HAIR
ANATOMY
We
have approximately 5 million hairs on our body. Of these,
about 150,000 (more if you're blond, less if you're a redhead)
are found on our scalp. The only parts of our body where
there are no hair follicles are the palms, soles, lips,
tip of the penis, clitoris, and labia minora (small "lips"
of the vagina (for more information refer to our Male
Genitalia Kit and our Female
Breast & Genital
Kit ).
Each
hair grows from a pocket in the skin called the hair follicle.
During its growing phase, the follicle has a bulb-shaped
bottom, the center of which is called the dermal papilla
(P).The papilla is fed by very small blood vessels, which
bring it food and oxygen and take wastes away. The papilla
is highly sensitive to hormones. It is here that hormones
and chemicals secreted by your body (or injested as a medicine)
work on the hair, making it grow faster, slower, or not
at all.
The
color of our hair is determined by pigmented cells growing
at the dermal papilla. These cells (melanocytes), contain
a chemical pigment (melanin) - stained dark purple in this
photo. The amount and density of melanin in these cells
determines the exact color of your hair. There are two types
of melanin, one coloring hair brown to black (eumelanin)
and one coloring hair blond to red (pheomelanin). The color,
shape and thickness is in a large part determined by genetics...just
look at your folks!
The
hair follicle is a sac-like structure that surrounds the
hair root. It is found below the skin, and has very small
blood vessels giving it nourishment. Sebaceous glands surrounding
the hair root secrete oil (sebum) while salt-water (perspiration)
is secreted from nearby sweat glands (not shown). The sebum
oil protects the hair and keeps it shiny and waterproof,
while the sweat is a way for the body to cool down if it's
too hot.
The
purpose of hair is to keep the warmth in. Warm-blooded mammals
require a warm body temperature to survive
(cold-blooded
animals, like snakes and other reptiles, metabolize and
survive at "room-temperature" and thus neither need, nor
have, hair). The
densely packed hair of mammals (fur) plays an important
role in keeping warm. A
tiny hair muscle attached to each hair follicle, called
the arrector pili, contracts when in a cold environment
(or when fearful). This causes the hair to be pulled downwards
so it stands up straight (goose-bumps or gooseflesh), and
warm air is actually trapped between each hair. This functions
like a layer of clothing, keeping warmth in.
Humans,
however, have lost most of our dense fur, so even though
we still get goose-bumps, and our hairs can stand on end,
the keep-warmth-in phenomenon no longer works.
Hair
is actually composed of a protein called keratin. It's the
same protein found in our nails and in our skin. A strand
of hair has three layers. The outermost layer is called
the cuticle. It is transparent and acts to protect the inner
layers. The innermost layer is called the medulla, and
is composed of large baggy cells that may have a hollow-like
appearance. Between these two layers is the cortex. The
bulk and strength of hair is from this compact area of cells.
