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Hair Anatomy and Growth
Hair Anatomy
Hair is present on many different regions of the body. Each region, such as the head, pubic area, chest, axillae (under the arms - armpit), and limbs, has hairs with microscopical characteristics attributable to that region. Although it is possible to identify a hair as originating from a particular body area, the regions of the body that are primarily used in forensic comparisons are the head and pubic areas. As hairs undergo the anagen phase (when a hair grows) and the telogen phase (when a hair is resting), the visible microscopic characteristics are sufficient to determine the phase of growth of the hair.
Naturally shed hairs, such as a head hair dislodged through combing, display undamaged, club-shaped roots.
A hair forcibly removed from the scalp will exhibit stretching and damage to the root area.
Forcibly removed hairs may have tissue attached.
The basic morphology of human hairs is shared by each individual in the population, but the arrangement, distribution, and appearance of individual microscopic characteristics within different regions of hair routinely allow a skilled hair examiner to differentiate hairs between individuals. An example would be the ability of an individual to recognize the face of a friend or relative in a crowd even though each person in the crowd possesses ears, eyes, a nose, and a mouth.
Scissor Cut Hair
Cutting with scissors produces a sheared or square cut end.
Razor Cut Hair
A razor cut is angular and very straight or clean.
Hair Affected by Burning.
Burned or singed hairs are charred and brittle and exhibit round vacuoles at the point of burning.
A Split Hair
Hair Growth
The Life Cycle of Hair
The replacement of human scalp hair occurs in a scattered mosaic fashion with no apparent wave-like or seasonal pattern.
The Life Cycle of Hair Can Be Divided Into 3 Phases
Hair spends a variable amount of time in each phase (listed below) determined by genetics, hormones, and area of the body. Laser energy has its greatest effect on the anagen phase (when hair grows). All of these factors must be considered when choosing a method of hair removal.
1) Anagen Phase:
The anagen phase is when a hair grows, and materials are deposited in the hair shaft by cells found in the follicle. Metabolically active and dividing cells above and around the dermal papilla of the follicle grow upward during this phase, to form the major components of the hair - the medulla, cortex, cuticle, and accompanying root sheath.
The anagen phase lasts approximately 2 to 7 years but may vary from a few months to about 10 years. On a healthy head, at any given time, approximately 80 to 90 percent of our hair is in the anagen phase. During this phase the scalp hairs grow about 1 to 2 cm per month. The hair on the arms, legs, eyelashes, and eyebrows have a very short active growth phase of about 30 to 45 days explaining why they are so much shorter than scalp hair.
2) Catagen Phase:
The catagen phase is the transition period between the anagen (growing) and telogen (resting) phases, which is when hair growth begins to shut down and stop activity. Approximately 2 percent of our hair is in the catagen phase at any given time. The catagen phase lasts about 3 weeks while a club hair is formed. This process cuts the hair off from its blood supply and from the cells that produce new hair. A new hair is formed and pushes the club hair up the follicle and eventually out.
3) Telogen Phase:
The telogen phase is when a mature hair is resting. It is the resting phase for hairs on the scalp, usually lasting 3 weeks to 3 months. The resting phase is much longer for hairs on the eyebrows, eyelashes, arms and legs. Approximately 10 to 18 percent of our hairs (approximately 100 to 1000 hairs) are in the telogen phase at any given time, and a minimal amount of force - as when one combs through the hairs on the head - will dislodge the hairs from the dormant follicle. It is normal to lose some telogen hairs each day. During the telogen phase, the hair is anchored in the follicle only by the root, which is club-shaped. The germ cells below the club-shaped root will give rise to the next generation of an anagen hair, which is when the hair growth process starts over again.
Hair is dead tissue. Once it is damaged it cannot be repaired, except by re-growing it. The only "living part of a hair" is the root, which is under the skin, called a follicle.
Recap
Each human hair grows in its own individual phase. Not all hairs are in the same phase at the same time.
- 1) Anagen Phase: When a hair grows.
Approximately 80% to 90% of our hair is in this phase at any given time. - 2) Catagen Phase: Hair growth begins to shut down and stop activity.
Approximately 2% of our hair is in this phase at any given time. - 3) Telogen Phase: A hair is resting.
Approximately 10% to 18% of our hair is in this phase at any given time.
References:
1) U.S. Department of Justice - Federal Bureau of Investigation - Forensic Science Communications - July 2000 - www.fbi.gov
2) Forensic Science Communications - January 2004 - www.fbi.gov
This summary is a general overview about the topic discussed and does not include all the facts, or include everything there is to know about any medicine and/or products mentioned. Do not use any medicine and/or products without first talking to your doctor. Possible side effects of medications, other than those listed, may occur. Full Disclaimer & General Safety Advisory