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Facts of the Matter Richard Brill |
Hair’s protein roots
just a beginning
to complex processHair is as complex as our fascination with it, yet hair is dead. This is not a value judgment -- it's just a fact. Hair is composed of specialized skin cells that died before they ever saw the light of day.
Hair is part of our mammalian heritage. It evolved to help keep the body warm, to protect the body from sunlight, thorns and scrapes and perhaps for other reasons as well. It may be that humans are no longer hairy because the use of shelter and clothing rendered full-body hair more of a parasite-harboring liability than an asset. Having been replaced by clothing as a body covering, hair today functions as status symbol, sex symbol, fashion and political statement.
Not really the "hairless ape," we are still covered with hair, except on the palms of the hands, the soles of the feet, and the lips.
The "hairless" parts of our bodies are populated by vellus hairs, which are short, immature hairs that contain little or no pigment. These hairs may be too fine and too short to be seen with the naked eye, but they are present on us all.
Terminal hairs are the long hairs that grow on the head, armpits and pubic regions, and often on the body, arms and legs, too. As opposed to vellus hairs, they are produced by follicles with sebaceous glands. In certain people who are genetically predisposed, the hairs in these follicles gradually become thinner and shorter until they look like vellus hairs, causing baldness or extreme thinning of scalp hair.
Polypeptide chains and the alpha helix
Hair is made from a remarkable protein called keratin that also makes up feathers, claws, nails, hoofs and the outer layers of skin. The difference in hair is the way the protein is arranged and structured. Belying its uncomplicated appearance, the structure of hair is much more complex than you might imagine.Keratin begins with amino acids, a collection of just 20 molecules that are the building blocks of all proteins because they can link end to end to form chains known as polypeptides. Each protein consists of a unique amino acid sequence, the blueprint for which is coded in sequences within the DNA molecule. The shape of the amino acid molecule causes the chain to coil like the stretchy cord of a telephone handset. Most proteins are formed from this structure that is known as the "alpha helix."
The Cortex
In the interior (cortex) of an individual hair shaft, three alpha helixes are twisted together to form a "protofibril." Eleven protofibrils are then bundled to form an eleven-stranded cable known as the "microfibril." Microfibrils are embedded in a protein matrix similar to the keratin in fingernails. Hundreds of such microfibrils are coiled and cemented into an irregular fibrous bundle called a "macrofibril." These macrofibrils are then grouped to form the cortex of the hair fiber.In the center of the coils lies the medullary canal, which is actually a part of the excretory system. It houses foreign debris, heavy metals, synthetics and medications that are thrown off by the body and eventually released when the hair is shed.
In addition to the strong peptide bonds that form the keratin proteins, there are weaker interactions between atoms within the coils of the cortex in the form of hydrogen bonds and sulfur bonds.
Hydrogen bonds form between oxygen and hydrogen atoms parallel to the axis of the alpha helix. Picture coils of a telephone cord connected by rubber bands. Hydrogen bonds account for much of the elasticity of hair, and are broken by water, so hair softens when wet.
Sulfur bonds form cross-links between sulfur atoms on adjacent polypeptide chains. Picture two handset cords held together with twist ties. Sulfur bonds are responsible for the hair's toughness and abrasion resistance as well as its texture. Knowing how to break and reform sulfur bonds using chemicals allows us to give hair a permanent wave.
Cuticle protects the cortex
The outer layer of the hair, the cuticle, covers the cortex along the length of the hair and protects it like the bark on a tree. The cuticle is made of overlapping layers of long scales that lie along the surface of the hair like tiles on a roof, with their free edges directed toward the tip.It's the condition of the cuticle scales that determines whether you're going to have a bad hair day. Cuticle that is in good shape is unbroken and lies flat, making hair look shiny and smooth.
Hair growth
Hair grows from a follicle, which is a tiny cup-shaped pit buried in the skin. It is well-supplied with minute blood vessels, which nourish the growing cells within.There are about 100,000 hair follicles on the typical head, but some people have as many as 150,000. On a baby's head there are about 1,100 follicles per square centimeter, by the age of 25 there are little more than half that many, by age 50 only half of those remain with only a slight reduction with age beyond that.
There are two regions to the follicle, the hair bulb and the mid-follicle region. The hair bulb, which lies inside the hair follicle, is the location of actively growing cells that eventually become the long fine cylinder of a hair.
New cells are continuously produced in the lower part of the bulb. As they grow they steadily push the previously formed cells upward. When the cells reach the upper part of the bulb they change, half becoming the lining of the follicle, the other half the shaft of the hair.
The pigment called melanin is produced in the follicle and moves upward, eventually to be embedded in the keratin coils of the cortex.
At the base of the hair follicle is the dermal papilla, fed by capillary blood vessels to supply oxygen, energy and the amino acids needed for growth. The dermal papilla is sensitive to male hormones (androgens) that regulate hair growth and in scalp hair may cause the hair follicle to gradually shift to the production of vellus hair instead of terminal hair in individuals who are genetically predisposed to this type of hair loss.
In the mid-follicle region the actively growing cells die and harden into what we call hair. As the cells below continue to divide and push upward, the hair grows upward too, out of the skin.
Hair growth cycle
The follicle is a tiny but powerful factory that works hard, but not continuously. Each follicle undergoes a cycle of activity that includes a brief vacation. At any one time only around 85% of our follicles are working, the remainder being in resting stages.The active growth (anagen) phase lasts for three to seven years without interruption. Pigment (melanin) is made in the hair bulb throughout this phase of the hair cycle, and shortens with advancing age.
How long anagen lasts is determined genetically, and varies from one person to another. It is the length of this time that determines how long the hair will grow before it falls out.
Following the growth phase a period of breakdown and change (catagen phase) lasts for between two and four weeks. During this phase the follicle stops producing hair and pigment and the base of the follicle moves upward toward the surface of the skin.
The phase of resting and shedding (telogen phase) lasts for three or four months. During this phase a new hair begins to grow from the follicle as the old hair is shed or pulled out, which happens easily and painlessly. These are the hairs that come out when you shampoo or brush your hair. Eventually a new hair emerges from the same opening at the surface of the skin as the old one, the follicle moves downward and the hair cycle begins again.
At any one time, around 10 percent of the follicles on an individual's head are in the telogen phase. Each follicle grows about 20 new hairs in a lifetime as each hair eventually falls out and is replaced by a new one.
Hair color
Color is perhaps the most obvious characteristic of hair, but as far as we know it has no biological function in humans, other than as a factor in sexual selection. Unlike the melanin in skin, it does not protect the hair from the harmful effects of sunlight.The color of hair is due to granules of melanin in the cortex, having been formed in special pigment-producing cells (melanocytes) in the hair bulb during the growing (anagen) phase of each hair.
There are two forms of melanin, the dark pigment that predominates in black and brunette hair (eumelanin) and a lighter pigment (phaeomelanin), that is found in red and blond hair.
The more eumelanin there is in the mixture, the darker is the hair. Most hair contains a mixture of the two pigments. The mixture (and the shade) varies from one person to another, but also among the hairs on one person's head. Eumelanin is actually yellow in color, so the range of colors produced by melanins is limited to shades of yellow, brown, red and black. The combination of pigments in the mixture is determined by the individual's genes.
Hair is another one of those wonders of nature that we take for granted even as we pamper it with billions of dollars in care and products and spend millions in advertising to make us more aware of it.
But when it comes right down to it, if you're having a bad hair day, blame your hydrogen bonds.
Richard Brill picks up where your high school science teacher left off. He is a professor of science at Honolulu Community College, where he teaches earth and physical science and investigates life and the universe. He can be contacted by e-mail at rickb@hcc.hawaii.edu
