Wound healing

Wound healing is the process where the body repairs damaged tissue after any traumatic injuries--anything from a paper cut to a stab wound. Acute wounds heal up quickly over days to weeks, whereas chronic wounds like bedsores, foot ulcers, or infections, can persist for months.

Now, some tissues regenerate more easily than others, and the regenerative capacity of tissue is classified as labile, stable, and permanent. Labile tissues like skin, as well as small and large intestine, heal extremely well, because they contain stem cells, which are undifferentiated cells that can divide and renew the cells that have died. Stable tissue, like the liver, recovers from injury by having mature differentiated cells divide or regenerate via hyperplasia. Permanent tissues like neurons and cardiac tissue have a weak regenerative capacity, because they lack these stem cells and cannot replicate via hyperplasia. Typically, injured permanent tissues are replaced by scar tissue or fibrosis--ultimately resulting in loss of function of the tissue.

Now when it comes to the skin - which is often the most visible tissue that’s damaged, wound healing occurs by primary intention, secondary intention, and tertiary intention. Healing by primary intention is when the wound edges come together--like what happens when two wound margins are stitched or sutured together. When this happens, stem cells in the epidermis, or uppermost layer of skin are brought close together and can regenerate the damaged tissue near the surface of skin--leaving a minimal scar. Healing by secondary intention occurs when the wound edges are left open. This can be the consequence of significant tissue loss, where the edges are too far from one another to meet, or when it is not safe to bring the wound edges together, like in the case of a contaminated wound. Examples of healing by secondary intention where the skin edges cannot meet include tooth extraction sockets or severe burn injuries; while an example of healing by secondary intention due to wound contamination is a laceration caused by soiled farm equipment. Since the edges of these wounds do not approximate, the wound is replaced primarily by granulation tissue, a mix of connective tissue and blood vessels, that grows from the base of the wound upwards. Healing by tertiary intention, or delayed closure, is when a wound is cleaned and purposefully left open at first, due to a high likelihood of being contaminated by bacteria--like during a dog bite injury. If these wounds are closed too early by primary intention they can trap bacteria inside the wound--leading to an abscess or walled off infection. Instead, these wounds are left open, cleaned, and observed for several days before they’re closed.Hemostasis

Now, to explain the steps of wound healing, let's imagine that you’re cutting a banana cream pie and you accidentally slip and cut your finger, ouch! During a penetrating injury like this, the skin is punctured through the top two layers of skin --the epidermis and the dermis. When this happens damage occurs to both the cells found in these layers and the area between the cells, called the interstitial space. The interstitial space contains proteins outside of the cell that act like the scaffolding between cells, so when this is damaged the cells separate--forming the open wound. Now, the first step of wound healing is hemostasis, and that’s where a blood clot forms at the site of trauma to prevent further blood loss. Hemostasis starts when blood vessels affected by trauma constrict. Platelets, which are tiny blood components involved in coagulation, immediately aggregate around the affected tissue and stick together to form a platelet plug. The platelet plug is further reinforced by a protein mesh called fibrin, which ultimately forms the blood clot.

Once an adequate blood clot has formed, the second step of wound healing begins - and that’s inflammation. Damaged cells release tiny proteins called chemokines and cytokines, which recruit immune cells like macrophages and neutrophils to the area, and make nearby blood vessels dilate and become more leaky or permeable. Immune cells start to seep into the affected area and they begin to clear foreign debris like dirt and digest dead or damaged cells caused by the injury, as well as destroy any microbes that may have gotten in. At the end, there’s a blood clot and dead macrophages that have consumed damaged cells and other debris, and this forms a scab, our body’s own bandage.

Now, the third step of wound healing is epithelialization, or migration, which is where basal cells, or the stem cells in the epidermis start to proliferate in order to replace the lost or damaged cells. This process takes about 48 hours, and ends when the epidermal layer is rejuvenated. But this newly formed epidermal layer is still quite weak because the stronger dermal layer below hasn’t yet regenerated.