Our body’s immune response to foreign invaders is an intricate and fascinating process involving different cell types, systems, defence mechanisms, and responses. This article aims to give a glimpse into the role that wound healing plays in fighting infection and regenerating tissue.
The lines of defence and their role in wound healing
Our body has two types of defences against pathogens - the non-specific defences (called “innate immunity”) and the specific defences (called “adaptive immunity”).
The non-specific defences are those that initially combat an invading pathogen and do not differentiate between different attackers (a.k.a. pathogens). The specific line of defence, on the other hand, adapts to the type of infection (as its name may suggest).
The non-specific defences can be divided into two parts:
The first line of defence: Consists of surface barriers that prevent pathogen entry into the body. Examples of this include our skin (which is an external boundary) and mucous layer (an internal boundary).
The second line of defence: Involves non-specific cellular and molecular responses that don’t differentiate between pathogens. A great example of such defence would be phagocytic leukocytes (a special type of white blood cell), which travel to the infection site and engulf the pathogen (this is known as phagocytosis). Following that, this step is assisted by inflammation, fever, and antimicrobial protein regulation.
In contrast, the specific line of defence involves:
The third line of defence: The specific immune response takes place, where lymphocytes (white blood cells) produce antibodies (protective proteins generated by the immune system in response to these foreign invaders) specific to the pathogen to fight back against the infection
Figure 1: A summary of the body’s lines of defence including the innate (non-specific immune response) and the adaptive immunity (specific immune response).
The importance of our skin
As the largest organ in the body, skin acts as a barrier between pathogens and the rest of your body, and is the first line of defence that protects your body’s internal structures when intact.
It consists of dry, thick, and rough regions composed of dead surface cells, while also containing biochemical defence agents where the sebaceous glands secrete chemicals and enzymes (i.e. biological catalysts that can speed up chemical reactions without getting consumed in the process) in order to inhibit microbial growth.
Besides that, the pH of the skin can differ depending on different conditions. If our skin detects a high pH level, it can secrete lactic acids and fatty acids to lower it. The fatty acids consequently inhibit microbial growth on the surface as it creates a dry, acidic, and salty environment.
Figure 2: Representation of the layers of the skin, including the epidermis (the outermost layer which is made of epithelial cells), the dermis (the middle layer containing nerves, blood and lymph vessels, and hair follicles), and hypodermis or subcutaneous tissue (the bottom layer made out of fat cells).
The skin has three layers - the epidermis, dermis, and subcutaneous tissue layers.
The thin layer of skin located on the surface level is called the epidermis. This layer is made of epithelial cells packed with keratin. Given that keratin is a key material making up our hair and nails, it therefore makes the surface of our skin tough and resistant to bacterial degradation.
Next is the thicker dermis layer, where the hair follicles, sweat glands, nerves and blood vessels are located. Finally, underneath the dermis lies a layer of fatty tissue – the hypodermis or subcutaneous tissue. This layer is made of adipose and connective tissue, which serves as a connection between the other skin layers and the fibrous tissue of muscles and bones. Additionally, the hypodermis also stores fat and serves as cushioning and isolation for the skin.
The stages of wound healing
The process of wound healing revolves around preventing blood loss, defending and repairing the wounded area, as well as healing.
This phenomenon is achieved through three main stages:
1. Haemostasis and inflammation
This step begins right after we get a cut (or in other words, once bleeding starts).
Vasoconstriction occurs during this stage, whereby the blood vessels near the wound are constricted so that blood flow to the site of injury is massively reduced. To do this, our body increases the calcium concentration within smooth muscle cells. This, in turn, triggers muscle contraction by inducing a change in the shape of a component of muscle fibre filaments known as troponin. Meanwhile, the release of another type of protein called endothelin can also lead to vasoconstriction.
Figure 3: A diagram showing platelets and fibrin coming together to form an initial thrombus (a.k.a. blood clot).
Following that, a substance known as subendothelial collagen binds to platelet receptors to initiate the aggregation of platelets (a type of blood cell; a.k.a. thrombocytes) to form a primary thrombus. Subendothelial collagen is found within the subendothelial extracellular matrix, which is an extracellular space between the epithelium cells of the epidermis layer and the dermis (as shown in Figure 2). On the other hand, platelets are produced in our bone marrow and have a “sticky” texture, thanks to the presence of platelet clotting factors. This thereby helps them to be amazing at initiating blood clots to stop bleedings.
To facilitate the adhesion process, a receptor known as integrin gets activated, in which its primary role is to facilitate the interaction of platelets so that a “shell” at the site of injury is formed.
Once that is done, a cascade of coagulation factors is initiated, thereby activating an enzyme called thrombin. Following that, this enzyme catalyses the conversion of a soluble plasma protein termed fibrinogen to insoluble fibrin fibres. As a result, these fibrin strands form a mesh around the platelet “shell” to stabilise the thrombus.
Once the thrombus is formed, the blood vessels can then expand and become more permeable so that oxygen and nutrients can reach the wound. Other than that, white blood cells (also termed as “inflammatory cells”) such as macrophages and neutrophils also arrive at the site to help remove damaged cells and any pathogens that may still be present within that area.
2. Proliferation
In essence, this step is all about tissue regeneration.
To begin with, the wound bed is filled with connective tissue (called the granulation tissue). During the process, new blood vessels need to be constructed in order for oxygen and nutrients to be continuously supplied to the new tissue. Besides that, cells known as myofibroblasts participate in the tissue remodelling process by gripping the edges of the wound and pulling them together. In fact, you can almost equate this to the smooth muscle contraction process, and the whole purpose of it is to reduce the size of the wound.
Finally, re-epithelialisation takes place. Epithelial cells (cells that line the surface of your body) arise from the wound edges and keep on migrating towards its centre until the wound is fully covered.
Figure 4: The process of re-epithelialisation. This involves epithelial cells arising from the edges of the wound and then migrating towards the centre.
3. Maturation
This is the last stage, where the wounded tissue finally recovers. It involves the remodelling of collagen fibres from type III to type I, resulting in better skin flexibility and a fully closed wound. Meanwhile, apoptosis (programmed cell death) also occurs to get rid of any unnecessary cells.
Factors affecting wound healing
Basically, there are several factors that affect the rate of the wound healing process:
The location of the wound: An area of the body that has less vascularity could have a slower wound healing process because the supply of oxygen and nutrients is much more limited.
Temperature: Higher temperature increases the rate of chemical reactions and enzyme activity.
Wound hydration: Hydration allows a faster delivery of oxygen and nutrients to the infection site.
Maceration: This essentially means that the wound is in prolonged contact with moisture, and having too much hydration would consequently be detrimental as it can delay the healing process.
Necrotic tissue: Necrotic tissue is basically dead or devitalized tissue. Given that they may contain pathogens such as bacteria, it is imperative to remove them early because their presence may reduce the rate of wound healing or even can cause further infections.
An introduction to connexin-43
Connexins are a type of integral membrane protein (i.e. they are embedded within the lipid bilayer of cellular membranes) that form intercellular channels between cells which are located next to each other.
Gap junctions are formed when these channels come together, and they basically enable the movement of ions and metabolites between adjacent cells as well as permit intercellular communication. Besides that, these gap junctions consist of two hemichannels that interact with each other.
Figure 5: A schematic of gap junctions. Gap junctions are integral membrane proteins that form channels between two adjacent cells. These gap junctions are basically made of two hemichannels, with each hemichannel constituting a total of six connexin proteins.
There are a total of 21 members within the human connexin protein family, all of which are found in all tissues other than the differentiated skeletal muscle, mature sperm cells, and erythrocytes (a.k.a. red blood cells).
Out of these 21 proteins, connexin-43 is the most widely researched, and there has been evidence of connexin-43 being involved in wound healing. In particular, it has been shown that connexin-43 plays a role in regulating the inflammatory process (i.e. part of the first stage of the healing process). Therefore, a reduction in the expression of connexin-43 would result in the delay of the overall process.
Infected wounds and the role of fever
In simple terms, fever is the abnormal increase of our body temperature above its normal range (>37.8°C).
Figure 6: An illustration of how fever is initiated - from the pathogen detection stage to the triggering of a fever.
This occurs when pathogens enter the body, causing the activation of leukocytes (a type of white blood cell) and thus the release of signalling molecules known as cytokines (Figure 6). These cytokines, in turn, signal a region of our brain called the hypothalamus to produce prostaglandin, which is a hormone-like compound that can trigger fever (Figure 6).
Fever is important during wound healing as it can combat infection by inactivating enzymes in pathogens that contribute to their proliferation. What’s more, fever also activates a group of proteins termed as “heat shock proteins”, and they increase the rate of metabolism for a faster wound healing process.
Nevertheless, although experiencing fever initially can help combat infection, a prolonged fever can be detrimental to the body as it may cause our body’s own enzymes to denature.
Medical conditions that affect wound healing
To put things simply, certain diseases can compromise the wound healing process. Two good examples of this are haemophilia and diabetes.
Haemophilia
Haemophilia is an inherited disease that hinders the blood clotting process by impairing the formation of fibrin as well as the regular coagulation cascade. As a result, patients suffering from haemophilia may experience frequent bleeding episodes, some of which could be life-threatening especially if a vital organ (such as the brain) has uncontrolled bleeding.
Luckily, haemophilia can be easily treated by physicians, whereby this involves delivering the missing blood clotting factor intravenously (i.e. injecting them directly into a patient’s vein) so that the regular blood clotting process can be restored. Meanwhile, prophylaxis (i.e. preventative treatment) can also be practiced. For instance, patients can be educated on how to perform the infusions themselves so that they are able to prevent bleeding episodes on their own.
Diabetes
Diabetes is a disease whereby the patient has abnormally high blood glucose levels. Diabetes can cause impairment in wound healing as it can lead to dysfunctional epidermal cells and hypoxia (deprivation of oxygen supply to tissues).
What’s more, having diabetes also increases the risk of atheromas (a.k.a. plaques) developing within our blood vessels, thus reducing the blood vessel’s overall diameter. As a result, a “roadblock” is created, and this creates a restriction of blood flow as well as a higher pressure inside the blood vessel. Besides that, diabetes has been linked to deep vein thrombosis, a condition that involves the formation of blood clots within deep veins such as those in the legs. In other words, it is not a good sign either.
Figure 7: Diabetes increases the risk of plaque build-up in blood vessels. Due to the formation of a plaque, there is a restriction of blood flow within the artery. As a result, blood can either flow at higher pressure or not reach the desired tissues. Therefore the tissue affected does not receive the essential nutrients and oxygen required for performing its proper function.
Why should you care?
Our body utilises complex and intricate processes to protect us. Wound healing is one of the many examples of such mechanisms.
From detecting a wound within seconds to stopping blood loss to finally reforming the injured area, wound healing essentially helps us combat infections using white blood cells and other processes such as inflammation.
Hence, aiding and improving the body’s immune response is a hot area of research. In particular, different forms of skin transplantation such as 3D printed epithelial cells or electric bandages to accelerate the wound healing process are currently under research. With these advances, maybe one day we will be able to redefine how wounds can be treated!
Author
Covadonga Piquero Lanciego
BSc Biological and Biomedical Sciences
University of Dundee