is made up of dead, cornified (horny) cells. It consists of a shaft that projects above the skin, and a root that is imbedded in the skin. The photo below shows 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.
Diagram of Hair in Skin
References:
1) Forensic Science Communications - January 2004 - www.fbi.gov/
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