December 20, 2007
James L. Breeling
Hair is grown in the hair follicle, the biochemical factory that produces a hair (see Hair Science: How and Why Hair Grows). In the follicle a hair is living tissue while it is being assembled. Hair is a nonliving fiber after it leaves the follicle. If a hair shaft is mature and undamaged, it is a strong fiber due to the proteins that constitute most of its body and the chemical bonds that hold the proteins packed together.
The bulk of the hair shaft is made up of the inner core called cortex, protected by an outer layer called cuticle. The cortex makes up the largest part of the hair shaft. Its structural components are proteins called keratins, which are also found in skin cells and other cells throughout the body, where they give the cells shape and protection.
What Are Keratins?
Keratins are a family of more than 40 proteins that are found in skin cells, hair, fingernails and toenails, and cells of specialized tissues such as the cornea of the eye and internal organs. Major functions of keratins are to provide mechanical strength, structural integrity and adhesion points for formation of molecular networks.
The structure of the various keratins is complex; each variation in structure is associated with the function of a specific keratin in a specific tissue. Keratins are synthesized in keratinocytes, specialized cells that make up the tissues providing protection and structural support to soft tissues.
Keratins are characterized as Type I which are acidic and Type II which are basic (alkaline) or neutral. Both types occur in hair, which has important implications for the formulation of hair-styling and hair-care products. Ingredients of such products can have effects on either Type I or Type II keratins.
The measuring stick for acidity/alkalinity is pH, the standard for assessing if a solution is more or less acidic than water, or more or less alkaline that water. Water is neutral pH (written as pH 7.0), which is mid-point on the pH scale of 0 to 14. A solution with pH above 7.0 is called basic or alkaline; a solution with pH lower than 7.0 is called acidic. Example relevant to hair-styling products: The products called hair relaxers are alkaline (higher than pH 7.0); the commonly used (and dangerous) hair relaxer applied at home is lye. The activity of the alkaline hair relaxer is halted by applying an acidic solution such as vinegar.
Keratin-associated proteins in the hair cortex can also be characterized in two groups: (1) high-sulfur proteins that are essential to formation of the disulfide bonds that glue keratins together into dense networks, and (2) proteins with high content of the amino acids glycine and tyrosine which also form networks contributing to keratin networks.
Keratin-related proteins act as a biologic adhesive to bind the keratins. Especially important are the high-sulfur keratin-associated proteins which participate in chemical bonding. A biochemical interaction called disulfide bonding between the keratins and high-sulfur keratin-associated proteins forms a keratin network that provides the structural integrity for the texture and shape of the hair shaft.
Disulfide bonds can be broken by chemicals used in hair styling, leaving the hair shaft pliable for shaping into desired forms. The bonds re-form after the styling chemical is rinsed away, leaving the treated hair in its new "style". Very strong chemicals such as bleaches can permanently damage the hair shaft and result in a degraded change in hair appearance.
The cuticle of the hair shaft is the outer layer that protects the cortex. Under a microscope, the cuticle looks like the overlapping scales of dragon skin. Keratin proteins also provide the structural integrity of the cuticle. While the cuticle is tough, it can be damaged by chemicals such as bleach or by overly vigorous brushing of the hair. When damage causes cuticle to break, the result can be (1) trichorrhexis nodosa also called "weathering", where hair easily breaks at weakened points along the hair shaft, or (2) trichoptilosis, also called "split ends" where the end of the hair shaft is shredded into a brushy appearance. Chemical and physical damage can cause trichorrhexis nodosa in otherwise healthy and normal hair. A congenital form of trichorrhexis nodosa occurs in families that have the genetic trait for the condition.
A microscopic look at hair as it grows from the follicle reveals a very complex structure that is organized in concentric layers with the cortex and cuticle at the center. Layers wrapping the cortex and cuticle include the medulla, inner root sheath, outer root sheath and the adventitial (outermost) sheath. The cortex and cuticle are the layers that emerge from the follicle as a hair shaft.
Hair Characteristics and Their Cosmetic Effects
Scalp hair has been described as a person's "crowning glory" by beauty consultants and advertising copy writers. Basically, that's another way of saying that hair is very important to a person's overall appearance-to the "self" that a person wishes to present to the world. Calling hair a crowning glory does not do much to describe the characteristics that make each person's hair unique. There are characteristics that do describe hair, and each of them is associated with cosmetic effects. Those characteristics are:
Let's start with hair color.
Why Does Hair Have Different Colors?
Hair color, like the other hair characteristics (caliber, wave, curl, frizziness) is an inherited trait. When the gene pools of a man and woman are combined in a child, the color of the child's hair will be directed by patterns of genetic inheritance. The patterns are complex but predictable if family genetic histories are known.
The color of hair (and skin) is determined by the pigment in specialized cells called melanocytes. As a child develops from embryo to fetus to infant, genes order certain cells to become melanocytes, then order the melanocytes to produce the pigment melanin. Melanocytes located in the hair follicle synthesize the melanin of hair color. Two types of melanin can be produced: eumelanin is the pigment of brown and black hair, pheomelanin is the pigment of blond and red hair. Melanocytes do not mix the synthesis of pigments as one mixes two colors of paint in a bucket. Only one type of pigment is synthesized per person. Variations in the shade of hair are due to greater or lesser amounts of pigment in the hair shaft.
Pigmentation of hair takes place only when hair is growing from the hair follicle; this growth phase is called anagen, one of the three phases (anagen-catagen-telogen) of hair growth (Click on Hair Science: How and Why Hair Grows). The pigmentation is "fixed" as the melanin is incorporated into the hair cortex. A hair remains the same color until it is eventually shed at the conclusion of the growth cycle.
As a person ages, less melanin is produced and hair transitions from color to gray to white (absence of color). Hair color contributes to a person's appearance in ways other than color alone. For example:
Hair caliber (diameter of the hair shaft) influences the appearance of hair density or fullness. Given about the same number of hair follicles per square centimeter, the person with hair of greater caliber will often appear to have a fuller head of hair than the person whose hair is of smaller caliber (so-called "fine hair"). Greater caliber does not always give an appearance of greater hair density-for example, a person with high contrast between hair color and scalp skin color may appear to have less dense hair if the scalp skin "shows through", no matter what the hair caliber. Hair styling must take hair caliber into account. Styles that work for small-caliber hair may not be appropriate for large-diameter "heavy" hair. An appearance of greater hair density can be created by use of hair-care products that increase hair volume (see What is long-lasting hair volume? and Hair Styling Product Ingredients for Long-Lasting Volume).
Natural curliness of hair ranges from loosely open loops to tightly coiled ringlets. Curliness creates an appearance of greater hair density.
Fashion trends may rule curliness "in" or "out" as a desired hair characteristic. When curliness is "in", people with straight hair often seek curliness by home use of hair-care products or by permanent waving at beauty parlors. When curliness is "out", people with curly hair may use hair "relaxing" agents to straighten curly hair.
Wave is a form of hair curl which also contributes to an appearance of greater hair density. Hair wave can be incorporated into an appropriate hair style, or may be removed by cutting or by temporarily "relaxing" with a hair-care product.
Frizzy hair is not just hair with extra-tight curl. It is hair with unique characteristics most often associated with hair of black people of African ancestry (called African-American in the United States). Frizziness should not be confused with "wooly hair", a hereditary defect that causes tightly curled "wooly" hair in persons of Caucasian ancestry.
Ethnic Differences in Hair and Their Cosmetic Effects
Light blond hair tends to suggest Northern European ancestry, black curly hair Mediterranean ancestry, straight black hair Asian ancestry, and tightly curled "frizzy" hair African ancestry.
Is that a true statement? In a general way it is true, but it would be more correct to say that a person's ancestry can be one determinant of a person's hair characteristics-hair color, caliber, curliness, etc. These "ethnic" characteristics of hair can have consequences important for hair care and hair styling. Ethnic characteristics do not necessarily reflect nationality; for example, blond "Caucasian" hair can be found in people from the Atlantic Ocean to the Ural Mountains in Russia.
Caucasian ancestry is associated with blond, red and brown hair that may be straight, wavy or curled. The hair shaft ranges from round to oval in shape, and is smaller in diameter (caliber) than black Asian hair. Caucasian hair of red color has the fewest scalp hair follicles per square centimeter of all colors of hair; this can have the effect of appearing to be thinning hair, especially if there is high contrast between hair color and scalp color.
Asian ancestry is associated with hair that is black in color, straight, round in cross-section, and of greater diameter (caliber) than Caucasian hair. Although there are fewer hair follicles per square centimeter for Asian hair than for Caucasian, Asian hair may have an appearance of greater density due to the larger diameter of hair shafts. Asian hair can be curled with application of chemical, mechanical and thermal treatment, but traditional Asian hair styles take advantage of the black sheen and dense appearance of Asian hair.
African ancestry is associated with hair that is black in color, tightly coiled, often with a "frizzy" appearance. The frizziness is due to the cross-section and curl of the hair shaft-more flat than round, and somewhat springy in texture. African ancestry is also associated with fewer follicles per square centimeter than Caucasian hair, but hair density can appear to be greater due to frizziness. African-American hair often requires specially formulated hair-care and hair-styling products that address the unique characteristics of African-American hair. Unique characteristics that have implications for hair care and hair styling include: