Your Skin, The Largest Organ
- Layers of the skin
- Facultative pigmentation
- Constitutive pigmentation
Skin covers all body surfaces. The skin of an average
adult weighs 8-10 pounds and has an average area of
about 22 square feet. The purpose of this outer covering
for the body is to protect against injury, infection,
heat, cold, and store water, fat and vitamins. The human
skin is rejuvenated about once every four weeks.
Thinking of your skin as an organ, rather than something
that we can use and abuse, puts things in proper perspective.
Your skin is a wonderfully resilient organ and for the
most part can survive virtually any form of punishment.
The skin is the body's boundary, tough enough to resist
all sorts of environmental assaults, yet sensitive enough
to feel a breeze.
A versatile organ, skin creates the first line of defense
against possible invasion by bacteria and germs, while
maintaining the body's internal environment to within
a few degrees of normal throughout our lifetimes. The
skin also secretes fluids that lubricate it and barricade
toxic substances, while maintaining this environment.
The skin can absorb some soluble substances
The Skins Function
The skin is divided into three layers, the epidermis
or outer layer which produces the tan; the dermis or
middle layer which contains collagen and other materials
vital to the skin's strength, its ability to repair
itself and fight off infections; and the subcutaneous
tissue or bottom layer which serves as insulation, a
food reserve and binds the skin to your body. The layers
of the epidermis which are involved in the tanning process
are the horny (outer) layer and the germ (inner) layer.
Cells from the germ layer are constantly reproducing
and pushing old cells up through the horny layer where
in approximately one month they are sloughed off. At
the base of the epidermis, cells called melanocytes
(about 5% of the epidermal cells) exist. These are the
pigment cells involved in the tanning process. The melanocytes
use the amino acid tyrosine to produce melanosomes (dark
brown granules of pigment) which contain melanin that,
when oxidized by UVR, provide the adaptive coloration
of the skin. When exposed to ultraviolet radiation,
the melanocytes release extra melanosomes thus making
the skin darker and completing melanogenesis which is
defined as the UVR-induced production and oxidation
of melanin, i.e., the process of developing facultative
pigmentation, better known as cosmetic tanning. Facultative
Pigmentation is simply the level of an acquired tan
developed by an individual exposed to ultraviolet light
where as Constitutive Pigmentation is our natural skin
Every individual has only a given amount of melanin
which is determined by an individual's skin type. Although
a person may gradually increase the amount of melanin
production through tanning, the person cannot change
from one skin type to another.
One function of the skin is to protect its underlying
tissues from invisible radiation i.e. that produced
by the sun. The sun emits three kinds of ultraviolet
(UV) rays, UVA, UVB and UVC. Although invisible, you
can see the results of ultraviolet rays in such things
as the growth of plants and the tanning of our skin.
UVC is the shortest, most harmful wavelength of ultraviolet
rays, but is virtually stopped by the Earth's ozone
layer and pollution. UVB is the medium wavelength and
although overexposure can cause erythema (sunburn),
a controlled amount is necessary to initiate tanning
in the skin.
UVA is the longest wavelength and is responsible for
the completion of the tanning process. Tanning is actually
the body's natural defense mechanism to protect itself
from the sun's rays.
The outer surface of dead cells (horny layer) is the
first shield against any invader. These cells, called
keratinocytes or skin cells (about 90% of the epidermal
cells), arise from the living dividing basal cells (named
for their location at the base of the epidermis). New
cells rise, pushed from the base by rapidly dividing
basal cells. These new cells produce greater and greater
quantities of a protein called keratin. The fibrous
keratin accumulates within the cells until it nearly
replaces their living cellular machinery. This journey
to the surface takes approximately four to five weeks.
Now they have withered, died and bound themselves firmly
to one another, forming a tough nearly impermeable outer
shell to the epidermis. Perpetual shedding of this horny
layer prevents many microbes from penetrating the skin.
As the epidermis goes about the business of renewing
the horny layer, it sheds the dried out cells at a rate
of one million every forty minutes. This horny layer
becomes thicker and tougher in response to UV to protect
the skin from overexposure. The remaining 5% of cells
found in the epidermis are mostly made up of Langerhans
and Merkel cells. Langerhans cells, also known as "immune
cells," help fight-off organisms trying to invade the
body. Merkel cells, known as "touch receptors," relay
touch sensations to the dermis as contact nerve endings.
Ultraviolet B initiates the tanning process by stimulating
the melanocytes, releasing melanin into the surrounding
cells. As these melanin granules migrate to the skin's
surface, there is a chemical reaction that occurs between
the tyrosine, the melanin and the UVA rays that turns
the skin a light brown or brown giving us the tanned
The degree of coloring achieved depends on the amount of melanin one has,
the duration of the exposure and the individual's reaction to the ultraviolet
The sun is not selective in the proportions of UVA
and UVB emitted. Therefore the skin is vulnerable to
too much UVB which can cause sunburn, as well as other
types of damage to the skin.
Another system at work in the epidermis is our immune
system. The epidermis houses special cells that join
the immune system in defense against disease. Langerhans
cells give agents of the immune system information regarding
the nature of foreign substances entering the body through
the skin. Extremely high doses of UV can damage Langerhans
cells, preventing them from sending the appropriate
warning signals to the immune system. Lymphocytes are
also located in the epidermis. They are the other defender
in this delicate cellular world. Lymphocytes are also
damaged by prolonged overexposure to UV.
This highly complex inner world of the skin mandates
responsible treatment by its owner as well as those
of us entrusted with the cosmetic care of this largest
of human organs.