If you are anything like me, you would know the experience of sobbing over a tiny paper cut after being too careless with the side of a page or the scrapes on a knee after falling on gravel.
Our skin experiences constant wear and tear, yet it manages to pull itself together and create new flesh to cover our squishy insides. The question is HOW exactly does our skin slowly recover? And since we’re on the topic, how does scarring occur?
This article will discuss the healing of open wounds and the process they undertake.
Layers of Skin - the Dermis and Epidermis
Figure 1. A diagram illustrating the layers of skin
But before we go into more detail, we need to know the basics first.
The epidermis is the outermost layer of the skin, lacking any blood veins and consisting of 10-30 layers of dead skin cells which are constantly pushed up and replaced by dividing cells below them in the dermis. It serves as the barrier between the outside world and your insides (See Figure 1).
On the other hand, the dermis contains hair follicles, nerve endings, blood vessels, and collagenic fibers (remember that last one). The dermis is what gives the typical qualities people envision when they think of skin: flexibility and durability.
TIP: "Epi-" is a prefix originating from Greek, which could mean “upon,” “on,” or “over”. Thus, the epidermis is on top of the dermis.
The 4 Stages of Wound Healing
These 4 stages have overlapping transition periods (see Figure 2).
Figure 2. A timeline (log scale) showing the overlap of the 4 stages of wound healing.
1. HEMOSTASIS
Goal of hemostasis: prevent blood loss and entry of pathogens.
Hemostasis starts almost immediately after a wound is caused; when the inner layer of your skin, the dermis, and blood vessels are punctured. With the skin broken, you are in danger of blood loss, and your body lacks a physical barrier to defend you from bacteria and viruses.
To remedy this, a fibrin clot (a fancy term for a blood clot) is formed.
Here’s a breakdown of what happens during hemostasis:
Chemical signals instruct the blood vessels at the sight of injury to contract [vasoconstriction] to prevent blood loss.
The damaged vein exposes collagen, which sends several chemical signals. These tell the spleen to release platelets which travel through the circulatory system to the wound where they are activated and become sticky (See Figure 3). This allows them to clot together and form a tentative barrier to block the bleeding. This forms a platelet plug. This process is known as primary hemostasis.
Fibrin protein molecules form long sticky threads that interlace to form a fibrin mesh that helps cells travel into the damaged area and reinforce the platelet plug, eventually turning into the final study fibrin clot – also known as a scab. This is known as secondary hemostasis.
Figure 3. Diagram demonstrating the movement of platelets during hemostasis
The time length of hemostasis depends on the severity of the wound. It could take only a few minutes to a few hours for non-major wounds. In the example of a scraped knee, the effects of hemostasis are visible when the bleeding stops and a scab forms.
Not only do platelets prevent blood loss, but they further assist in closing wounds by contracting and pulling the injured vein’s two sides together. This is easier to see in a paper cut when a scab doesn’t obviously form and the skin can be seen being pulled together.
To learn more about collagen, click here!
2. INFLAMMATION
Goal of inflammation: To clean up the injury site so that new tissue can be formed.
Once blood loss is controlled, the body shifts its focus to cleaning the wound site—a process known as inflammation.
Inflammation occurs after the fibrin clot has been completed, and blood, salt, protein, and water from blood vessels pool into the surrounding area. Normally, you notice your wound turning red, hot, and swelling.
It is during this stage that blood vessels dilate [vasodilation] so that white blood cells (eg, macrophages and neutrophils) can traverse easily to enter to clean the site of injury. They remove pathogens, foreign substances, and damaged cells from the area through phagocytosis.
Phagocytosis: a process where white blood cells engulf and destroy foreign or unnecessary substances through endocytosis.
TLDR: a white blood cell eating foreign or unwanted substances.
This prevents infection from bacteria or viruses and prepares the area for new tissue growth.
Generally, the inflammation reaches a peak 24-48 hours after initiation, before rapidly declining in numbers. This stage lasts around 6 days to 2 weeks for fairly minor wounds depending on the severity.
While it tends to be the most painful part of wound healing, inflammation is natural and should not be worried about unless it is prolonged or extreme.
3. PROLIFERATION
Goal of proliferation: To fill with new tissue and to cover the wound.
After the bleeding has stopped and the site is clear, your body can start repairing and filling the wound with new skin cells.
Proliferation is also known as ‘granulation’ for the new granulated tissue that forms.
Granulation: (in biology) to have a roughened surface.
Filling the wound: In what is called collagen deposition, chemicals signal cells such as fibroblasts to produce collagen (type 3), which acts as scaffolding and structural support for connective tissue to fill the wound, helping cells to travel into the injury. Over time, a small network of minute blood vessels regrow and deliver oxygen. This results in the tissues’ pinkish-red color with a rough and uneven texture.
Contraction: Dermis tissue contracts to help close the wound further. It appears and feels as if the wound has tightened.
Covering the Wound: Proliferation ends by regrowing epidermal cells (cells that act as a barrier between the inside of your body and the outside world) through identical cell division (mitosis) to create new tissue and close the wound.
By the end of proliferation, the wound is tender but healed over.
Granulation tissue derives its name from its appearance; it develops capillaries that tend to protrude from the surface of a healing wound to produce minute red granules.
4. MATURATION
Goal of maturation: To recover the skin to its original state
The fourth and final stage lasts the longest and starts when the wound has been covered in collagen and tissue.
At this point, collagen synthesis slows down, and the collagen deposited by fibroblasts is reorganized and converted into different types. This includes the transformation of type 3 collagen from a delicate and flexible matrix to the tensile strength and stability of type 1 collagen, which builds a sturdy scar.
Remodeling can take anywhere from a month to 2 years or more to complete depending on one's personal health and the severity of the wound.
Generally, the scar tissue formed during the maturation process is roughly 20% weaker than pre-injured skin with reduced elasticity.
Summary
In short, the body goes through a four-step process to heal wounds. It: (Figure 4):
Stops the bleeding (hemostasis).
Cleans the area (inflammation).
Rebuilds and fills the wound (proliferation).
Strengthens the new tissue (maturation).
Figure 4. Diagram of Stages of Wound healing
Role of Collagen in Wound Healing
What happens when a wound dries up? – Understanding Scarring
When cells die from a wound, the body panics just a tad and rushes to produce collagen to cover the area left behind by the cell. More collagen means a higher chance of scarring.
Collagen isn’t a bad substance; it is an essential part of skin and wound healing and plays a role in skin elasticity and durability. However, excessive amounts make skin rigid, thick, tight, and unable to function properly.
Generally, if a wound becomes covered with epithelial tissue within two weeks, minimum fibrotic collagen is deposited and no major scarring occurs. Alternatively, if the wound is not filled within two weeks, a high amount of fibrotic collagen is deposited, increasing the likelihood of scarring.
Furthermore, collagen fibers in healthy skin tissue are arranged similarly to a basketweave. However, collagen in scar tissue is arranged in one direction, resulting in reduced elasticity and durability, and interfering with cell processes.
How to Help a Wound Heal
Now that we know how wounds heal, how can we help our body speed up the process?
Wounds should be kept clean to prevent infection and warm for adequate circulation of blood to deliver nutrients. Wounds should also be kept moist. A wet bandage covering acts as a medium that eases the migration of cells during proliferation. Furthermore, water is used in several crucial chemical reactions that cells need to function; a dry cell ends up dying from being unable to complete these processes. Thus, a dry wound means dead cells, leading to a slower recovery rate and scarring.
Conclusion
Now you know the complicated process of wound healing! Next time you get a wound, think about the amazing ways human bodies are designed to function and adapt to everyday actions, including paper cuts.
Fun Facts
Cartilage is the slowest healing tissue because of its lack of blood circulation [avascular].
The hemostasis phase is debated to be a preliminary phase of inflammation.
Wounds that require over 4 weeks to heal are known as chronic wounds.
Reference list
Abdullzaher, E.M. ed., (n.d.). Wound Healing. [online] Physiopedia. Available at: https://www.physio-pedia.com/Wound_Healing [Accessed Aug. 2024].
FuseSchool - Global Education (2018). Platelets & Blood Clotting | Biology | FuseSchool. YouTube. Available at: https://www.youtube.com/watch?v=gExUCrpAKyQ [Accessed 3 May 2020].
Granulation Tissue. (n.d.). [online] Atlas of Head and Neck Pathology, p.1. Available at: https://medicine.osu.edu/-/media/files/medicine/departments/otolaryngology/atlas-of-head-and-neck-pathology/g/granulationtissue2.pdf?rev=51795c2257614f4a83ccb7a3627de158&hash=F27478FF823D33778D9CD86F0713F677#:~:text=GRANULATION%20TISSUE- [Accessed 31 Aug. 2024].
Hasudungan, A. (2023). Wound Healing - Stages of healing and pathology. [online] www.youtube.com. Available at: https://www.youtube.com/watch?v=Q3cVCiraAgw [Accessed 31 Aug. 2024].
HealthPartners Blog. (2021). Wound healing stages: What to look for. [online] Available at: https://www.healthpartners.com/blog/wound-healing-stages-how-to-tell-if-a-wound-is-healing-infected-or-chronic/.
Kaur, N. (2023). 4 stages of healing a wound. [online] Credihealth. Available at: https://www.credihealth.com/blog/stages-of-healing-a-wound.
Sargen, M. (2019). Biological Roles of Water: Why is water necessary for life? [online] Science in the News. Available at: https://sitn.hms.harvard.edu/uncategorized/2019/biological-roles-of-water-why-is-water-necessary-for-life.
ttsz (2021). Skin layers. Epidermis, Derma, Hypodermis. [Online Image] iStock. Available at: https://www.istockphoto.com/vector/skin-layers-epidermis-derma-hypodermis-structure-of-the-human-skin-gm1344816437-423067465 [Accessed 29 Sep. 2024].
Vera, M. (2016). Phases of Wound Healing: The Breakdown. [online] WoundSource. Available at: https://www.woundsource.com/blog/phases-wound-healing-breakdown.
www.aad.org. (n.d.). Scars: Who gets and causes. [online] Available at: https://www.aad.org/public/diseases/a-z/scars-causes.
www.elastoplast.com.au. (n.d.). What Are the Phases of Wound Healing? [online] Available at: https://www.elastoplast.com.au/first-aid/wound-care/what-are-phases-of-wound-healing.
Commenti