Hair Unit 4
I. The study of hair • Hair is considered to be class evidence. • Without follicle cells, it cannot be used to identify a specific individual.
Hair is easily left behind… • It adheres to clothing, carpeting, upholstery… • It transfers from one substance to another, one location to another, quite easily.
Hair has a tough outer covering enabling it to resist decomposition. • The physical characteristics of hair can indicate racial background.
Chemical tests can indicate drug history, the presence of toxins, and heavy metals as well as the individual’s nutritional history.
When a follicle cell is present, DNA evidence may be available. • Because DNA evidence is individual, its presence makes the hair sample individualized.
A. The function of hair • All mammals have hair. • In many, the hair is quite dense and is called fur • It is reduced in most humans as it isn’t as critical a feature as it is with other mammals.
When humans are born, they have their greatest number of hair follicles: 5 million – 2% on their head. • As we age the density of our hair decreases.
Hair: • Reduces friction • Acts as a sensory organ • Protects against sunlight • Regulates temperature in association with the muscles in skin.
When conditions are cold, muscles pull the hair upright creating pockets that trap air – providing a warm, insulating layer next to the skin.
When conditions are warm, muscles relax, flattening the hair releasing the trapped air.
B. The structure of hair • Hair has two parts: • A follicle and a shaft
The follicle is a “club-shaped” structure in the skin. • At its end is a network of blood vessels (called the papilla) that supply nutrients to the hair helping it to grow
The bulb surrounds the papilla…an oil secreting sebaceous gland is within the bulb helps condition the hair. • The erector muscle that regulates temperature attaches to the bulb
The hair shaft is made of keratin…a protein produced in the skin. • This makes the hair strong and flexible
Hair shafts are made up of three distinct layers: • The inner medulla • The cortex • The outer cuticle
1. Cuticle • The transparent outer layer of the hair shaft • made of scales that overlap and protect the inner layers. • Scales point from proximal end to distal end
The direction the scales point indicates where the younger and older ends of the hair are located. • This is important in analyzing the hair for toxins, drugs or metals to generate a timeline or indicate a particular location
Human and animal hair differ in their cuticle scales. • Human scales are flattened and narrow – called imbricate
Rodents and bats have a coronal cuticle – scales that look like a stack of crowns. • Cats, seals and mink have spinous scales and look like petals
coronal imbricate spinous
2. Cortex • The largest part of the hair shaft in humans. • The part of hair that contains the melanin (pigment granules) that give hair its color
Pigment distribution varies among individuals. • Larger pigment granules give the cortex an uneven distribution of color when viewed microscopically.
3. Medulla • This is the center of the hair. • It can be a hollow tube • It can be filled with cells • It can be absent • It can be fragmented • It can be continuous • It can be doubled
a. Medullary classifications • Continuous: one unbroken line of color • Interrupted (intermittent): pigmented line broken at regular intervals
Fragmented (segmented): pigmented line unevenly spaced. • Solid: pigmented area filling both the medulla and the cortex • None (absent): no separate pigmentation in the medulla
C. Types of hair • Hair varies in shape, length, diameter, texture and color. • Hair varies from region to region on the body.
Cross sections of hair can be circular, triangular, irregular or flattened. This influences the curl of the hair.
Hair can have a coarse or a fine texture. • Because of all these inconsistencies, 50 hairs are usually collected from suspects’ head and 25 pubic hairs are generally collected
Scientists have identified 6 types of hair on the human body: • head •underarm •pubic • body (auxiliary) • beard/mustache • eyebrow/eyelash
1. cross-sectional shape • Head hair: circular or elliptical • Eyebrow/lash: circular with tapered ends • Beard: thick and triangular
Body: oval or triangular (dependent on regularity of shaving) • Pubic: oval or triangular
2. Other characteristics • Arm/leg: blunt tip, may be frayed at ends due to abrasion • Beard: coarse, may have double medulla • Pubic: diameter varies; buckling may be present
D. Life Cycle of Hair • Anagen stage: lasts ~1,000 days. 80 – 90% of hair is in this stage A period of active growth – cells around the follicle are rapidly dividing and depositing materials within the hair
Catagen stage: accounts for ~2% of all hair growth and development. Follows anagen as hair grows and changes
Telogen stage: ~10 – 18%of all hairs are in this stage. The follicle is dormant or resting. Hair is easily lost at this time.
E. Treated hair • Bleaching hair removes pigment, makes hair a yellowish color, brittle and disturbs the scale on the cuticle.
Artificial bleaching results in a clear demarcation on the hair. • Bleaching by the sun leaves a more gradual mark.
Dying hair changes the color of the hair shaft, both the cuticle and the cortex take on the dye color.
Hair grows at a rate of ~1.3 cm per month. • If hair is color-treated -measuring the length of the color-treated portion and dividing by 1.3, the length of time since the hair was colored can be calculated.
F. Racial differences • There are broad characteristics that can be used to differentiate a hair sample as belonging to an individual of a particular race.
European hair is generally straight or wavy. The pigment granules are small and evenly distributed, the cross-section is oval or round with a moderate diameter with minimal variation. The color may be blond, red, brown or black.
Asian hair is straight, the pigment granules are densely distributed, the cross-section is round with a large diameter, the shaft tends to be coarse and straight with a thick cuticle and a continuous medulla.
African hair is kinky, curly or coiled, the pigment granules are densely distributed, clumped and may differ in size and shape; the cross-section is flattened with a moderate to small diameter and considerable variation.