 How
Skin Works
Skin is one of the most amazing organs in
the human body. It is hard for us to think about it as an organ.
However. we tend to think of organs as boxy things. Your heart,
liver, kidneys - those are obviously organs. But skin is an
organ too, especially if you look at the dictionary definition
of "organ", like this definition from the Merriam
Webster Collegiate Dictionary:
Organ -
a) differentiated structure
(as a heart, kidney, leaf, or stem) consisting of cells
and tissues and performing some specific function in an
organism
b) bodily parts performing
a function or cooperating in an activity the eyes and
related structures that make up the visual organs
By that definition, skin is definitely an organ.
Skin is made up of very specific cells and tissues, and their
collective purpose is to act as the boundary between "you"
and "the world". One of the neat things about skin
that makes it different from a lot of other organs is the
fact that it does have to deal with the real world. Therefore
it is loaded with sensors, and it also has a very tough layered
design so that it can handle realities of the environment
like abrasion and sunlight.
If you take a look at a cross section of typical
skin (like the skin on your arm or leg) you find that it is
made up of two main layers: the epidermis on the outside and
the dermis on the inside. The epidermis is the barrier, while
the dermis is the layer containing all the "equipment"
- things like nerve endings, sweat glands, hair follicles
and so on. Here's a picture to help you see what is going
on:
In the subcutaneous layer (you may have heard
of subcutaneous fat - this is where it lives) you can see
the blood vessels (shown as two thin red and blue lines).
These vessels branch infinitely (not shown) into the dermis
to supply the sweat glands, hair follicles, sebaceous glands
and erector muscles with blood. They also fan out into the
dermis's capillary bed. It turns out that the dermis is loaded
with capillaries. Capillaries satisfy the nutritional needs
of the cells in the dermis, and they also help the skin perform
an important cooling function in humans. The epidermis has
no direct blood supply, but instead is supported and fed by
the dermis.
The dermis is where the action is functionally.
As shown in the diagram, the dermis contains sweat glands,
hair follicles (each with its own tiny little muscle so that
your "hair can stand on end"!), nerve endings and
so on. There are several different types of nerve endings:
* Heat sensitive
* Cold sensitive
* Pressure sensitive
* Itch sensitive
* Pain sensitive
 All
these different nerve endings let you sense the world. They
also help you protect yourself from burns, punctures, etc.
by warning you when something is damaging your skin.
The epidermis is your interface to the world,
and it is actually quite interesting. It has two main layers,
the inner of which is living and the outer of which is dead.
The dead skin cells of the outer layer are what we can actually
see, and they are constantly flaking off and being replaced
by new cells being pushed outward.
The living, inner layer is called the malpighian
layer. The malpighian layer creates the dead cells that we
can see. It is in direct contact with the dermis, which feeds
and supports it. The malpighian layer is our focus of attention
actually, because it is here that the sun affects the skin
during tanning. The malpighian layer is itself layered like
this:
* In direct contact with the dermis is the
basal layer. If you have ever heard of a basal cell carcinoma
(cancer), this is where it starts.
* Above the basal layer is the spinous layer.
* Above the spinous layer is the granular layer.
Above the granular layer is the stratum corneum.
The stratum corneum is the outer layer of dead cells - the
cells that we see as our skin. The cells in this layer are
filled with a protein called keratin. Keratin is a very interesting
protein because it is tough - horns, hair, hoofs, fingernails
and feathers all gain their strength from keratin. The same
stuff that your fingernails are made of actually forms your
visible skin (but in a much thinner and more flexible layer).
That is what makes your skin so tough. In parts of the body
that get a lot of wear, like the palms and the feet, the stratum
corneum is thicker to handle the abrasion.
Living among the basal cells in the malpighian
layer is another type of cell called a melanocyte. Melanocytes
produce melanin, which is a pigment that is the source of
tanning. The melanocytes are actually where a tan comes from.
Here is what the Encyclopedia Britannica has to say about
melanocytes:
"The appearance of the skin is partly
due to the reddish pigment in the blood of the superficial
vessels. In the main, however, it is determined by melanin,
a pigment manufactured by dendritic cells called melanocytes,
found among the basal cells of the epidermis. Their numbers
in any one region of the body, which range from about 1,000
to more than 2,000 per square millimetre, are roughly the
same within and between races; the blondest whites have as
many as the darkest blacks. Colour differences are due solely
to the amount of melanin produced and the nature of the pigment
granules. When the skin becomes tanned on exposure to sunlight,
the melanocytes do not increase in number, only in activity."
("Integumentary Systems, Pigmentation", Britannica
CD. Version 97. Encyclopaedia Britannica, Inc., 1997.)
Not only do melanocytes produce a tan, they
are also responsible for the form of cancer called melanoma.
Melanoma is caused by UV radiation damage to melanocytes.
Repeated exposure to UV can cause cancerous mutations.
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