12.1: Growth of Hairs

• Hairs – a particular structure common only to mammals; they are the fibrous growths that originate from their skin.

  • Follicle –  the structure within which hairs grow; it is a roughly cylindrical tube with a larger pit at the bottom.

  • Keratin –  a tough protein-based material from which hair, nails, and horns are made in animals.

  • Keratinization – the hardening process of hair growth.

  • The follicle contains other structures, such as blood vessels, nerves, and sebaceous glands — the latter producing oils that coat hairs, helping to keep them soft and pliable.

  • Pili Arrector – hair muscles.

• 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.

• Melanocytes – specialized cells in the follicle producing small colored granules that give hairs their particular color.

  • Two types of melanin:

    • Eumelanin – dark brown pigment.

    • Pheomelanin – lighter pigment.

• 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.

• 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.

12.2: Microanatomy

A single hair has a:

• Root – the portion that formerly was in the follicle, the proximal portion of the hair.

• Shaft – the main portion of the hair.

• Tip – the distalmost portion of the hair.

Three main structural elements of the hair:

1. Cuticle – a series of overlapping layers of scales that form a protective covering.

   1. Imbricate – human scale pattern.

2. Cortex – consists of spindle-shaped cells (fusiform) that contain or constrain numerous other structures.

   1. Pigment granules are found in the cortex and are dispersed variably throughout the cortex.

   2. Cortical Fusi – small bubbles that may appear in the cortex; when they do appear, they may be sparse, aggregated, or evenly distributed throughout the cortex.

   3. Ovoid Bodies – odd structures that look like very large pigment granules.

3. Medulla – appears as if a small explosion occurred in the middle of the hair and may be found singly or in multiples.

Animals have three types of hair:

1. Guard Hairs – large, stiff hairs that make up the outer part of the animal’s coat.

   1. Guard hairs may have a widening in the upper half of the shaft, called a shield.

   2. Subshield Stricture –  a narrowing of the hair to slightly less than the normal, non shield shaft diameter.

2. Fur Hairs – thinner and softer which fill in the rest of the coat providing warmth and bulk.

3. Vibrissa – the technical term for whiskers, the short to long, stiff, often white hairs around the snout and muzzle.

• Some nonhuman hairs are color banded — showing abrupt color transitions along the shaft of the hair, including the tip.

12.3: Body Area Determination

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.

• Transitional Body Hairs – hairs that do not fit into these categories; such as those on the stomach, between the chest, and the pubic region.

• Buckling – an abrupt change in direction of the hair shaft with or without a slight twist.

• Shouldering – an asymmetrical cross-section of hairs.

12.4: Various Characteristics of Hair by Ancestry

12.5: Damage, Disease, and Treatments

• 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.

• 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.

• 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.

12.6: Hair Comparisons

The wording of microscopic hair examination results usually takes the following forms:

• …a hair sample from the crime scene and another taken from (the defendant) were “similar” and “consistent.”

• …the pubic hairs from the crime and (the defendant’s) pubic hairs exhibited “similar microscopic characteristics.”

• …that hairs from the crime scene were “similar” to (the defendant’s).

• … Hair from the crime scene exhibited “the same microscopic characteristics” as (the defendant’s) hair.

12.7: DNA and Hairs

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