Trace Evidence
Anatomy of Hair
Hair is an appendage of the skin.
Layers of the Skin
Epidermis
The outer layer of skin. The thickness varies in different parts of the body.
As cells move closer to the surface, they flatten out and dye waiting to be sloughed off.
Dermis
The middle layer of skin. It contain blood vessels, nerves, sweat glands, and hair roots.
Pores are small holes that bring sweat and oil to epidermis
Hypodermis
The inner and thickest layer of skin. It can be missing in some areas where skin is very
thin. It contains adipose cells - cells that store fat.
Structure of Hair
Follicle - the organ from which hair is grown
The length of a hair extends from its root or bulb embedded in the follicle, continues
into a shaft, and terminates at a tip end.
The shaft is subjected to the most intense examination by the forensic scientist.
Hair shaft consists of the cuticle, cortex, and medulla.
Root
The root is embedded in the follicle. It produces the hair and allows for hair growth. It
A single root contains enough DNA for analysis.
Follicular Tag is the translucent tissue around the root containing DNA that is seen when
hair is pulled from the head.
Stages of Hair Growth
Hair grows about 1 centimeter (width of pinkie finger) a month.
Anagen Phase
This is the initial growth phase during which the hair follicle is actively producing hair.
The root bulb is flame shaped. This stage may last up to 6 years.
Catagen Phase
This is the transitional phase where growth slows down and the follicle breaks down.
The root bulb shrinks. It takes 2-4 weeks to complete this phase.
Telogen Phase
This is the 'resting' phase. Growth stops and hair naturally falls out of the skin. The root
bulb is club shaped. No more than 10% of follicles are in the telogen phase at any one
time.
Parts of the Hair Shaft
Cuticle
The cuticle is the scale structure covering the exterior of the hair. Humans exhibit an
'overlapping shingles' pattern. The overlapping scales point toward the tip of the hair.
The scale characteristics of the cuticle may be important in distinguishing
between hairs of different species but are not useful in distinguishing between
different people.
The cuticle is made of a protein called keratin which gives hair its hardness.
Keratin
Hair is composed of the protein keratin, which is also the primary component of
finger and toe nails. It is resistant to chemical decomposition so it is able to
retain structural features over a long period of time. This is the property of hair
that makes it such a prime example of physical evidence.
Cortex
The cortex is the main body of the hair shaft.
Pigment granules called melanin that give hair its color are found in the cortex. The
color, shape, and distribution of these granules provide the criminalist with important
points of comparison among the hairs of different individuals.
Human hairs are generally consistent in melanin throughout the length of the hair shaft.
Bleached hair and grey hair has no melanin. Dyed hair has dye in the cuticle and cortex.
Medulla
The medulla is the canal-like structure of cells running through the center of the hair.
The medulla is the most predominant hair feature in many species but it is not present
in all hairs. The presence of a medulla varies even hair to hair.
Types of Medulla: Continuous, interrupted, fragmented, or absent
Human medulla is cylindrical and interrupted commonly. The exception are Asians
which have a continuous medulla.
Medullary Index
The thickness of the medulla varies by species. Humans have a medulla that is
thinner than animals. The ratio for humans is less than 1/3 and for animals it is
1/2 or more.
The forensic characteristics of the medulla for comparison are found in the medulla
shape and index. It is best used to determine if the hair originated from an animal or
human and possibly what species of animal.
Hair Evidence
Reference Sample Collection
50 full length head hairs are needed from the areas of the scalp.
24 full length pubic hairs are needed when dealing with a rape case.
Collection is accomplished by pulling the hair out or clipping it at the skin line.
Body Parts
Scalp hairs have little diameter variation and uniform pigment distribution
Pubic hairs have wide variation in shaft diameter and continuous medulla
Facial hairs are coarse in texture and triangular in cross-section.
Hair evidence can determine the place of body origin of a sample.
Race
Caucasian hairs can be straight or wavy, fine or coarse but have pigments that are
evenly distributed.
Negroid hairs tend to be flat or oval in cross section with unevenly distributed pigments.
Mongoloid hairs have a continuous medulla.
Race cannot easily be determined from hair evidence.
Age and Gender
The age or gender of a person cannot be told by hair evidence except from an infant.