Causes and Types
Diet for Good Hair
Straight and Curly Hair
Styling or Reshaping Hair
Hair Growth Cycle
Normal Shedding of Hair
Hair comes in many different forms: scalp hair, facial hair, body hair, and so on. In different parts of the body, hair varies both structurally and biologically. For example, scalp hair is different than the fine, fleecy, vellus hair found on our face.
Hair also varies in shape, length, rate of growth, and response to stimuli. Factors such as hormones, temperature, light and nutrition affect the rate of hair growth and the strength of the hair. The lengths of the phases in the hair life cycle also vary in different parts of the body. For example, scalp hair has the longest growth phase, and grows about 0.35 mm each day.
There are on average 100,000 to 150,000 hairs on the human scalp. Hairs grow from follicles that are sac like structures under the skin. At any time 90% of the hair are in the growth phase, which lasts an average of 3 to 6 years, after which it enters the resting phase, which lasts for 5 to 6 weeks. At the end of the resting phase, the hair falls out naturally and is replaced by a new hair. Some hair loss is quite normal. A hair drops out because a new one has developed and grown underneath it. Healthy adults shed 50 to 150 hairs each day.
Hair is composed of three basic layers:
- the cuticle, the outer protective layer
- a second, thicker layer called the cortex
- sometimes a third, inner layer called the medulla
The cuticle is the continuous layer of waxy substances covering over the outer surfaces of the epidermis, serving as the protective scales. The cortex is the second layer that provides strength to the hair shaft, and determines the color and texture of hair. The medulla is the inner layer that is present only in thick, large hairs.
The outer protective cuticle can be damaged by chemical or mechanical means, such as hair coloring or blow-drying. When the cuticle is damaged by such means, the protective scales are peeled away. This exposes the rest of the hair shaft. In some cases, even the innermost layer--the medulla--is exposed for further damage. Frayed scales of the cuticle impart a dull appearance to hair. An intact cuticle is responsible for the strength, shine, smoothness, softness and manageability of healthy hair. A layer of sebum coating the cuticle also adds to the shine. Conditioning agents can help to improve the look and manageability of damaged hair but cannot repair the shaft.
The root of the hair is inside the hair follicle. The root looks like an onion at the base of a plucked hair. In the root, the dermal papilla is the only true dermal part of the follicle. These cells play essential roles in regulating hair growth, hair cycle, and the size of the resultant hair. Surrounding the dermal papilla are epithelial keratinocytes and a smaller number of melanocytes, cells that give hair the color.
Melanocytes are special cells located in the skin and the eye that synthesize melanin pigments. They give skin including freckles and age spots and hair their color. Melanocytes produce pigments in the form of tiny granules of melanin. The melanoctye cells then transport the granules into neighboring keratinocyte cells. As old skin dies and new cells grow in from below, the newly pigmented keratinocytes work their way over 4 to 5 days to the topmost layer of skin and the protruding hair shaft. There they produce the color of the skin, as well as that of the hair.
There are two kinds of melanin: eumelanin and pheomelanin. Eumelanin is the most abundant type of human melanin, found in brown and black skin and hair. Pheomelanin is present in lighter hair--blond and red hair. White or gray hair color occurs when the pigment-producing cells no longer produce melanin or the melanocytes are absent.
Coloring the hair is a rapidly growing practice. Estimates are that 50% of women color their hair and men are also changing their hair color. All age and ethnic groups are experimenting with hair colorants. Damaged hair and basic ethnic differences in the physical properties of hair have an effect on the penetration of the dye into the hair shaft.
Straight and Curly Hair
Different hair-fiber types result from differences in hair follicles. Thick, straight hair is produced by large, straight hair follicles. If you cut one of these hairs in half, you will see that it has a circular cross-section. Flat hair follicles make curly hair that has an elliptical cross-section. Curled follicles produce irregular kinky hair.
Hair shape is also an ethnic characteristic. Caucasian hair has an elliptical cross section giving it a slight curl while oriental hair has a circular cross section causing straight hair. Black hair has a flattened elliptical cross-sectional shape. This cross-sectional asymmetry accounts for the irregular kinky appearance of Black hair. Hispanic hair has a cross-sectional shape somewhere between a circle and a flattened ellipse giving it a wavy appearance.
Other factors affecting the shape of hair are the chemical bonds that form within hair proteins. Curl is a factor of disulfide bonds within the keratin molecules.
Elasticity of the hair varies but in general hair can be stretched 30% of its original length in water and experience no damage. Water is absorbed very rapidly causing the hair shaft to swell. The moisture content, elasticity and tensile strength is lower in Black hair than in Caucasians.
Styling or Reshaping Hair
Hair can be reshaped by styling, such as by permanent-wave styling or straightening solutions. These styling techniques perform three general steps:
|1 ||The perm or straightening solution breaks disulfide bonds that determine the amount of curl in the natural hair.|
|2 ||The hair is mechanically reshaped into the desired configuration by using curlers, for example.|
|3 ||Neutralizing solution neutralizes the styling chemicals that broke the disulfide bonds, so that the disulfide bonds can be reformed in the new configuration.|
This kind of treatment frequently weakens the tensile strength of the hair shaft.
There are other, less stringent methods of reshaping hair. The methods usually involve breaking the weaker hydrogen bonds that form between keratin molecules. For example, water disrupts some of these bonds. If hair is held in the desired shape while it dries (using curling rollers, sock rollers, etc.), temporary changes can be made. High humidity can cause the hair to manifest its natural shape.
Hair Growth Cycle
Hair originates in a follicle, which is a cavity in the skin that holds and protects the active or living cells that become the nonliving strand of hair. Each hair follicle operates independently of the other hair follicles. Because of this, each individual hair may be in a different phase of growth.
There are three phases in the hair growth cycle:
- active growth phase, or anagen phase, lasting about 3 to 6 years in scalp hair;
- regressing phase, or catagen phase, during which cell proliferation ceases, the hair follicle shortens, and an anchored club hair is produced;
- resting phase, or telogen phase.
Hair shedding usually occurs in the regressing phase, but can also occur in the resting phase.
The hair root produces the cells that form the living part of the hair. This pushes the cells that already exist up and out from the follicle. As the distance from the follicle increases, the hair loses its nuclear DNA and becomes a strand of cross-linked proteins. This protein strand (the hair you comb or brush) is not living tissue. The only living parts of hair are the cells within the hair follicle in the skin. This active-growth phase is called the anagen phase scientifically. It lasts about 3 to 6 years in scalp hair.
After a certain period of growth, the hair follicle goes into a regressing phase. New cells are not created at this stage. Instead, the hair follicle actually shrinks about 80%. Part of the hair root is destroyed, and the active dermal papilla breaks off from the rest of the hair follicle. This regressing phase is called catagen, and lasts one or two weeks.
In the resting phase, telogen, like the regressing phase, the protein hair strand remains connected to the hair follicle, but it doesn't grow. After five or six weeks, the dermal papilla reconnects to the base of the hair follicle and the bloodstream. The hair reenters the active-growth phase and a new hair begins to form.
The old hair strand is usually shed near the end of the resting phase. If it does not shed, the new active-growth phase pushes the old hair out (sheds it) to make room for the new strand of hair.
Normal Shedding of Hair
Hair is normally shed during the resting phase in the hair growth cycle. When hair is shed, the protein hair strand is pushed out of the hair follicle to make room for a new hair. This is a part of a gradual replacement cycle. Normally, 50 to 150 scalp hairs are shed each day in the hair-growth cycle.
Different types of hair are shed after different periods of time. For example, eyebrow hairs last 3 to 5 months. Scalp hairs last 3 to 6 years before being shed.
Baldness or alopecia results when replacement hair (new growth) fails to keep up with hair loss. Baldness may be due to hereditary factors, pathological conditions, ageing, radiation injury to the dermal papilli, or a number of other causes.
There are different ways of hair removal including shaving and waxing that are used on various body parts. Facial hair in women is usually dealt with by bleaching, waxing or electrolysis. However, a prescription cream that slows the growth of hair when applied topically is available to decrease unwanted facial hair. Eflornithine works by inhibiting an enzyme, ornithine decarboxylase, in human skin that ultimately affects the rate of hair growth. It does not remove hair and must be used continuously. It slows the hair growth so that less frequent hair removal is needed. Side effects may include redness, stinging, burning, tingling or a rash. It has not been evaluated for safety in pregnant or nursing women so it should be avoided by these women.
The compound eflornithine is also effective in preventing cancer formation in many organ systems, inhibiting cancer growth, and reducing tumor size.
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