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Normal
hair
Straight
wavy and curly hair are all normal examples of normal hair presentation.
Healthy hair will have an outer cuticles made from dead, keratinized
cuticle cells that over lap each other in a roof tile effect. The
outer cuticle is the strongest part of the hair fiber and protects
the inner cortex. The inner cortex of the fiber is also made from
dead keratinized cells that have been flattened and squeezed together
(see hair biology). The difference between straight, wavy, and
curly
hair is in the shape. Straight hair is circular, or nearly circular
in cross section. Wavy hair has a more oval cross section and curly
hair has a very flat oval cross section.
 The circular cross section of straight hair means it has optimum
strength in any direction transversely across the fiber. Because
wavy and curly hair fibers are oval in cross section, they are stronger
and less flexible in one direction but more flexible across the
flatter side of the oval cross section. The result is curls in hairs
adjacent to each other are all aligned together so the curls all
run the same way. Why do the curls all run the same way? Because
the hair follicles in the skin are all aligned in the same direction.
Not surprisingly, curly hair is made by curly hair follicles
and straight hair by straight hair follicles. The curl of curly
follicles
means that when the cells of the hair fiber are incorporated, those
that are on the inner curve are flattened less than those on the
outer curve. Cells added to the hair fiber on the outer curve are
flattened and stretched more. The result is to fix a curl into
the
hair fiber across the oval cross section. The curl is permanent
and if the hair fiber is pulled and let go the fiber will spring
back into its original curl just as tensile steel springs do.
While normal hair can have various presentations, there are a
variety of diseases that involve defective hair fiber production.
As well as diseases causing fiber defects, any damage to the outer
layer of cuticle of healthy hair may expose the inner cortex to
chemicals and also reduce the overall strength of the hair fiber
and its resistance to physical processes like brushing and pulling.
A normal healthy hair is as strong as copper wire of the same diameter.
The weaker hair fiber may lead to development of a wide range of
hair shaft defects some of which are described in this section of
the web site.
Normal
hair versus defective hair fiber references
- Hess WM, Seegmiller
RE, Gardner JS, Allen JV, Barendregt S. Human hair morphology:
a scanning electron microscopy study on a male Caucasoid and a
computerized classification of regional differences. Scanning
Microsc. 1990 Jun;4(2):375-86.
- Dawber RP. An update of hair shaft disorders.
Dermatol Clin. 1996 Oct;14(4):753-72.
- Caserio RJ. Diagnostic techniques for hair
disorders. Part I: Microscopic examination of the hair shaft.
Cutis. 1987 Sep;40(3):265-70.
- Caserio RJ. Diagnostic techniques for hair
disorders. Part II: Microscopic examination of hair bulbs, tips,
and casts. Cutis. 1987 Oct;40(4):321-5.
- Rogers M. Hair shaft abnormalities: Part
I. Australas J Dermatol. 1995 Nov;36(4):179-84; quiz 185-6.
- Rogers M. Hair shaft abnormalities: Part
II. Australas J Dermatol. 1996 Feb;37(1):1-11.
- Whiting DA. Structural abnormalities of
the hair shaft. J Am Acad Dermatol. 1987 Jan;16(1 Pt 1):1-25.
- Hutchinson PE. Diagnosis of hair disease.
Practitioner. 1980 Nov;224(1349):1159-67.
- Crounse RG. The diagnostic value of microscopic
examination of human hair. Arch Pathol Lab Med. 1987 Aug;111(8):700-2.
- Shelley WB. Hair examination using double-stick
tape. J Am Acad Dermatol. 1983 Mar;8(3):430-1.
- Bottoms E, Wyatt E, Comaish S. Progressive
changes in cuticular pattern along the shafts of human hair as
seen by scanning electron microscopy. Br J Dermatol. 1972 Apr;86(4):379-84.
- Dawber R, Comaish S. Scanning electron
microscopy of normal and abnormal hair shafts. Arch Dermatol.
1970 Mar;101(3):316-22.
- Caputo R, Ceccarelli B. Study of normal
hair and of some malformations with a scanning electron microscope.
Arch Klin Exp Dermatol. 1969;234(3):242-9.
- Nikiforidis G, Balas C, Tsambaos D. Mechanical
parameters of human hair: possible application in the diagnosis
and follow-up of hair disorders. Clin Phys Physiol Meas. 1992
Aug;13(3):281-90.
- Niyogi SK. Abnormality of hair shaft due
to disease. Its forensic importance. J Forensic Med. 1968 Oct-Dec;15(4):148-51.
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