0:00:07.401,0:00:11.408
The largest organ in your body[br]isn't your liver or your brain.

0:00:11.408,0:00:16.284
It's your skin, with a surface area[br]of about 20 square feet in adults.

0:00:16.284,0:00:19.420
Though different areas of the skin[br]have different characteristics,

0:00:19.420,0:00:22.035
much of this surface performs[br]similar functions,

0:00:22.035,0:00:26.152
such as sweating, feeling heat and cold,[br]and growing hair.

0:00:26.152,0:00:28.105
But after a deep cut or wound,

0:00:28.105,0:00:31.746
the newly healed skin will look different[br]from the surrounding area,

0:00:31.746,0:00:36.481
and may not fully regain all [br]its abilities for a while, or at all.

0:00:36.481,0:00:41.097
To understand why this happens, we need to[br]look at the structure of the human skin.

0:00:41.097,0:00:43.045
The top layer, called the epidermis,

0:00:43.045,0:00:47.219
consists mostly of hardened cells,[br]called keratinocytes,

0:00:47.219,0:00:49.135
and provides protection.

0:00:49.135,0:00:52.192
Since its outer layer is constantly being[br]shed and renewed,

0:00:52.192,0:00:54.085
it's pretty easy to repair.

0:00:54.085,0:00:57.244
But sometimes a wound penetrates[br]into the dermis,

0:00:57.244,0:01:00.664
which contains blood vessels[br]and the various glands and nerve endings

0:01:00.664,0:01:03.565
that enable the skin's many functions.

0:01:03.565,0:01:06.753
And when that happens, it triggers the[br]four overlapping stages

0:01:06.753,0:01:09.798
of the regenerative process.

0:01:09.798,0:01:14.840
The first stage, hemostasis, is the skin's[br]response to two immediate threats:

0:01:14.840,0:01:16.446
that you're now losing blood

0:01:16.446,0:01:20.256
and that the physical barrier of [br]the epidermis has been compromised.

0:01:20.256,0:01:23.007
As the blood vessels tighten to minimize[br]the bleeding,

0:01:23.007,0:01:25.258
in a process known as [br]vasoconstriction,

0:01:25.258,0:01:28.952
both threats are averted by forming[br]a blood clot.

0:01:28.952,0:01:33.140
A special protein known as fibrin forms[br]cross-links on the top of the skin,

0:01:33.140,0:01:38.449
preventing blood from flowing out[br]and bacteria or pathogens from getting in.

0:01:38.449,0:01:41.902
After about three hours of this,[br]the skin begins to turn red,

0:01:41.902,0:01:44.828
signaling the next stage, inflammation.

0:01:44.828,0:01:47.726
With bleeding under control[br]and the barrier secured,

0:01:47.726,0:01:52.079
the body sends special cells to fight any[br]pathogens that may have gotten through.

0:01:52.079,0:01:54.963
Among the most important of these[br]are white blood cells,

0:01:54.963,0:01:56.732
known as macrophages,

0:01:56.732,0:02:02.004
which devour bacteria and damage tissue[br]through a process known as phagocytosis,

0:02:02.004,0:02:05.718
in addition to producing growth factors[br]to spur healing.

0:02:05.718,0:02:08.082
And because these tiny soldiers [br]need to travel

0:02:08.082,0:02:10.187
through the blood to [br]get to the wound site,

0:02:10.187,0:02:13.597
the previously constricted [br]blood vessels now expand

0:02:13.597,0:02:16.253
in a process called vasodilation.

0:02:16.253,0:02:18.443
About two to three days after the wound,

0:02:18.443,0:02:23.238
the proliferative stage occurs, when [br]fibroblast cells begin to enter the wound.

0:02:23.238,0:02:25.681
In the process of collagen deposition,

0:02:25.681,0:02:29.749
they produce a fibrous protein [br]called collagen in the wound site,

0:02:29.749,0:02:33.934
forming connective skin tissue[br]to replace the fibrin from before.

0:02:33.934,0:02:37.717
As epidermal cells divide to reform[br]the outer layer of skin,

0:02:37.717,0:02:40.610
the dermis contracts to close the wound.

0:02:40.610,0:02:43.217
Finally, in the fourth stage [br]of remodeling,

0:02:43.217,0:02:47.601
the wound matures as the newly deposited[br]collagen is rearranged and converted

0:02:47.601,0:02:49.428
into specific types.

0:02:49.428,0:02:52.116
Through this process,[br]which can take over a year,

0:02:52.116,0:02:55.231
the tensile strength of the new skin[br]is improved,

0:02:55.231,0:02:58.639
and blood vessels and other connections[br]are strengthened.

0:02:58.639,0:03:02.104
With time, the new tissue [br]can reach from 50-80%

0:03:02.104,0:03:04.566
of some of its original healthy function,

0:03:04.566,0:03:08.818
depending on the severity of the initial [br]wound and on the function itself.

0:03:08.818,0:03:11.098
But because the skin[br]does not fully recover,

0:03:11.098,0:03:15.751
scarring continues to be a major clinical[br]issue for doctors around the world.

0:03:15.751,0:03:18.592
And even though researchers have made[br]significant strides

0:03:18.592,0:03:20.693
in understanding the healing process,

0:03:20.693,0:03:23.871
many fundamental mysteries[br]remain unresolved.

0:03:23.871,0:03:27.453
For instance, do fibroblast cells arrive[br]from the blood vessels

0:03:27.453,0:03:30.039
or from skin tissue adjacent to the wound?

0:03:30.039,0:03:32.619
And why do some other mammals, [br]such as deer,

0:03:32.619,0:03:37.296
heal their wounds much more efficiently[br]and completely than humans?

0:03:37.296,0:03:39.831
By finding the answers to these questions[br]and others,

0:03:39.831,0:03:44.794
we may one day be able to heal ourselves[br]so well that scars will be just a memory.