Hair structure & growth cycle PDF / PPT

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Cosmetologists should study the properties of the hair and
scalp so that they can differentiate between normal and
abnormal hair loss, offer a variety of chemical services to
clients, and aid a client in caring for their scalp and hair well-
Trichology- study of hair and its diseases and care.
Structure of the Hair
Human hair is divided into two parts:
(“epi”- means above or outer; “derm”- means skin so
epidermis is the OUTER layer of skin)
 Hair root (roots are underground)- located below the
 Hair shaft- located above the epidermis
Structures of the Hair Root
*Hair follicle- tube-like depression or pocket in the skin or scalp
that contains the hair root
*Hair bulb- lowest part of a strand of hair; thickened, club-shaped
*Dermal papilla- small, cone-shaped elevation located at the base
of the hair follicle that fits into the hair bulb
*Arrector pili muscle- small, involuntary muscle in the base of the
hair follicle; when it contracts, we get goose bumps
*Sebaceous glands- oil glands in the skin that are connected to the
hair follicles; secretes sebum (fatty, oily substance)
Structures of the Hair Shaft
*Hair cuticle- outermost layer of hair; consists of a single,
overlapping layer of transparent, scale-like cells; protects inner
structure of hair; creates shines and smoothness
*Cortex- middle layer of the hair; 90% of total hair weight comes
from the cortex; elasticity and color are the result of protein in the
*Medulla- innermost layer of hair; generally only thick, coarse hair
contains a medulla; has no known purpose
Chemical Composition of the Hair
*Keratinization- process by which newly formed hair cells in the
hair bulb mature, fill with keratin, move upward, lose their
nucleus and die
*COHNS- major elements that make up the human hair:
Side Bonds of the Cortex
* Side bonds-link the polypeptide chains (long chain of amino acids
linked together by peptide bonds) together; responsible for the strength
and elasticity of human hair
* Hydrogen Bond- weak, physical cross-link bond that is easily broken
by water or heat
* Salt Bond- weak, physical cross-link bond that is broken by alkaline or
acidic solutions
* Disulfide Bond- strong, chemical side bond; can be broken by extreme
heat, some high-temp styling tools, permanent waves and chemical
Hair Pigment
* Melanin- the tiny grains of pigment in the cortex that give hair its
natural color
* Eumelanin- provides natural dark brown to black hair color
* Pheomelanin- provides natural colors ranging from red to ginger to
yellow and blonde tones
*Two main types of hair found on the body are vellus hair and
terminal hair.
*Vellus hair (lanugo hair)- short, fine, unpigmented, downy;
appears on parts of the body that are normally considered
hairless (forehead, eyelids, bald scalp);almost never has a
medulla; helps with the evaporation of perspiration; women
normally retain 55% more vellus hair than men
*Terminal hair- long, coarse, pigmented hair found on the
scalp, legs, arms, and bodies of both males and females; it
usually has a medulla
*The three stages of hair growth are the
1. Anagen Phase
2. Catagen Phase
3. telogen Phase
*Each strand of hair on the human body is at its own stage of
*Once the cycle is complete, it restarts and a new strand of hair
begins to form.
*The rate or speed of hair growth is about 1.25 centimetres or 0.5
inches per month, or about 15 centimetres or 6 inches per year.
*The anagen phase is known as the growth phase. This is the
phase where the hair physically grows approximately 1 cm per
*It begins in the papilla and can last from two to six years. The
span at which the hair remains in this stage of growth is
determined by genetics.
*The longer the hair stays in the anagen phase, the longer it will
grow. During this phase, the cells in the papilla divide to
produce new hair fibers[contradictory], and the follicle buries
itself into the dermal layer of the skin to nourish the strand.
About 85% – 90% of the hairs on one’s head are in the anagen
phase at any given time
*The catagen phase, also known as the transitional phase, allows
the follicle to, in a sense, renew itself. During this time, which
lasts about two weeks, the hair follicle shrinks due to
disintegration and the papilla detaches and “rests,” cutting the
hair strand off from its nourishing blood supply.
*Signals sent out by the body (that only selectively affect 1
percent of all hair of one’s body at any given time) determine
when the anagen phase ends and the catagen phase begins.
*The first sign of catagen is the cessation of melanin production
in the hair bulb and apoptosis of follicular melanocytes.
Ultimately, the follicle is 1/6 its original length, causing the hair
shaft to be pushed upward.
*While hair is not growing during this phase, the length of the
terminal fibers increase when the follicle pushes them upward.
*During the telogen or resting phase (also known as shedding phase)
the follicle remains dormant for one to four months. Ten to fifteen
percent of the hairs on one’s head are in this phase of growth at any
given time.
*In this phase the epidermal cells lining the follicle channel continue
to grow as normal and may accumulate around the base of the hair,
temporarily anchoring it in place and preserving the hair for its
natural purpose without taxing the body’s resources needed during
the growth phase.
*At some point, the follicle will begin to grow again, softening the
anchor point of the shaft initially. The hair base will break free
from the root and the hair will be shed. Within two weeks the new
hair shaft will begin to emerge once the telogen phase is complete.
The process results in normal hair loss known as shedding.
* The Journal Of Cell Science At Glance