Hair Growth Biology

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Hair grows from primary follicles. Actively growing hair follicles penetrate the entire epidermis and dermis. There are approximately 5 million total body hair follicles, of which 100,000 to 150,000 are scalp follicles. In adults, 90% of the hair follicles are in the growing (anagen) stage and the remainder are in the resting (telogen) stage. Follicular density decreases with age (1135/cm2 at birth to 485/ cm2 at ~30 years to 435/cm2 at 80 years). Scalp hair grows at a rate of 0.37 to 0.44 mm/day and normal scalp hair loss or shedding in adults ranges from 50 to 100 hairs per day (2).

The growth of hair in humans is controlled by complicated mechanisms that can differ among various body locations. Morphologically, there are three types of hair: vellus, terminal, and intermediate. Vellus hairs are short, fine (<0.3 mm in diameter), soft, usually nonpigmented, and unmedullated. Terminal hairs are large (>0.3 mm in diameter), darkly pigmented, and medullated. Ninety percent of the hairs on the chest, trunk, shoulders, legs, and arms of men are terminal hairs, whereas only 45% of hairs in the same regions on women are terminal (3). Intermediate hairs occur on the scalp, and they demonstrate a morphology between those of terminal and vellus hairs. Intermediate hairs are medullated and contain a moderate amount of pigment (i.e., less than that found in terminal hairs) (4).

There are four types of hair follicles: terminal, vellus, miniaturized, and senescent. Terminal follicles bear terminal hairs at some time during the life of an individual, whereas vellus follicles do not bear terminal hairs at any time during an individual's life. Miniaturized follicles are those terminal follicles that have lost their ability to produce terminal hairs and instead produce vellus hairs. Senescent follicles are any of the three types of follicles that no longer produce hairs and have lost histological evidence of the ability to produce hairs.

The character of human hair is constantly changing from the prenatal period to old age; and under given physiological conditions, the same hair follicle can successively form different types of hair. Despite differences among individuals, follicle development for all types of hair is virtually the same.

Hair undergoes repeated cycles of active growth and rest. The relative dura- -a tion of each cycle varies with the age of the individual and the region of the body where the hair grows. The length of the cycle is often modified by a variety 4

of physiological and pathological factors. The cyclic phase of the hair follicle is

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Hair Loss Prevention

Hair Loss Prevention

The best start to preventing hair loss is understanding the basics of hair what it is, how it grows, what system malfunctions can cause it to stop growing. And this ebook will cover the bases for you. Note that the contents here are not presented from a medical practitioner, and that any and all dietary and medical planning should be made under the guidance of your own medical and health practitioners. This content only presents overviews of hair loss prevention research for educational purposes and does not replace medical advice from a professional physician.

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