Hairs – a particular structure common only to mammals; they are the fibrous growths that originate from their skin.
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Anagen – actively growing phase of the hair; the follicle produces new cells and pushes them up the hair shaft as they become incorporated into the structure of the hair.
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Melanocytes – specialized cells in the follicle producing small colored granules that give hairs their particular color.
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Catagen – transitional phase of the hair; the follicle begins to shut down production of cells, the cells begin to shrink, and the root condenses into a bulb-shaped structure called a root bulb or a club root.
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Telogen phase – the resting phase for the follicle. Cell production has ceased completely; the root has condensed into a bulb and is held in place only by a mechanical connection at the base of the root/follicle.
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A single hair has a:
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Three main structural elements of the hair:
Cuticle – a series of overlapping layers of scales that form a protective covering.
Cortex – consists of spindle-shaped cells (fusiform) that contain or constrain numerous other structures.
Medulla – appears as if a small explosion occurred in the middle of the hair and may be found singly or in multiples.
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Animals have three types of hair:
Guard Hairs – large, stiff hairs that make up the outer part of the animal’s coat.
Fur Hairs – thinner and softer which fill in the rest of the coat providing warmth and bulk.
Vibrissa – the technical term for whiskers, the short to long, stiff, often white hairs around the snout and muzzle.
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The typical body areas that can be determined are the head (or scalp), pubic, facial, chest, axillary (armpits), eyelash/eyebrow, and limb; typically, only head and pubic hairs are suitable for microscopic comparison.
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Area | Diameter | Shaft | Tip |
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Head | Even | Straight or curly; some waviness; may be very long | Usually cut |
Pubic | Varies | Buckling; sometimes extreme waviness or curl | Usually pointed; may be razor cut |
Facial | Wide; even | Triangular in cross-section; some shouldering | Usually cut; maybe scissors or razor cut |
Chest | Even to some variation | Wavy to curly; some more straight | Usually pointed |
Axillary | Even to some variation | Less wavy/curly than the chest | Usually pointed; may be colorless |
Limb | Fine; tapering | Slight arc | Usually pointed |
Eyebrows/lashes | Tapering | Arc; short | Pointed |
Ancestry | Diameter | Cross Section | Pigment Distribution | Cuticle | Undulation |
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Africa | 60-90 | Flat | Dense; clumped | Thin | Prevalent |
European | 70-100 | Oval | Even | Medium | Common |
Asia | 90-120 | Round | Dense to very dense | Thick | Never |
Scissor-cut hair has a clean, straight border, whereas razor-cut hair is angled.
Bleaching of the hair oxidizes the pigmentation and removes its color.
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Pili Annulati – to hairs with colored rings.
Monilethrix – makes hairs look like a string of beads.
Pili Torti – Monilethrix makes hairs look like a string of beads.
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Vermin (such as lice), dandruff, or fungus may also be present on hairs, and this fact should be noted. These factors add to the classification of the hairs as coming from individuals with these traits.
The wording of microscopic hair examination results usually takes the following forms:
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Sensitivity and specificity are performance measures of binary classification tests (true/false, correct/wrong, present/absent) and use the following terminology:
Sensitivity – True positive rate.
Specificity – True negative rate.
Precision – (positive predictive value, or PPV): Proportion of true positives against all positive results.
Negative predictive value (NPV) – Proportion of true negatives against all negative results
Accuracy – Proportion of true results
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