BENG 100: Frontiers of Biomedical Engineering
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Frontiers of Biomedical Engineering
BENG 100 - Lecture 22 - Tissue Engineering
Chapter 1. Introduction to Tissue Engineering [00:00:00]
Professor Mark Saltzman: So, this week we’re going to be talking about the subject of tissue engineering. It’s covered directly in Chapter 14 but not in a lot of detail. I provided a couple of other readings which are listed here on this slide, these are in the lecture notes. If you go that folder there’s two papers, one from 1993 and one from 2002. I would read them in that order, read the one from 1993 and then the one from 2002. You’ll see sort of how this field, which I’m going to describe today, has evolved over the last 15 years or so. Some of the challenges that were talked about in this article from 1993, you’ll see, are still present today. It’s turned out to be more difficult than biomedical engineers thought to accomplish, some of these things. Some of the things have gone faster, we’ve made more progress than we thought we would in 1993. If you read those two papers, read the bit from Chapter 14 about tissue engineering, and then Chapter 16 I’ve also given you. Chapter 16 is about biomaterials and artificial organs, but there might be some things in there which are useful to you in understanding this subject as well. Section will meet this Thursday in the regular time and place.
What’s the motivation for tissue engineering? This slide has a lot of numbers on it and the particular numbers are not important. I’ve posted it in the notes for the course so you can look at it at your leisure, but the main point here is that there are many diseases that drugs cannot treat effectively. They’re diseases that are prevalent, they don’t occur in small numbers. That’s why all the numbers are here so you can see how many incidences there are of these different–what is labeled on this slide organ and tissue deficiencies.
For example, you know that burns of the skin are quite common. Some burns are not severe enough that they need any other treatment other than maybe some antibiotic lotion and a dressing while it heals. Some burns are so severe, or so extensive, that they threaten the life of the patient who has them. If you lose enough of your skin, you lose a lot of water through that open area of skin. You can become dehydrated to the point where you can die of dehydration. Your skin is an important barrier for keeping water inside your body. Of course, your skin is a barrier for keeping other things outside the body, toxins, and in particular, microorganisms, viruses, and bacteria. So, your risk of infection is tremendous if you have large burns over a significant part of your body.
There’s no way to–that we currently know to speed up the healing of skin fast enough that patients that have severe skin burns can survive that injury. What’s done is that you need skin transplants; you need preferably an autograft of skin, taking skin from one place in your body and putting it in another. If you have severe burns over much of your body, then there’s no skin to graft. You might take an allograft, an autograft is a graft from you, from the individual to the individual, taking some of your tissue and moving them to another location; that’s ‘auto’, self graft. Allograft is from a similar individual but not you, somebody who might be matched but not perfectly matched immunologically to you and you get a graft from them. When you get a blood transfusion from somebody that’s not you but there’s some matching that’s gone on to make sure that you’re not going to be incompatible with the blood, that’s an allograft. Xenograft is a graft between species, so that would be a graft from another species to human, for example, or between species, so autograft, allograft, xenograft, good words to know for our discussion on tissue engineering.
You can imagine there’s a limited number of allografts, that is skin transplants that are available to treat burns. So, not all the patients that need them are able to get these kinds of grafts and so life is threatened in that situation. Not only burns but there are pressure sores, what are commonly known as bed sores for patients that have to spend significant time in bed because of an accident. Let’s say they’ve had a spinal cord injury and they have to spend a lot of time recovering from that, or they can’t move anymore on their own and they’ll develop sores which are very difficult to heal. Diabetics because they–because Diabetes can affect the sensory nerves in your extremities, often get sores in their extremities, particularly in their feet or lower leg, which don’t heal very well; partly because they can’t feel them as well anymore, and so they become injured and continue to get injured. Partly because the process of Diabetes slows down healing in general, these–they get ulcers or lesions on their skin that don’t heal. Just this top category alone lots of patients, different indications, not enough tissue to treat these tissue deficiencies of the skin.
Chapter 2. Challenges in Organ Transplantation [00:06:16]
If you went down this list you would find many others as well. Let me go down here, we’re going to talk about in some detail the liver. When you get liver disease that’s severe enough, the only medical option right now is a liver transplant. We’ll talk about why that’s the only option available for liver disease that becomes very severe, that could result from some metabolic disorder, from cirrhosis, or from liver cancer. In that case, a transplant is the only option. I’m sure you’ve all heard in the news about cases where there is a patient, often a child, who has some–a disease, waiting for a transplant and calls will go out to the community. This is a familiar story in our society. The reason for that is because there are a significant number of patients that are in this category. Not as many as in the case of skin, but a significant number.
There is something like tens of thousands of patients that are eligible for liver transplants and waiting for them. Why are they waiting? Because there’s not enough donor organs to satisfy the need of people that have transplants. That’s unlikely to change. For a liver transplant donor to become available, somebody has to die with a healthy liver, they have to have consented to have their organs used for another person, and they have to be matched immunologically with the patient who’s going to receive the donor. Somehow, those two things have to come together. All of these things have to work right in order for a liver donor and a recipient to match. They have to match not only biologically, but it all has to match in terms of timing as well, and that’s a complicated process.
Today, if you go and look on this website, we, ‘we’ meaning ‘we as a society’, have set up organizations and procedures to try to help with this matching process. One of those is called The Organ Procurement and Transplantation Network. They keep track of all of the patients that are on lists waiting for organ donations, they keep track of all the organs that are going to become available and a matching process occurs, and computers are involved, and all kinds of the resources you would expect to be involved in this thing. If you look for any particular kidney transplants, there are currently as of yesterday 75,000 individuals in the U.S. waiting for donor kidneys; in the liver there are 16,000. These are patients that today need an organ donation, their physicians have decided that they’re too sick to respond to any other treatment and they’re just waiting for an organ to appear.
I talked–in the very first lecture you might remember, I showed you this picture to talk about–there’s already very high technology involved with this medical effort. Part of the technology are things like this organ procurement network. This is run electronically on the internet, using all the high tech tools of communication that we have in order to connect physicians who are trying to treat patients with physicians who find that there’s a donor available, and by other high tech industries like the airlines who can quickly move organs from one place to another. If the donor of the organ died in California and the recipient is here in New Haven, then you’ve got to get the organ as quickly as possible from California to Connecticut. That can be done now sometimes.
Another technology, which is embedded in this, is that some hours have to pass between when the organ is harvested and the organ can be transplanted. You need to be able to preserve organs for that period, however long that period is. You’d like to be able to preserve them for a long period of time. What if we could preserve donor kidneys in the same way that we preserve blood? Blood–many of you have gone when the Red Cross has come here looking for blood donations. They take your blood, they store it in a special way such that it can be used up to three weeks past the point when you donated. That creates many opportunities for your blood to be useful. In the same way, if you donated organs and there were ways to preserve the organs that would be–make them more useful as well. There’s a whole technology behind this that I wanted to at least describe to you in words and you could think about. Biomedical engineers are currently working around the world on better ways to preserve organs, but it’s a difficult problem. It can be accomplished in blood and certain kinds of blood cells, can be partially accomplished with some tissues like skin, cannot really be done with livers or kidneys. Not yet, but there’s a great need for that.
Why–I wanted to say a little bit more about why you have to take this extraordinary measure of transplant to treat diseases like liver failure. When a liver becomes so diseased that it cannot recover its function on its own, lots of things go wrong. This chart is a table of some of the functions which are happening everyday in your liver. The liver is an important endocrine organ, we talked about the endocrine system, it makes hormones like IGF1, it activates Vitamin D which functions as a hormone, is important for structure of your bones, it produces the hormones that are used in your thyroid gland and many other hormones. If your liver starts to die you start to lose the ability to produce those hormones and you can imagine, then, that lots of things in your endocrine system start to go wrong. The liver also produces many of the clotting factors; these are proteins that are needed for clotting blood if you have an injury. If the liver starts to die you’re not making as many of these, you can have a bleeding disorder. It makes plasma proteins like albumin, which have many functions in the body. It makes bile acids, and bile acids are very important as an excretory organ for getting rid of molecules that the kidney is not able to eliminate into urine.
I could go on down the list. I put it here just to say that when your kidney–when your liver fails thousands of things start to go wrong. You could imagine treating any one of these with drugs. If the problem is activation of Vitamin D you could take Vitamin D, you could take pre-activated Vitamin D. If it was a thyroid hormone you could take a thyroid hormone. If it was any one of these things, there might be a medical solution for it. But when it’s this whole list of things, because your whole organ is failing, you can’t support that function with drugs alone. It’s just too many things going wrong at once.
Drug therapies are usually used for treating something where we understand what the molecule is involved is and where there’s only one or two molecules that are involved. It’s some metabolic disorder, you’re not making a particular molecule that you need. So, you take it in the form of a pill, or you have an enzyme that’s not working properly and so you take injections of that enzyme. In the case of cancer, there’s a single drug which can eliminate the unwanted cells in cancer. These are areas where drug therapy works.
Gene therapy, which we talked about a little bit, works in other kinds of situations. Where you’re missing an element of a cellular process; like in cystic fibrosis for example, your lungs and other tissues are missing a membrane protein that’s important in transport of chloride across your lung epithelium. Now, you can’t just take that protein because it won’t get inserted into the cells in the right way. We talked about membranes and how proteins are inserted in membranes. You can’t just give the patient that membrane protein and have it restored to the right location in the cell. Because it’s only one thing that’s going wrong, you can think about treating this with gene therapy, by giving those cells that are missing the protein a gene that allows them to produce that protein on their own and to correctly insert into the membrane. Gene therapy is possible to think about in diseases like cystic fibrosis and some diseases of the liver and other organs as well. In general, they’re diseases that are caused by a single gene defect.
When we’re thinking about liver failing lots of things are going wrong, many genes involved. Gene therapy isn’t really thinkable in that situation where the whole organ is starting to fail. Whole organ transplantation is the only current solution, a surgical solution for problems like heart failure, kidney failure, liver failure, where the whole organ is starting to fail. The motivation, then, for tissue engineering is to try to think about another way to treat these diseases where you would somehow manufacture or create a tissue in the laboratory that could substitute for the transplanted tissue from another patient. The idea is to try to make synthetic or semi-synthetic tissues that could take over the function of a failing liver, a failing kidney, a failing blood vessel, a failing heart. It is intended to treat these diseases that are too complex to think about complex in the number of things that are going wrong; too complex to treat with any of the other therapies we’ve talked about over the course of the–of our time here.
Chapter 3. Cell Culturing in Tissue Engineering [00:17:37]
This is a field that was really–has ancient roots, and those ancient roots are discussed in your book. The idea of engineering tissues was really only developed relatively recently, and first defined, really, in the 1980s. That first paper I gave you to read by Langer & Vacanti is really considered one of the first, most influential scientific papers in this growing field. Here’s one definition of what tissue engineering is. I told you what the goals of it are kind of in broad terms, but it’s the ‘application of principles and methods of engineering and life sciences toward fundamental understanding of structure function relationships in normal and pathologic mammalian tissues’. That’s not the more interesting part to me, the more interesting part to me is, ‘and the development of biological substitutes to restore, maintain, or improve tissue function.’
Why is it only in the 1980s that people started thinking about this? There’s really two reasons and one is an engineering reason, a broad reason, and the other is sort of life science and biology reason. We had come to the point in the 1980s where a lot of the biology that we have talked about now was fairly well known. In particular, the biology of cells in culture of taking cells out of organs and tissues, maintaining them in culture, getting them to do the things we wanted to do in culture was fairly well known by the 1980s, it’s a fairly mature technology.
One could think about using cell culture techniques to produce complicated structures. The other is an engineering advance and that is that we had developed, by the 1980s, enough understanding of artificial biomaterials that we could start to create materials that had unique properties which made them good candidates for use in the kinds of applications I’ll talk about. The innovations, or the kind of foundations of tissue engineering, are our ability to culture cells, to do that reliably, reproducibly, to understand what cells are going to do when we maintain them outside the body, and our ability to build materials that are biocompatible that can be implanted into people and that can serve as scaffolds for new tissue. I’ll say more about what that means.
Just to try to put this in more concrete terms, here’s two examples of how you might artificially engineer a tissue, or two kinds of applications that people might think about in a heart that’s failing. I use this as an example because we know a lot about the cardiovascular system now and so you’ll understand both of these examples. We talked about the important function of the blood vessels of the heart, the coronary arteries. We also talked a little bit about how these vessels can become diseased in life, particularly for patients in the 6th or 7th decade of their life. This vessel might become diseased through a process of arthrosclerosis; the lumen becomes narrowed such that blood no longer flows. The resistance gets too high for blood to flow well, so you’d like to replace this artery.
The only thing that the artery has to do is carry blood from one place to the other, so really all you need is a tube. People have tried for years to use different kinds of tubes to replace this section; the operation is called a cardiac-bypass. A surgeon will go in, he will remove this diseased section of tissue, or he’ll just put a bypass around it, that is, a tube that goes from this location and connects to this location. Like a bypass on the highway it’s a clear route that allows you to get around a route that’s no longer passable.
Well, these vessels carry a lot of blood. They have to be very reliable vessels because they need to carry blood all the time because your heart is constantly working, it needs a constant reliable source of blood. The best substitute for this is a natural one. So, one of the most successful operations here is to take a blood vessel from somewhere else in the patient’s body and use it to create a bypass. You’d like it to be an artery. Bypasses can be made, there’s another artery that’s nearby called the internal mammary artery that surgeons could take. You can take it from the tissue sort of near the heart, it doesn’t really serve as important a function, and you can bypass it in here. Now you have a new artery in place to continue to flow blood.
That works, but what if you have more than one site of blockage? What if you have a blockage here, and a blockage here, and a blockage over here, and a blockage over here so you need multiple bypasses? You don’t have enough arteries in order to do that. Then, the surgeon will go and take the saphenous vein from your leg. It’s a large vein in your leg that returns blood that’s flowed down to your leg back up to the vena cava. There are enough other veins in your leg that you can lose that vein without any loss of function in your leg. They can now take the vein and cut it up into segments, and use that to bypass all these segments of artery that are clogged. That works as well, but veins aren’t the same as arteries, you remember that. If you take a vein which has a different anatomical structure and put it in the position of an artery that is exposed to the high pressures that you know already occur there, sometimes that works. Sometimes the vessel changes and remodels and adapts so that it functions, and sometimes it doesn’t. Sometimes those veins also become diseased and they no longer serve as an effective bypass.
Then, what do you do? If both of those options fail you can try to use synthetic materials. Synthetic materials are used to replace blood vessels in many parts of the body. If you have an aorta that has an aneurism, a surgeon can put a synthetic piece of, basically Dacron, a synthetic material. It will serve as a functional aorta and do a very nice job. It turns out that we know from experience, now, that that only works if the vessel is big enough, because for big vessels all of these kinds of synthetic materials that we make. While they might have the right mechanical properties to serve as an aorta, they’re not biological. So, blood will clot when it hits that unnatural surface.
It’s okay if blood clots on the surface of a big artery because it’s big enough that you could have a lining of a clot here along the inside, and it would still be enough diameter to flow. If it’s a small artery and it clots blood doesn’t pass through any longer. We know now that the limit of what one can do with synthetic materials is about half a millimeter to a millimeter, and that’s the size of these coronary arteries. You can’t use synthetic materials there. What we’d like to do is grow an artery outside the body. What if we could that by taking the synthetic material and growing on it a lining of natural cells? The synthetic material would provide the mechanical properties needed for it to function as an artery. If we could grow natural arterial and endothelial cells all over the surface of this, the blood wouldn’t know that it’s a synthetic material. It would only see the cells which are forming the natural barrier, the natural wall of a blood vessel, so that’s tissue engineering–an example of tissue engineering to create an artificial artery to use in bypass. I’m going to talk more about the details of where we are in that process and what the obstacles are next–on Thursday.
Let’s say you didn’t get there in time and a particular patient–they didn’t have a bypass in time so they have a blockage. They have a mild cardial infarction or a heart attack because the blood becomes comprised to some area. This area of the heart doesn’t get blood for some period of time, and so this tissue in the wall of the heart dies. Now, in general if that happens there’s only a limited ability of your heart tissue to heal after you have a heart attack. A scar is formed here and this part of the heart doesn’t function in the same way that it did before.
Remember how it functions, is it has to transmit electrical signals and it has to contract in response to those electrical signals. It has to do that with a regular frequency, so it has to both conduct the electrical signal and contract. This damaged cardiac muscle doesn’t do that anymore. Well, sometimes that’s okay. You might not need all of your cardiac muscle to be functioning in order to have a reasonable efficient heartbeat, but sometime the area of this is so large that it compromises the ability of your heart to deliver the cardiac output you need to stay alive.
What if you could make an artificial cardiac muscle? What if you could take some kind of a material and grow cardiac muscle cells on it? Grow them in such a way that they filled up this material and they started to function like heart, that is, they functioned in the right electrical way and they functioned in the right mechanical way. Then, maybe a physician could come in and just sew this patch that you’ve created outside the body into place on the tissue. Maybe that patch would start to function like that part of the heart wall that didn’t function. Now, where would the cells come from that you did that with if we think about this particular application, we’re going to create a patch for repairing myocardial defects. Where would the cells come from? What would be your first choice of cells that you use for this? Justin?
Student: From the heart.
Professor Mark Saltzman: From the heart. From what heart?
Student: From the patient’s heart.
Professor Mark Saltzman: From the patient’s own heart. What if you could take a small biopsy of tissue from somewhere else in the heart? You could take a small sample of those cells, and you could take them outside the body and grow them in culture. Well, this is where our ability to culture cells comes in very handy because we could take a small sample, isolate the cells we want, propagate them in culture until we had millions of those cells, put them into the patch, put the patch back into the patient. Now, the patient has an autograft of cells, but placed in the right configuration so that they function as a tissue. What are the problems with that? What’s the problem with that?
Student: The entire organ is diseased.
Professor Mark Saltzman: If the disease was too big you couldn’t do that. Why couldn’t you do that? Because you couldn’t grow enough cells, you think, or–
Student: Because [inaudible] doesn’t use tissue [inaudible].
Professor Mark Saltzman: Yeah. You might–there might be some practical limits to it but you’re getting close to what the real problem with that would be. Think about the timing of it. How long would it take you to take this biopsy and grow the cells and make a new artificial–probably would take weeks to do. Could the patient survive those weeks while you’re making the tissue engineered solution? Maybe, maybe not, it would depend on how extensive the failure of the heart was, as Justin described.
What would be another solution to that? Well, let’s not take cells from the patient but let’s create a bank of heart cells. That might be possible, but you have to be able to preserve heart cells. You have to have a bank of cells with all the right immunological markers labeled, you have to have them in enough quantity that they would be available rapidly. There’s problems with that but maybe you could do it that way. Maybe you could use stem cells. We talked about stem cells before and some of their capabilities, but one of the real advantages of what people think of as using stem cells to treat diseases in adults is just exactly what we’re talking about here. Being able to create an environment or an engineering solution in which stem cells could be used to create functional tissue. In this case you’d want stem cells that could very quickly become cardiac muscle cells that could be differentiated in the cardiac muscle cells. Ideally, you’d want stem cells from the same person, you’d like to isolate them from the same person somehow, or from a donor that was matched. You’re starting to see both the opportunity, I hope, and the problems that have to be solved in creating these new solutions.
I list on this slide a few of the characteristics of tissue engineering that we’re going to talk about, not in quite this organized a way, but I hope that by the end of the week you’ll be able to look back at this list and say, ‘Oh, I understand what that characteristic means in terms of what we’re trying to accomplish in tissue engineering.’ The first is what I’ve been talking about up to this point. The tissue engineering really represents kind of a modern or contemporary logical extension of conventional medical and surgical practices. If we’re going to make an engineered tissue, in many cases, we’re going to give them to a surgeon and they’re going to do what they know how to do, which is replace an organ or a tissue. You’re just giving another resource to surgeons to do what they know how to do already. We’re using it to extend medical practices to diseases that can’t be treated by drugs or gene therapy or other kinds of medicine.
Chapter 4. Tissue Engineering in the Regulation of Healing Processes [00:32:34]
What I want to talk about for most of the rest of the time today is this second bullet here, that tissue engineering involves control or regulation of normal healing processes. That if we want to understand and use tissue engineering we need to know something about how the body heals itself and take advantage of those natural mechanisms of tissue regeneration. We’ll talk over the course of the next two lectures about tissue engineering as an attempt to replace the cellular component of diseased tissues. Whereas in drug therapy we’re thinking about the molecular component, now we’re replacing cells, that it uses cellular processes sometimes to control drug delivery. We’ll talk about how you could use tissue engineering as a drug delivery system, and that tissue engineering produces new models for the study of human physiology.
I won’t say anything more about that this week, so let me just say that now, that what if you could produce heart tissue like this for transplantation? Another use for it would be if I can make this tissue outside the body that behaves like the human heart, I can use it to study how the human heart works outside the body in a much more reliable way, or a much more sophisticated way than I could with ordinary cell cultures. Because now you not only have the cells of the heart but you have the cells of the heart arranged in the right kind of configuration, so I could understand–I could use it to understand how the heart develops. I could expose it to different drugs and ask, ‘What are the toxicities of these drugs to the normal heart?’ without doing it in patients, and with doing it in a system where I have a lot of control and a lot of ability to look inside and see what’s happening.
Let me go back to this point now, tissue engineering involves control or regulation of the normal healing process. We’re all familiar with at least some aspects of healing because you’ve all had a cut in the skin that has maybe caused you some distress in the short term. Maybe it was a cut that was deep enough, and this shows you the structure of the skin. Here’s the outer layer of the skin here that forms this sort of thick barrier layer, the epidermis that things can’t penetrate; water can’t penetrate easily. You put water on top of your skin, it beads up there, it doesn’t get absorbed. Likewise, water inside doesn’t come out and that’s why you don’t get dehydrated. Microorganisms can’t go through because of this barrier.
Underneath is a more connective tissue layer called the dermis and inside this dermis there’s cells that are producing an extracellular matrix of collagen, largely. We talked about extracellular matrixes several weeks ago. Within that collagenous matrix there are blood vessels, arteries red here, veins blue. Those blood vessels bring blood to the skin to provide oxygen, and to get rid of carbon dioxide in these cells in your living skin. You get a cut in this tissue and what happens if it’s a deep enough cut you bleed. You got to do something to stop the bleeding. You’ve had this and so you put some pressure on and you wait for clotting to occur, but you’ve disrupted these vessels. They have clotted so that a clot has formed inside these vessels so that the bleeding stops.
Usually a clot forms at the surface. If you look at this after you’ve got the bleeding stopped you have a red looking area. It looks different from your skin, that’s the beginning of the healing process, is that your body has coagulated blood at that site to replace the tissue that’s been lost. You’ll have a red spot there that turns into a reddish looking scab. That’s your body responding to stop the immediate insult, the bleeding, and to fill up the space of this tissue that’s been damaged. Cut’s too big, this can’t happen, you can’t just put pressure on it and stop it, and so what do you do? You go to the emergency room, you get some stitches. They bring the sides of that wound back together with stitches, they hold it closed. The same thing happens but in the smaller space now, because they’ve just brought the tissue together so that your body could produce this provisional tissue. The provisional tissue at the beginning is just a blood clot.
After that point many things start happening within the skin, and a complex process of cells sending out signals and cells responding to signals in a coordinated and regulated way occurs. What this diagram shows is some little fingers of tissue entering into this coagulated provisional matrix that was formed by the blood clot. These cells are initially cells that are called fibroblasts which live in the dermis. Normally, they’re just sitting there sort of in a quiescent state, not reproducing very rapidly, making collagen, providing matrix to keep up sort of the normal structure of your skin. When there’s an injury, they get turned on and they migrate into the wound. They migrate into this provisional matrix and they start changing it. They start digesting the clot and they start making new extracellular matrix of collagen. They do that so that there can be a pathway for these blood vessels to regrow. The blood vessels need to regrow, renourish the skin so that you can start to get the normal composition back, the normal cellular components, and you’ll see these blood vessels starting to form sprouts that enter into this provisional matrix.
Now, you’ve watched the process of a scar healing on your own body and you’ve seen the stages that it goes through. It will go from the red appearance to a duller red, and eventually become a white, or it doesn’t look red at all. After some period of time you won’t even see where the cut was at all. The end process of these fibroblasts coming in, replacing, remodeling the matrix of blood vessels regrowing back in, establishing new cells from the epidermis start to grow in over this matrix in the end. First they’ll grow over the top and so you might see a little bump here. Eventually the whole tissue through this elegant self-regulated process heals itself so you can’t see where the cut was, we’ve all had that experience.
Probably we’ve also had the experience when we got a worse cut and it didn’t end up looking right. You might still have a scar from some cut that you got on your hand or your arm where it was too severe. Now, what happened there? The same process of remodeling happened but it happened in a more aggressive or more vigorous way because the signals that control this were large. It was a large wound, big problem, lots of cells got activated, they made a lot of collagen, they remodeled this provisional matrix very aggressively. Why? So, that you could heal, and so that you wouldn’t have a wound here that was open to infections or loss of blood or loss of water.
Your body knows how to respond to very severe injuries, more severe than we usually encounter in our lives. The result is you’ll get a scar form that because the response was too aggressive. Our ancestors, who lived much more dangerous lives than we did, who might get attacked by a lion or a bear, needed to have a very aggressive healing response so they didn’t die if they got attacked under some circumstances. We don’t get attacked by lions or bears so often anymore and so we don’t necessarily need that aggressive healing response. We might like a gentler healing response that leads to a better functional–that doesn’t have a scar, for example.
How can you control healing in that case? Well, one of the ideas of tissue engineering is to provide materials that would guide, or regulate, or change that natural response of healing. You have a wound and the wound is going to heal on its own in some way. You would like it to heal in way that you control so that the outcome is more to your liking. Now, maybe the outcome is that it heals much faster, maybe the outcome is that it heals without a scar; maybe the outcome is that it heals and new kinds of cells are woven into the tissue in place of the ones that were there formerly. Maybe this wound was created by a surgeon who removed a cancer here, and you’re trying to replace the cancer cells with normal cells.
What are the options in terms of doing this? Well, one is that you could put a material in here and the material goes into the wound. That material has some function, and maybe that it serves as a better provisional matrix then the clot would have. ‘Better’ means what? Maybe it regulates how cells grow in or how cells attach, or how cells synthesize collagen after they enter. Maybe you could design a material that changes the healing response locally. That’s one way that you might engineer a tissue. Now, here I haven’t put any cell cultures in, I’ve just put in a fancy material that I designed. We’ll talk about what those might be a little bit later.
Maybe you don’t need material at all, but maybe you could replace this with cells. Maybe I could just take cells, maybe recovered or harvested from some other place in the patient. Maybe these are skin cells that I got from a normal location and I put them back into a wound in order to speed up the process of healing at that site. Maybe they’re stem cells that came from somewhere, or maybe they’re just cells. Often times the best combination seems to be to put cells in together with some material. Here, the material might function in a couple of different ways. It might function in this way we talked about here, by giving signals to the normal tissue to regulate the healing response that’s going to occur anyhow. That material might contain cells that you have derived somehow, that you want to hold in that site; that you want to hold locally and provide an environment where they can grow and take over the function of the tissue.
These are some sort schematic ways to think about how one might use tissue engineering to control the normal healing process. I’ve shown this, I’ve given you examples of this that–where I motivated it with thinking about the skin, but this doesn’t have to be the skin. This could be your liver. Maybe I put a material into your liver that allows it to regenerate. Maybe I put liver cells derived from some other source back into the liver, and they take over the function of the liver. Maybe I put materials on cells and create a new liver tissue. So, you could extend this idea to almost any tissue or organ.
Here’s an example of that, where just the material is used to guide the healing process. I think this is conceptually a very simple idea and one that works to some extent. This is a nerve. ‘Proximal’ means closer to the brain, so maybe this is a nerve in your leg and it’s got severed by some kind of injury or it got crushed and somehow the nerve is damaged in this region here. This nerve is sending sensory signals from the distal part, back up the nerve into the brain; it’s sending motor signals down from the brain to the more distal regions. If this tissue–if this nerve gets damaged by trauma somehow, then you’re not going to get sensory signals sent through it, you’re not going to get motor signals. So, you can’t feel, you can’t move and all of the tissue that’s distal or downstream of that site. How do–how would you repair that?
Well, it turns out that surgeons know that if it’s a small region and they go back in and they take the damaged ends of the nerves and they put them back together, you will recover much of the function of that nerve. Maybe not 100%, but if they do it quickly and they do it well that these cells will regrow the connections that are lost in this section of trauma. You will, over a period time, begin to feel in that tissue again and you’ll be able to move. If the damage is too big, you can’t physically bring them back together. Your leg is a fixed link and if the area of damage is too big you can’t bring these tissues back together. They found that you can use an autograft, that is take a nerve from somewhere else. If I cut the nerve out of another spot and then put it in this region in between the two ends, the nerves will regrow again. Problem with that is you don’t have a lot of nerves that you’re not using. So, in order to repair this nerve you had to sacrifice a nerve somewhere else.
Maybe you could do that same thing with a material, so what would the material look like? The material might look like this, and this is just a cylindrical cuff and it’s a complete cylinder. It’s shown as a cutout here just so that you can see inside, but it’s a complete cylinder that’s hollow and you put one end of the nerve into one end, and one end of the nerve into the other. Now, you’ve placed these nerves, not in physical contact, but you’ve created a compartment through which they can communicate. You don’t have this end of the nerve dangling out in space and this end of the nerve dangling out in space. Remember it’s not going to be a nice picture like this where the nerve is existing on its own, but there’s muscles and blood vessels and other kinds of things around in this area. By putting a channel with a nerve in both ends you’ve isolated the area of the nerve you want to regenerate from all the rest of the tissue. Does that make sense? You hope that you get some regrowth through here.
What scientists have found is that if you do this you will get some regrowth. You don’t get complete regrowth of a thick nerve, but you get a thin nerve with less fibers, less nerve tissue, less function but you get some recovery. So it works, and we’ve known for about 20 years or so that it works but it doesn’t work well enough, so how could I make it better. Well, one way you could make it better is by changing the material. Maybe the properties of the material weren’t right, maybe the cuff on the outside is too impermeable and you’d like to have molecules exchanging in some way. Maybe it’s not permeable enough and so you need to regulate it that way. Maybe you don’t want a nerve cuff that’s filled with just air or just water, but maybe you’d like a nerve conduit that’s filled with a material that promotes the growth and reconnection of these fibers. Maybe you’d like some kind of a gel in there that nerves could regenerate through.
So, the obvious place to go for that is to use the kinds of materials that the body uses for repairing tissues. Fill up this channel not just with water or saline, or some kind of a solution, but fill it up with an extracellular matrix, like the kind of extracellular matrix we talked about weeks ago, that all of your tissues have. A supporting gel like structure, formed from proteins and carbohydrates that are secreted by cells, and through which cells normally grow and live. Taking natural proteins like collagen, forming gels out of them and putting these gels inside the material increases the number of fibers that grow across a gap like that.
This is just an example of–a simple example, no cells involved here of a material that’s designed in order to enhance the healing of a wound and how one might engineer properties of that material to make it better at supporting the healing function. That’s what I wanted to say about tissue engineering involves control or regulation of the normal healing process. I want you to keep that in mind as we go through the examples that we’ll talk about in Thursday’s lecture. I’m going to stop there. Questions? See you on Thursday.
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