This article is an extension of a previous one.
Our biggest organ is assaulted by the harsh outside environment every day. The skin, protecting our whole body and covering up to 2 square meters, is an ever-changing, regenerating shield capable of healing even the most deadly of wounds.
The human skin consists of epidermis, dermis and hypodermis, each containing several layers of distinct tissues. Although this article isn't about the skin specifically, seldom is there a wound not involving healing of the skin. Due to its contact with the environment, it plays an important immunity role as well, keeping the bacteria, viruses and other pathogens from entering our body.
The epidermis is the outermost layer of the skin composed of dead skin cells at the top, called stratum corneum, and the germinative layer beneath, where skin cells are still alive but become filled with keratin. This is also where melanin, also called the skin pigment, shelters the rest from harmful UV light.
Dermis is the layer beneath epidermis that consists of connective tissue and cushions the body from stress and strain. It contains nerve endings, hair folicles, sweat glands, lymphatic and blood vessels and much more. It is the busiest part of the skin, for certain.
Hypodermis, or the subcutaneous tissue is what connects the skin to the bone and muscle. It mainly contains subcutaneous fat and elastin, serving as both padding and insulation for the body.
Figure 1: anatomy of human skin. Source: Oldřich Eliška: Lymfologie: teoretické základy a klinická praxe, 2018. ISBN-10: 80-7492-387-8
Types of wounds
Barring internal hemorrhage and corner-case scenarios, every wound perforates the skin, and thusly it is mostly synonymous with skin healing. This is a dynamic process which includes highly organized and coordinated cellular, humoral (blood-related) and molecular mechanisms and can last up to several years, depending on the wound type and size.
Open wounds are cuts, punctures and other deformations where the skin is split and cracked, with the underlying tissue exposed to surrounding pathogens. In contrast, closed wounds have intact skin surface, but the tissue below is damaged. These wounds usually manifest as bruises and are caused by blunt force trauma. In either case the wound heals more quickly if it is without infection, without missing parts and if the area of the affected tissue is small. A wide but orderly cut will usually heal faster than a patch of scraped-off skin. This is referred to as primary healing – wounds without complications. Secondary healing occurs when immune system needs to intervene, either due to infection or to remove dead cells and tissue parts.
Four stages of wound healing
As soon as the wound is made, its healing begins. The first stage is the hemostasis phase, which has the goal of closing the wound by clotting as fast as possible. The moment blood starts to leak out of the body via ruptured blood vessels, a cascade of factors constrict the surrounding tissue and the plateles start to aggregate. Fibrin strands begin to form, turning the constricted and trapped blood around the wound into gel in a process called coagulation, which keeps the wound sealed and allows the tissue to relax.
Inflammatory phase begins right after the perforated blood vessels leak their content into the surrounding tissue, causing a swelling. This allows lymphocytes and macrophages to remove damaged cells and pathogenes from the area. Inflammation is a natural part of this step, and promotes the recruitment of more leukocytes (white blood cells), growth factors, nutrients and enzymes.
Proliferative phase begins minutes to hours after the onset of the wound and its main goal is to repair the damaged tissue: new network of blood vessels is being built beneath the blood cloth, still present at this point in the form of a scab. The scab should always be left to protect the wound for as long as it is firmly attached to it. This minimizes the amount of time and effort myofibroblasts, the stem cells in skin, need to close the wound. When they are done, epithelial cells can reapply a coating of the skin.
While the wound is now closed, the inside tissue is still in disarray: the blood vessels are positioned suboptimaly, there is tissue rubble around, and the remodeling phase has the goal of ordering it as smoothly as possible. Cells used to repair the wound, but not needed anymore are removed by apoptosis and devoured by macrophages, and collagen gets remodeled into a more organized structure. This begins about 21 days after the injury and can last for more than a year.
Although discussing four separate stages of wound healing, it is important to note that they are overlapping between each other, depending on the state of the surrounding tissue. The further away from the wound we get, the less damage the tissue sustained, and thus would need less time to complete each step. As this part of the wound could already be entering the proliferative phase, the tissue torn up by a knife cut could still be in the hemostasis phase.
Figure 2: the complete timeline of wound healing. Source: Häggström, Mikael (2014). "Medical gallery of Mikael Häggström 2014". WikiJournal of Medicine. DOI:10.15347/wjm/2014.008
Pathogens, age, immune status, insufficient oxygen, obesity, alcoholism, diseases, especially diabetes, and smoking are all negative factors in wound healing. Any wound failed to close in 4 weeks is considered a chronic wound, and mandates the care of a physician. Although the process is far from simple, the body knows what to do as long as the wound is clean and orderly.
1. Guo, S., DiPietro, L. A. Factors Affecting Wound Healing. J Dent Res. 2010 Mar; 89(3): 219–229.
2. Häggström, Mikael (2014). "Medical gallery of Mikael Häggström 2014". WikiJournal of Medicine. DOI:10.15347/wjm/2014.008
3. Nguyen DT, Orgill DP, Murphy GT (2009). "4 The Pathophysiologic Basis for Wound Healing and Cutaneous Regeneration". In Orgill DP, Blanco C (eds.). Biomaterials for Treating Skin Loss. Elsevier. pp. 25–57. ISBN 978-1-84569-554-5
4. Oldřich Eliška: Lymfologie: teoretické základy a klinická praxe, 2018. ISBN-10: 80-7492-387-8
5. Rasche H (2001). "Haemostasis and thrombosis: an overview". European Heart Journal Supplements. 3 (Supplement Q): Q3–Q7
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