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The largest organ in your body
isn't your liver or your brain.

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It's your skin, with a surface area
of about 20 square feet in adults.

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Though different areas of the skin
have different characteristics,

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much of this surface performs
similar functions,

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such as sweating, feeling heat and cold,
and growing hair.

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But after a deep cut or wound,

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the newly healed skin will look different
from the surrounding area,

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and may not fully regain all 
its abilities for a while, or at all.

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To understand why this happens, we need to
look at the structure of the human skin.

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The top layer, called the epidermis,

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consists mostly of hardened cells,
called keratinocytes,

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and provides protection.

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Since its outer layer is constantly being
shed and renewed,

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its pretty easy to repair.

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But sometimes a wound penetrates
into the dermis,

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which contains blood vessels,
and the various glands and nerve endings

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that enable the skin's many functions.

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And when that happens, it triggers the
four overlapping stages

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of the regenerative process.

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The first stage, hemostasis, is the skin's
response to two immediate threats:

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that you're now losing blood,

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and that the physical barrier of 
the epidermis has been compromised.

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As the blood vessels tighten to minimize
the bleeding,

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in a process known as 
vasoconstriction,

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both threats are averted by forming
a blood clot.

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A special protein known as fibrin forms
crosslinks on the top of the skin,

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preventing blood from flowing out
and bacteria or pathogens from getting in.

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After about three hours of this,
the skin begins to turn red,

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signaling the next stage, inflammation.

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With bleeding under control
and the barrier secured,

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the body sends special cells to fight any
pathogens that may have gotten through.

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Among the most important of these
are white blood cells,

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known as macrophages,

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which devour bacteria and damage tissue
through a process known as phagocytosis,

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in addition to producing growth factors
to spur healing.

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And because these tiny soldiers 
need to travel

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through the blood to 
get to the wound site,

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the previously constricted 
blood vessels now expand

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in a process called vasodilation.

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About two to three days after the wound,

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the proliferative stage occurs, when 
fibroblast cells begin to enter the wound.

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In the process of collagen deposition,

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they produce a fibrous protein 
called collagen in the wound site,

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forming connective skin tissue
to replace the fibrin from before.

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As epidermal cells divide to reform
the outer layer of skin,

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the dermis contracts to close the wound.

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Finally, in the fourth stage 
of remodeling,

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the wound matures as the newly deposited
collagen is rearranged and converted

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into specific types.

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Through this process,
which can take over a year,

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the tensile strength of the new skin
is improved,

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and blood vessels and other connections
are strengthened.

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With time, the new tissue 
can reach from 50-80%

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of some of its original healthy function,

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depending on the severity of the initial 
wound, and on the function itself.

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But because the skin
does not fully recover,

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scarring continues to be a major clinical
issue for doctors around the world.

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And even though researchers have made
significant strides

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in understanding the healing process,

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many fundamental mysteries
remain unresolved.

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For instance, do fibroblast cells arrive
from the blood vessels,

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or from skin tissue adjacent to the wound?

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And why do some other mammals, 
such as deer,

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heal their wounds much more efficiently
and completely than humans?

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By finding the answers to these questions
and others,

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we may one day be able to heal ourselves
so well, that scars will be just a memory.