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Biology 103
2001 Second Web Report
On Serendip


Joelle Webb

As visually oriented animals, most humans make an effort to have a presentable appearance when they go out into the world. In our attempts to "dress to impress" we spend a substantial amount of time fussing with our hair. Though, over the course of evolution, natural selection has managed to greatly reduce the amount of hair on our bodies, humans have devised a number of ways to tame their manes. Technological advances allow us to braid, cut, crop, dry, dye, gel, highlight, pin, perm, shave and straighten our hair (among other things). The majority of these grooming rituals are practiced by females. Being of that group, I wished to find out about the basic structure of hair, the nature of growth cycles that occur and the biological basis for racial variations of texture.

Hair is defined as "slender outgrowth of the skin of mammals 3 . Even with five million hair follicles, an adult human cannot be considered a hairy animal in comparison to other mammals 1. One fifth of these follicles can be found on the scalp; most noticeably, this site also usually harbors the longest hair on the human body. Hair is found at various densities and lengths over most of the human body except on the faces of the hands and the bottom of the feet. Microscopic features of the hair like its color, shape and root appearance make them distinguishable between species and individuals 3. They serve as an identifier on a microscopic level, when scientists examine singular strands of hair, and an overt level, when you notice your friend's new haircut.

The structure and composition are important factors in identifying how they can be used for identification. Close to the skin surface of the human body, cells under the surface of the skin "differentiate and become keratinized to form the hair cortex (Co) and surrounding hair cuticle (Hc) of the hair shaft at the center of which is situated the medulla (M)" 1. The three tiered Inner root sheath (IRS) surrounds the hair shaft while incorporating a product called trichohyalin 1. Before emerging to the skinís surface the IRS disintegrates, leaving only the hair cortex and a protective cuticle visible. A micro-organ called the dermal papilla (DP) resides at the base of the hair shaft and controls its growth cycles. This module is also vital to the subject of this paper because it regulates the thickness of the hair fiber and the rates at which the follicles produce new cells. The Dermal papilla is central to understanding the physical properties of hair that we see with our naked eye embodied in each strand. If the size of a DP is greatly reduced the hair follicle is rendered inactive 1. Though the hair may seem like a simple structure, it is composed of many complex parts.

A website called performed a study on "Hair follicle growth rates by age and body region 2." Data was not divided into male/female studies but data collected from each sex was compiled together. The results showed, as expected, that hair growth slows with age. Not surprisingly, scalp hair grew the fastest while eyebrows had an overall slower growth rate. Another study was done by the same website on the regeneration of plucked hair. Scalp hairs, which tend to be the longest and grow quickly, took the longest amount of time to grow back. A similar study with participants from various races would be helpful but these studies show that even hairs on one human body do not behave exactly alike. In the same way one might argue that biological variations associated with race are possible.

Many studies on hair growth have been done by criminologists and dieticians because hair provides a long history for human activities. Some clues that can be picked up from examining a strand of hair include evidence of drug use, nutritional deficiencies and damage from chemotherapy. A forensic technique for hair analysis currently being developed is one that would identify the race of the owner 4. This would require the categorization of people by race and the establishment of racial characteristics. Further, some scientists argue that there is no biological basis for races thus nullifying the entire classification process. Some proponents suggest that testing would not attempt to accurately determine the race of a person, but identify which groups they are more or less likely to be members of 4. Alternately, the Federal Bureau of Investigation has created a model group of characteristics for each of three groups: Mongoloid, Caucasoid and Negroid 3. These archetypes are based on head hairs from representative members of each race. (Head hairs are the easiest to analyze since they are commonly found at crime scenes and easily harvested.) Creating racial distinctions based on hair type is an emotionally charged controversy, but scientists canít deny physical differences in hair construction which are connected with racial groupings.

Morphological differences in curvature of hair and other characteristics can be associated with specific populations and are products of the original biological structure. For African Americans, the complicated texture of hair and decreased manageability make complex grooming practices and products preferred. There are four basic physical properties that are specific to the hair of people of African descent 5. The shape of the shaft is flat or elliptical and contains weak areas. Overall low tensile strength demonstrates the fragility of each strand. Consequently, "it can be five times more difficult to comb curly, wet African-American hair than straight, wet Caucasian hair 5." The fourth property is related to the absence of sebum in the hair shaft that results in spiraling and low moisture content. The difficult nature of their hair has led many women, particularly black females, to treat their hair with chemicals and heat. These include perms and flat irons which use heat to straighten hair. Correspondingly, almost ĺ of African-American women complain of breakage. People with brittle dry hair look for treatments to make their hair more manageable and unfortunately end up making it even more susceptible to breakage.

Scientists have identified microscopic characteristics that apply to Caucasoid and Mongoloid head hair strands. Responsibility for similar properties is given to analogous dermal papilla within a race. Obviously, there is not and never will be a pure race, but one can infer that hair similarities between people of the same origin are biological in nature.

WWW Sources

1) Hair Biology , Hair Follicle Structure

2) Hair Biology , Hair Follicle Growth Rates

3) Hairs, Fibers, Crime, and Evidence , Part 1: Hair Evidence

4) The Further Mis-measure , The Curious Use of Racial Categorizations in the Interpretation of Hair Analyses

5), Dermatology Times

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