The largest organ in your body isn't your liver or your brain. It's your skin, with a surface area of about 20 square feet in adults. Though different areas of the skin have different characteristics, much of this surface performs similar functions, such as sweating, feeling heat and cold, and growing hair. But after a deep cut or wound, the newly healed skin will look different from the surrounding area, and may not fully regain all its abilities for a while, or at all. To understand why this happens, we need to look at the structure of the human skin. The top layer, called the epidermis, consists mostly of hardened cells, called keratinocytes, and provides protection. Since its outer layer is constantly being shed and renewed, it's pretty easy to repair. But sometimes a wound penetrates into the dermis, which contains blood vessels and the various glands and nerve endings that enable the skin's many functions. And when that happens, it triggers the four overlapping stages of the regenerative process. The first stage, hemostasis, is the skin's response to two immediate threats: that you're now losing blood and that the physical barrier of the epidermis has been compromised. As the blood vessels tighten to minimize the bleeding, in a process known as vasoconstriction, both threats are averted by forming a blood clot. A special protein known as fibrin forms cross-links on the top of the skin, preventing blood from flowing out and bacteria or pathogens from getting in. After about three hours of this, the skin begins to turn red, signaling the next stage, inflammation. With bleeding under control and the barrier secured, the body sends special cells to fight any pathogens that may have gotten through. Among the most important of these are white blood cells, known as macrophages, which devour bacteria and damage tissue through a process known as phagocytosis, in addition to producing growth factors to spur healing. And because these tiny soldiers need to travel through the blood to get to the wound site, the previously constricted blood vessels now expand in a process called vasodilation. About two to three days after the wound, the proliferative stage occurs, when fibroblast cells begin to enter the wound. In the process of collagen deposition, they produce a fibrous protein called collagen in the wound site, forming connective skin tissue to replace the fibrin from before. As epidermal cells divide to reform the outer layer of skin, the dermis contracts to close the wound. Finally, in the fourth stage of remodeling, the wound matures as the newly deposited collagen is rearranged and converted into specific types. Through this process, which can take over a year, the tensile strength of the new skin is improved, and blood vessels and other connections are strengthened. With time, the new tissue can reach from 50-80% of some of its original healthy function, depending on the severity of the initial wound and on the function itself. But because the skin does not fully recover, scarring continues to be a major clinical issue for doctors around the world. And even though researchers have made significant strides in understanding the healing process, many fundamental mysteries remain unresolved. For instance, do fibroblast cells arrive from the blood vessels or from skin tissue adjacent to the wound? And why do some other mammals, such as deer, heal their wounds much more efficiently and completely than humans? By finding the answers to these questions and others, we may one day be able to heal ourselves so well that scars will be just a memory.
Najveći organ u vašem telu nije jetra ili mozak. To je vaša koža, sa površinom od skoro 2 kvardatna metra kod odraslih. Iako različiti delovi kože imaju različite karakteristike, većina ove površine vrši slične funkcije, poput znojenja, osećaja toplote i hladnoće, kao i rasta dlaka. Ali nakon duboke posekotine ili rane, sveže zalečena koža će izgledati drugačije od okoline i možda neće neko vreme, ili uopšte, povratiti sve svoje sposobnosti. Kako bismo razumeli zašto se ovo dešava, moramo pogledati strukturu ljudske kože. Gornji sloj, epidermis, sastoji se većinom od stvrdnutih ćelija, keratinocita, i pruža zaštitu. S obzirom da se njegov spoljašnji deo stalno guli i obnavlja, prilično se lako oporavlja. Ali ponekad rana probije u dermis, koji sadrži krvne sudove i razne žlezde i nervne završetke koji omogućavaju mnoge od funkcija kože. Kada se to desi, pokreću se četiri faze regenerativnog procesa, koje se preklapaju. Prva faza, hemostaza, je odgovor kože na dve neposredne pretnje - to što gubite krv i to što je fizička prepreka u vidu epidermisa sada ugrožena. Dok se krvni sudovi skupljaju kako bi smanjili krvarenje, u procesu pod nazivom vazokonstrikcija, obe pretnje se eliminišu tako što se stvara krvni ugrušak. Poseban protein zvani fibrin stvara veze na vrhu kože i time sprečava krv da teče van a bakterije i patogene da uđu unutra. Nakon otprilike tri sata ovog procesa, koža počinje da crveni i time najavljuje sledeću fazu, upalu. Kada je krvarenje pod kontrolom i prepreka osirurana, telo šalje posebne ćelije da se bore sa patogenima koji su možda prošli. Najbitnije među njima su bela krvna zrnca, zvana makrofagi, koja proždiru bakterije i oštećuju tkivo kroz proces zvani fagocitoza, pored proizvodnje faktora rasta kako bi se podstaklo zarastanje. A pošto ovi mali vojnici moraju da putuju kroz krv kako bi došli do mesta ranjavanja, do tada suženi krvni sudovi se šire u procesu zvanom vazodilatacija. Oko dva do tri dana nakon nastanka rane, dešava se faza proliferacije, kada fibroblasti kreću da ulaze u ranu. U procesu taloženja kolagena, oni proizvode vlaknasti protein na mestu rane, zvani kolagen, i tako se stvara vezivno tkivo od kože kako bi se zamenio fibrin od pre. Kako se ćelije epiderma dele da obnove spoljašnji sloj kože, dermis se skuplja da zatvori ranu. Konačno, u četvrtoj fazi preoblikovanja, rana zri dok se sveže nataloženi kolagen raspoređuje i pretvara u određene tipove. Kroz ovaj proces, koji može da traje preko godine, zatezna čvrstoća nove kože se poboljšava i krvni sudovi i druge veze se ojačavaju. Novo tkivo će vremenom dostići od 50 do 80% nekih od svojih normalnih funkcija, u zavisnosti od težine prvobitne rane i same funkcije. Ali s obzirom da se koža ne oporavlja potpuno, ožiljci ostaju jedan od najvećih medicinskih problema za doktore u svetu. Iako su istraživači postigli znatne pomake u razumevanju procesa lečenja, mnoge ključne misterije još nisu rešene. Na primer, da li fibroblasti dolaze iz krvnih sudova ili iz tkiva kože naspram rane? I zašto neki drugi sisari poput jelena leče svoje rane mnogo efikasnije i potpunije nego ljudi? Pronalazeći odgovore na ove i druga pitanja, jednog dana ćemo možda moći da se izlečimo tako dobro da će ožiljci postati samo sećanje.