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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.