Notes from the Video

What do you know about wound healing?

Wound healing can be a very complicated topic!  There are cells, phases, growth factors and a lot of details that can leave you dizzy.  Today we're going to make it really easy with this video entitled Wound Healing 101.

What do we need to understand wound healing?

If you're a surgeon or you're going to be a surgeon or you're interested in surgery know that surgeons make wounds and we take care of wounds!  So if you're going to be making wounds you should really know the processes behind how a wound heals and then how you might be able to either help a wound heal or know what you can do to avoid poor wound healing and complications.

When we think about wound healing it's best to think about it with these three things in mind

1. Three phases of wound healing
2. Predominant cell type in each phase 
3. Specific events in each phase

Check out the timeline, it's set up at time of injury, three days, seven days, three weeks and months to years.

First, the THREE phases

One important thing to understand is that all of the phases overlap.

The first phase is the inflammatory phase and this kicks off right after injury and it lasts for about a week.

Overlapping with this around day three day two to three is what's called the proliferative phase and that goes for a few weeks and that extends overlaps with what's called the remodeling phase and that can last for up to two years as the wound and scar matures.

What cells are involved in each of these phases?

Right after injury platelets present for hemostasis but the predominant cell type in the inflammatory phase is the neutrophil.

The neutrophil scavenges debris and gets rid of all the nasty stuff and this cell is responsible for sterilizing the wound which occurs in the first 48-72h.

Next, monocytes come into the wound, they will elaborate a bunch of different growth factors and this overlaps with fibroblasts little bit later in that first week.

In the proliferative phase fibroblasts will be the predominant cell type and start laying down type 3 collagen.

Finally in the remodeling phase we have maturation of the wound.

Inflammatory Phase

Boom let's say you're on the trauma service and you get this guy that came in after this motor vehicle accident and he has this big laceration on his knee, everything's going all right but he has this big laceration on his knee.

So what's happening within the wound in the inflammatory phase?

After this injury first you're going to get intense vasoconstriction and exposure of this sub-endothelial matrix that causes platelet aggregation and formation of that platelet plug.  That's going to be your primary hemostasis.  Now primary hemostasis is that kickstarter for wound healing as bradykinin is elaborated and it will dilate those blood vessels and capillaries so these neutrophils can enter the wound.

Bradykinin will also increase vascular permeability so those neutrophils and monocytes can get out of the bloodstream and into those injured tissues.  With respect to hemostasis and complement cascades, factors c3a and c5a will be chemoattractants and increase vascular permeability even more for cellular entry.

Finally histamine secreted from mast cells and leukotrienes, these are going to break cellular cell contact in the endothelium and really allow those neutrophils and monocytes to get into the wound.

There are a lot of things that are happening, these don't happen in sequence they're basically happening all together.

How about that neutrophil?

You gotta love the neutrophil, so this is the most important cell in the first 24 hours of an injury in a wound and is responsible for sterilizing the wound.

Neutrophils are scavengers, they're going to get rid of debris, they're going to get rid of bacteria, they're even going to get rid of foreign bodies. 

In that first 24 hours neutrophils will clean up the wound .

Right around 48 to 72 hours monocytes are coming in from the local environment and also monocytes that have been attracted from those chemoattractants , for example complement factors c3a c5a, enter the wound and the monocytes are going to become macrophages.   

Macrophages are the primary regulator of wound healing and they're the predominant cell type at 48 to 72 hours.

Macrophages secrete a ton of growth factors everything, TGF alpha and beta, platelet derived growth factor,  fibroblast growth factor, epidermal growth factor, vascular endothelial growth factor.

All of these are secreted by the macrophage and so the macrophage is really responsible for elaborating these growth factors so they can recruit fibroblasts into that wound and start to lay down some collagen.

What, from a surgeon's point of view, is a good example of why it's important to know that the macrophages are predominant at 48 to 72 hours?

Delayed primary closure

This is an excellent tie into basic science and why you need to know the basic science of wound healing.

In delayed primary closure, let's say you have a really sick patient who's septic, you open up their belly and you that they have diffuse peritonitis, maybe they have stool or fecal debris all over the abdomen, maybe it's diverticulitis.  So you do a sigmoid colectomy, you bring out an end ostomy, it's a dirty wound,  a class 4 wound and now it's time for wound closure, what do you do?

In delayed primary closure we're going to close the fascia, if we can do it with minimal tension, but we're going to leave the skin closure open.  Now the questions becomes, when do we close the skin?

We can either leave the wound open and let it heal by secondary intention or we can have a delayed closure of the wound.

The optimal timing for skin closure is between 48 and 72 hours when the neutrophils have sterilized the wound, cleared all of the debris and the macrophages are predominant.

We find that if you do a DPC, a delayed primary closure, at 48 to 72 hours you can have really good outcomes with decreased surgical side infections.

Proliferative Phase

Next is transitioning from this inflammatory phase to the proliferative phase, what is the marker of the proliferative phase?

The marker of the proliferative phase is collagen deposition in the formation of that collagen matrix as well as fibroblast proliferation. 

When we get into the proliferative phase the fibroblast becomes the predominant cell type between days three and five and the function of the fibroblast is to lay down collagen and strengthen this wound.

How do fibroblasts work?

You can go back to that lecture on primary hemostasis where we talk about laying down of that hemostatic plug or that platelet plug and you get this matrix of fibrin and fibrinogen.  The fibroblasts use this when they start to migrate to the wound, again because of those macrophages, and they use this as a scaffold to start to lay down this collagen in extracellular matrix.

Now you can see that in the extracellular matrix, that's composed of fibrin, fibrinogen, macrophages, platelets and glycosaminoglycans.  The most predominant one here is hyaluronic acid, a molecule that absorbs a ton of water, so it's like a sponge. 

Fibroblasts can use all of these together as a scaffold, for starting to lay down the collagen.  Now laying down the collagen and having fibroblasts proliferate is a really complex process.  The formation of this ECM or extracellular matrix is dependent on not only the cells, not only the macrophages and the fibroblasts but it's also dependent on integrins and membrane receptors within the extracellular matrix.

There are more than 27 types of collagen. 

They all have a right-handed triple helix and all of the different types are differentiated by changes in that triple helix, a bend here or a bend there, the difference between type 3 collagen and type 1 collagen.

Now the reason I bring up type 3 and type 1 collagen is type 1 collagen is the most predominant collagen in the skin, both in unwounded skin and in wounded skin but in the early phases of wound healing type 3 collagen is very high.

A good way to think about the ratio of type 1 to type 3 collagen is that there is 4 times as much type 1 collagen as there is type 3 collagen in skin and wound scar.

Now even though there is relatively increased amount of type 3 collagen early in the healing process it's always less than the total amount of type 1 collagen in the mature scar.

Remodeling Phase

After a couple of weeks in the proliferative phase laying down that collagen we get into the remodeling phase.

It goes into action between two and three weeks in a woundand this is where you get further collagen cross linking, regression of the capillaries, the

wound becomes less pink because of the regression, you get maturation of the scar which can take up to a couple of years.

Now there are a bunch of enzymes involved in this process and they can be the collagenases or gelatinases or the matrix metalloproteinases(MMPs), now you don't need

to remember all those but just know that there are a lot of different enzymes responsible for that collagen crosslinking, transitioning from type 3 to type 1 collagen and of course regressing those capillaries and maturing the scar.

What is scar?

What's the difference between normal skin and scar tissue?

Scar is where you have disorganized collagen deposition and also disorganized tissue architecture compared to the skin around it so finally in the remodeling phase one thing to remember and this is frequently tested on your exams is tensile strength.

So at different stages in the wound healing process the wound is going to gain strength.

You can see in the graph that the increase in tensile strength is quite rapid in the first few days of a healing wound but it's still very low as a percentage of the strength of the tensile strength in normal skin and even after waiting six to eight weeks you see that a scar is only going to heal up to 80 percent of the strength of normal skin so it will never be the same.

Growth factors, some important ones to remember...

Platelet-derived growth factor (PDGF) and transforming growth factor beta (TGF-b)

Platelet-derived growth factor or PDGF is secreted by alpha granules in the platelets, macrophages, endothelial cells and fibroblasts.

What does PDGF do?

It is a chemoattractant and attracts neutrophils macrophages and fibroblasts to the wound.

It also stimulates fibroblasts to synthesize new extracellular matrix and increases the collagenase enzyme which is active

in the remodeling process.

How about TGF-b?

Transforming growth factor beta is secreted by platelets, macrophages, fibroblasts and keratinocytes you can and it stimulates collagen synthesis and decreased extracellular matrix degradation.

It also attracts fibroblasts and macrophages to the wound, so it's kind of this self-fulfilling prophecy, you get TGF-b that's secreted, macrophages and fibroblasts

and that attracts more macrophages than fibroblasts in the wound and we've learned that in the inflammatory phase once the neutrophils are done cleaning up the wound the macrophages are the predominant cell type and then of course when we get into the proliferative phase the fibroblasts take over and start laying down that collagen.

Fibroblast Growth Factor  (FGF)

This is secreted by endothelial cells, fibroblasts and macrophages and this stimulates endothelial cells to divide and form new capillaries. 

Remember, wounds need blood to heal and this is also why wounds are a little bit red or pink and so FGF stimulates the formation of these new capillaries.

Vascular Endothelial Growth Factor (VEGF)

This is secreted by platelets, macrophages, fibroblasts and keratinocytes and this stimulates the proliferation of endothelial cells for angiogenesis.

Both of these are incredibly important in increasing the blood supply to get that wound healed.