Angiogenesis in Wound Healing

00:01 All right, then.

00:02 Let's move along.

00:03 We have moderated acute inflammation.

00:07 We're into the stages now, where we are really trying to get the definitive healing to go on.

00:15 In this process when we cannot get regeneration, we need to lay down scar.

00:21 And the first step in that is angiogenesis.

00:25 Here's a roadmap.

00:26 You can see we've already covered the top three topics, moderating acute inflammation, recruiting and activating macrophages, regenerating parenchyma, if we can.

00:37 The next two are what about not being able to regenerate and that starts as he said with angiogenesis.

00:45 Angiogenesis is recognized by us as pathologists as granulation tissue.

00:54 A word granulation tissue is not a granuloma.

00:59 Remember, granulomas are a nodule of activated macrophages.

01:03 Granulation tissue is this early provisional stroma with a lots of new blood vessels upon which we're going to lay down scar.

01:13 So granulation tissue is no more like a granuloma than it is like your grandma.

01:19 Okay, just make that really clear.

01:23 This is granulation tissue.

01:25 It's going to peak in tissues that have been injured at about five to 10 days.

01:30 It's very loose connective tissue.

01:33 It's got a lot of edema, because the vessels are incredibly leaky.

01:37 It's got a few residual inflammatory cells, but mostly what granulation tissue is, are these small, new, delicate capillaries that we've just built.

01:48 And why is it called a granulation tissue? Well, if you're anything like me, I cannot leave a scab alone.

01:53 So when I have a scab, I pick it.

01:55 And underneath are these little pink granules, they kind of they leak blood, because there was a very delicate capillaries.

02:02 And the surgeons of your recognize that in a wound that had lots of good granulation tissue, that wound was going to do well, it was going to heal in.

02:11 But if there wasn't good granulation tissue, because of the little, you know, the capillaries weren't forming, well, that was going to likely get infected.

02:19 So it's called granulation, because the little apparent granules, a pink capillaries that you can see under a scab.

02:25 Okay. That's a sidelight.

02:28 In any event, this is what it looks like.

02:31 What it is? New Delicate Vasculature Has very loose extracellular matrix, as you can see there.

02:38 There are residual chronic inflammatory cells.

02:40 In fact, the macrophages mostly M2 macrophages at this point, are going to be responsible for coordinating all the next few steps.

02:49 And in fact, they're the major source driving the formation of the granulation tissue.

02:53 So they need to be there to do this.

02:56 It's very edematous, because the vessels are leaky.

03:00 And it bleeds very easily.

03:02 As what happens when you pick that scab and the little vessels bleed.

03:06 It's the scaffolding.

03:07 It's the provisional matrix upon which we're going to lay down scar.

03:11 Okay, so that's, it's all of those things.

03:14 So how exactly do we get these new blood vessels? What is the mechanism of angiogenesis? All right, on the right hand side of your screen, it's a big white area.

03:25 That's a dead zone, that is where we've had tissue injury and all kinds of death and destruction occur there.

03:31 There will be macrophages that are out there, because they're cleaned up the debris, and they're going to be the ones driving the next stages.

03:37 So the very first stage of angiogenesis, we have to break down the membrane, the basement membrane, at the nearest blood vessel that's intact.

03:49 So we need to be able to have a sprout of new blood vessels that will grow into the dead zone.

03:54 And that proteolysis of extracellular matrix is driven by molecules, mediators, elaborated by the macrophages.

04:01 So you see the macrophages there, and those little dots, those are going to be various factors that are going to drive the breakdown of extracellular matrix.

04:10 What are those? Some of the factors there are others, but the two main ones are basic FGF, basic fibroblasts derived growth factor made by macrophages.

04:23 And then you have vascular endothelial growth factor or VEGF, also made by macrophages and other cells.

04:29 But that's going to start the first process of breaking down the basement membrane, and allowing us to have a bud of new capillary growth.

04:38 Next step, we need to have the endothelial cells that are there at that new bud migrate in the direction of where we want new blood vessels to form.

04:48 So we want them to migrate to chemotaxis.

04:51 And you see the macrophages out there spewing some more little bubbles, and those are going to be the factors that drive this process.

04:59 And what are the those? Wow, it's basic FGF and VEGF again.

05:05 So far, we only have to memorize two additional factors, for the most part.

05:11 Basic FGF and VEGF, cause the proteolysis of the bsal membrane and also now drive the chemotaxis of the endothelial cells out into the dead zone.

05:21 Okay, well, it's just not enough to have them migrate.

05:26 Clearly, they can't get stretched thinner, and thinner, and thinner.

05:28 We need to make more of them in order to make a blood vessel of new capillary.

05:32 So they have to proliferate.

05:34 And again, you see the macrophages out there and the dead zone spewing pixie dust to make this happen.

05:40 What's in the pixie dust this time? Wow, same stuff, basic FGF and VEGF, and that's going to be driving proliferation of the endothelial cells.

05:50 Cool.

05:51 Okay, it's not just enough that we have a whole bunch of endothelial cells growing and marching into the dead zone.

05:57 They need to form a tube so that we can bring in blood.

06:00 So we need to actually form a mature capillary.

06:05 That maturation has to also happen with kind of an inhibition of growth.

06:09 We want things to stop.

06:11 How do we make that happen? The lumen formation, the maturation, and inhibition of growth, Do you think it's basic of FGF and VEGF? No, unfortunately, it's not.

06:21 It's two other factors, or it's one other factor called angiopoietin-1 that's elaborated by perisites, P-E-R-I sites that are part of the maturing capillary.

06:34 And there are receptors for angiopoietin-1 called Tie-2 that live on the endothelial cells.

06:41 So that maturation process, different set of factors, unfortunately.

06:47 In the process of angiogenesis, we've got a mature blood vessel that's out there, but we are now going to have a very highly metabolically active tissue.

06:55 We're going to lay down a lot of new tissue out there.

06:58 We need a fair amount of nutrition to make that work.

07:02 So one of the ways that we can improve that deposition of nutrition is to make the vessels a little extra leaky, so that more nutrition gets out fairly easily, without having to be transported.

07:15 To do that, we need to increase permeability through gaps or through transcytosis moving contents that are in the lumen out into the newly forming scar tissue.

07:29 Again, macrophages are driving this with little bits of of pixie dust, a little balls, and what's in there? Again, basic FGF and VEGF.

07:40 So for most of the factors involved in angiogenesis, we can actually blame it, or attributed to basic FGF and VEGF.