Frontiers of Biomedical Engineering

BENG 100 - Lecture 23 - Tissue Engineering (cont.)

Chapter 1. Introduction [00:00:00]

Professor Mark Saltzman: So, we're going to continue talking about tissue engineering today. I introduced the subject on Tuesday and I'm going to talk about some more examples that sort of illustrate this new approach to creating medical therapies. I just wanted to remind you before about the two readings that are already posted on the website and to encourage you to look at those to get a sense for how the field has developed over the last 20 years or so.

Last time I used this sort of outline as a guide of, at least, my view of the characteristics of tissue engineering. We talked about it at the beginning of class last time, there's a logical extension of conventional medical and surgical practices. In that sense a solution, potentially a solution to the limited number of donor organs that are available to treat diseases where transplantation of tissue is the only option. Many approaches to tissue engineering involve regulating your body's natural response. We can heal, in many cases, and regenerate.

The liver is a--for example is a fabulously regenerative organ. If you go into a laboratory animal, like a rat, if you remove two-thirds of their liver, their liver will re-grow within a period of a couple of weeks. The liver has an amazing regenerative capacity. There are human diseases where that regenerative capacity is overwhelmed. Infection can be that kind of a disease, hepatitis infection, for example; cirrhosis is that kind of infection. When you get past the body's natural ability to heal itself, then transplantation is the only option. In tissue engineering, we might be thinking about a different strategy, could we provide some support for the body's own ability to heal? Could you provide a little bit of extra liver tissue that would allow the body an opportunity to compensate for its loss?

Chapter 2. Tissue Engineering for Replacement of Diseased Tissues [00:02:28]

Tissue engineering involves control or regulation of the normal healing process. Today I'm going to start by talking about how it attempts to replace or supplement the cellular component of diseased tissues. When a tissue is diseased, for example, when we talked about the coronary arteries and needing a bypass, you're adding extra cells or extra tissue there because medical therapies, at least the ones we have now, are not effective at re-establishing the blood flow pathway. You're adding new cells and they have to be cells that are in the correct configuration. They have to be cells like a vessel, and they have to be performing the functions that cells ordinarily provide. I talked about how we couldn't use completely synthetic materials to replace that blood vessel, even though a synthetic material would provide a conduit for blood. Because the material properties aren't right, it isn't the same as the natural cellular layer, then blood coagulates on the surface. It doesn't provide that normal function that cells provide. We want to think about ways to replace or supplement that cellular component.

Let me talk about one example. This is an example that I've been working on in my own lab, together with two different laboratories at the medical school, Jordan Pober in Immunology and Al Bothwell in Immunology. It really uses a strategy that's been used in laboratories around the world to try to re-establish tissues in animals. The technique is fairly straightforward and shown in this cartoon, that cells are cultured. These are special cells in a couple of different ways. One is they are human cells, and they are harvested from human umbilical veins. The umbilical vein, which is normally discarded after delivery, the baby is born, the umbilical cord and the placenta are ordinarily discarded. If you keep that tissue, you can isolate the cells that form the lining of that tissue. It's a vein and what you get from this isolation are endothelial cells. It's abbreviated here HUVEC, that stands for 'human umbilical vein endothelial cells.'

People have learned over the past few decades how to maintain these cells in culture, how to propagate them, how to increase them in number using the same techniques we talked about very near the beginning of the course. They've also discovered how to genetically modify these cells so that they perform better in culture. One of these modifications is to add a gene called Bcl-2. Bcl-2 is a survival gene that allows these cells to live beyond when you would expect them to live. If you just take human umbilical vein, endothelial cells, and you propagate them and culture you can passage them. Remember the process of passaging from one tissue culture to another, always getting more and more cells. You can do that for some limited period of time, but if you give them this survival gene, Bcl-2, you can do that for a much longer period of time, you can get more cells.

Cells are grown from human tissues. The idea is to somehow get these cells into an animal, or into a patient, and get them to reform blood vessels inside the patient. The way that this is done is that these cells are suspended in a three-dimensional gel. This is a gel of collagen and it looks just like this scanning electron micrograph. On the side, if you looked at very high magnification, it's just a web or a network of protein fibers formed from the natural protein collagen. We talked about collagen's role in the extracellular matrix earlier. Here, this is isolated collagen, purified, and then formed into a gel, and cells are suspended within it. That's what this picture down here is to show you. This is a high magnification; a cell in here would be almost the same size as this full picture would be a cell, and so you can imagine each cell that's suspended in this three-dimensional material is surrounded by a webbing of protein fibers. It has a thickness; so unlike normal cell culture where we grow them flat on a plastic dish, these cells are suspended within a three-dimensional gel.

Now there's something else in this gel here, and that's shown here, these are particles, particles made of the material alginate. In section a few weeks ago you all made particles like this. We made them with food coloring in them, but you learned the process, it's quite straightforward. Imagine that instead of putting food coloring in it like we did, you put in a protein. You selected that protein to be one that encourages blood vessels to form. It's a protein that your body naturally uses to cause blood vessels to form more readily. That protein is called vascular endothelial growth factor, or VEGF. It's not important the details but it's a signal that the body uses to form new blood vessels. We've isolated it, purified it, and encapsulated it in these tiny particles, which are then suspended with the cells in the gel.

What you have here is something that you might call a neo-tissue, what we would like to become a tissue when we implant it back in the animal. It's not a tissue yet, it's a collection of cells and extracellular matrix, and signaling molecules that are arranged in a three-dimensional structure. The hope is that if you put these back into a patient or put them back into an animal, like the mouse shown here, that you would grow new blood vessels. That's the hope. How does this work?

This is some early work that was done by Jordan Pober here in the School of Medicine, and there's a lot of pictures here. This is from a paper that I could give you if you're interested in reading in more detail, but I want you to focus on this one here. What they did was they--instead of taking this gel and putting it right into the animal, they maintained it in culture for a while. The idea was that in culture you can control the conditions that the neo-tissue develops within. Maybe you can control them in such a way that the tissue gets a start toward being what you would like it to be. In this green picture here, I don't know if you can see it. It's posted online and you could blow it up and take a look at it. What you see is that these cells have organized into tube-like structures. You'll see tubes clearly here in this gel where you don't see them so clearly in the other green images here.

These are the cells I talked about before, the ones that are genetically modified with the survival protein, so that they survive in culture. It turns out this survival protein also encourages their formation into tube-like structures that are starting to look like blood vessels. If you wait the proper period of time and then implant these gels into a mouse, so here they just made a small incision in the skin and slipped the gel underneath the skin and left it there, then these tubes developed into blood vessels. Now that would be cool enough on its own, but it does something more, in that this gel that's now filled with blood vessels does two things. One is that the vessels start to differentiate, some become like capillaries, some become like veins, some become like arteries, and they start to connect to the vascular system of the mouse.

If you wait long enough and you look by electron microscopy inside these gels, you will find many, many blood vessels that look like this. This is a cross-section in this Panel B below. You can see the endothelial cells have formed into a capillary, it's a tube. Not only is it a tube but here's a mouse blood cell inside of it. It's a tube that's now carrying mouse blood through it. You've created new pathways for blood to flow. Now that's a neat trick and you could imagine using that to treat disease.

One way you might use it to treat disease is to implant these things into areas of the body that--where there aren't enough blood vessels. For example, many adults get vascular deficiencies in their legs. These could result from trauma, some from just the natural process of aging, some from diseases like Diabetes and the blood vessels in your legs don't work anymore. If you're not delivering enough blood to your leg the tissues become ischemic or deprived of oxygen. What if you put one of these patches into that area where you weren't getting enough blood flow and you could create new blood flow. That's what this experiment shows here, treating an ischemic limb with one of these patches. What you see particularly in high magnification is that lots and lots of new blood vessels form and they connect back to the major vessels that are already in the legs. You've created new blood flow pathways.

One could imagine using this in the heart, for example, as well. When you have tissue that's deprived of oxygen because of a heart attack, for example, or loss of flow, that's one idea; another idea is that you could use these to create sort of miniature organs. What if instead of just putting endothelial cells in there we put other cells in there as well. What if you put liver cells in together with the endothelial cells and the signaling molecules? What if you put cells from the pancreas in there? Then maybe this gel would develop not only into a vascularized tissue, but into a vascularized tissue that contained liver cells or cells from the pancreas, or cells from the kidney. Maybe those cells would start to function like a little tissue. Maybe you could make a system that supports the liver, in the case the natural liver is developing. This is one of the central ideas of tissue engineering.

Chapter 3. Synthetic Materials in Tissue Engineering [00:13:30]

One of the other--that I think illustrates one of the reasons why tissue engineering has become a viable alternative at this point in the scientific culture because we know how to culture cells, we know how to genetically manipulate cells, and we know how to put them in these kinds of micro-environments like I've shown. The other thing that we've learned how to do is--and I mentioned this last time, is to make synthetic materials that could serve as artificial extracellular matrices. This slide shows a number of materials that were made in laboratories here at Yale. One of them is a network of fibers; now this should look reminiscent to the collagen picture I showed you a few slides ago.

It looks like a network of fibers that are all wound together. Now the fibers here are much bigger then collagen fibers. In fact, a cell here would look very tiny on this scanning electron micrograph, where it filled up the whole thing before, so we're thinking about fibers at a different scale. These are large fibers. In fact, these are the same kinds of fibers that are used, same size and in some cases, the same actual fibers that are used to make the materials in your clothing. Clothing is made of fibers that are woven together. This is a non-woven mesh, so the fibers are randomly arranged, but they're fibers not unlike the kinds of fibers you would find in cloth material that we make into clothing.

At high magnification these are cylindrical fibers, there's lots of space in between them, and so you could fill these fibers up with cells and cells would cling to the fibers and potentially grow within this scaffold. This is a different kind of architecture of a scaffold but made of the same sort of material, synthetic polymers. Here instead of making fibers, a porous structure was made that looks like a sponge. There are the walls of the material that give it substance, and there are lots of spaces in between much larger then cells that you could fill up with cells.

Why use synthetic materials when I already showed you an example of how you could use a natural material? Well there's a couple of advantages. One is that we know a lot about how to process synthetic materials now. We can make them into almost anything that you want. You can control their structure on the microscopic scale, as I've shown here. You can control their structure on the larger scale as well, which I'll show you in a few minutes. The other advantage is that these particular--these are different than the kinds of fibers that are woven into clothing because these intentionally degrade over time.

These are synthetic polymers that will slowly dissolve over a period of months. One could make a scaffold, build a tissue around it, implant it in the body, and then this scaffolding would slowly disappear. What you hope is that as the--during the time period when this scaffolding is disappearing genuine tissue is being formed in its place. That's an advantage of these materials as well, that because they're polymers we can control, to a fairly high degree, how fast they degrade and what products they degrade into.

This shows you an example of tissue engineering where these kinds of materials, I showed you on the last slide, so microscopically they'd look like this up on the top panel here. Macroscopically, or with your eye, this is just a photograph from a camera. Y ou could make these thick materials, full of all this webbing of fibers, and you could form them into different shapes, a triangle, a square, or a cross in this shape here. Now because this is a synthetic solid material, it maintains its shape even if you put it into water.

Because it's a porous material, as I showed you before, you could fill this up with cells. The experiment that was done here is that these materials, these porous materials were seeded with cells from cartilage, and the cells that make up cartilage are called chondrocytes. Samples of cartilage were obtained, digested down to individual cells, those cells were grown and cultured until you had a lot of them, and then cells were put here, put here, put here, and then they were implanted back into animals. Here's what comes out of the animal several months later, and what you see is material that looks very much like cartilage.

A couple of things are interesting; one is that these things have maintained their shapes even though they weren't implanted into animals for many months. The shape that you make the original material in leads to a tissue that's that same shape. If you look microscopically inside these tissues, you'd find that they look like cartilage in many respects, they look like natural cartilage. That is the chondrocytes, or the individual cells that we put on here, multiplied inside the body, inside the scaffold, and they filled it up, they filled up the space. The tissue that you get looks like the scaffold that you use.

This is a microscopic image of some of those, so imagine you cut this one down the middle and then you looked under a microscope. This is what normal cartilage looks like. You can see the chondrocytes here, the white regions, and the chondrocytes excrete and extra cellular matrix which is stained red in this particular preparation. If you look at this cartilage that was formed by tissue engineering, you'll see some structures here. Some long structures, these are residual fibers. The polymer fibers haven't completely degraded at this point, but you see a lot of cells, a lot of healthy cells. You see they're almost at the same density that they are in the natural cartilage, so the cells have multiplied to reach sort of a natural state.

They're secreting an extracellular matrix, they're making their own micro environment by secreting molecules and so they also stain red. Now it's not as deep a red as here, they're not making as much as natural cartilage, but they're on their way. Here's an example of taking a material, being able to fashion it into a particular shape that you want, being able to control the microstructure so that you can put cells in it in such a way that cells develop into something that's starting to look very much like tissue.

Now this was done in the late 1980s I believe. At this time, the scientists that were doing it were trying to make the point that there was a lot of potential for this kind of strategy. One place where you might want to make new cartilage is in plastic surgery. People that have lost tissue due to cancer or to a birth defect, maybe they're missing some cartilage in their nose. There are diseases where children are born without functional cartilage in their ribcage. There are even some traumas where people might lose an ear, and your ear is basically skin covered with cartilage. They did this experiment mainly to make a point and so I'll show it to you.

Instead of making a triangle or a square, or a cross, they made a polymer scaffold that looks a little bit like a human ear and they filled this scaffold--this is 100% polymer now, there's no tissue in here, but then they added cartilage cells. They added chondrocytes and they implanted it back into an animal at a site where they could easily see it. They implanted it into the ear of a rabbit. Over time, what happened was the cartilage cells that were in this scaffold grew into cartilage in the same shape as the scaffold that they used. Here's what looks like a human ear growing on the ear of a rabbit. A dramatic demonstration, and when they took this ear out of the rabbit, obviously it's not a real ear, it doesn't hear, but it has--it's all the cartilage that forms the outer part of your ear. If you took a section of this later and looked inside, it would look just like the cartilage that's inside your ear.

You could use this same strategy to make other kinds of tissues as well. I'm going to return to the example of coronary arteries because that's such a big medical problem around the world; coronary heart disease. Heart disease is either the number one or number two killer around the world. Certainly in countries that are developed like the U.S., heart disease and cancer are the leading causes of death. A lot of the death from heart disease comes from failure of your coronary arteries. One of the most performed surgical operations in the United States is coronary artery bypass surgery. You probably all know somebody who has got it, maybe a grandparent or a family friend. It's not an uncommon surgery and it's not perfect yet, and we talked about the ways that it's not perfect last time.

Chapter 4. In Vitro Cultivation of Replacement Blood Vessels [00:23:13]

What if you could grow, outside of the body, blood vessels that you could use to replace the vessels inside of a patient? People have been working on this for a long time. Here's an initial attempt. This is a tube that was made of a polymer, and in this case it was a polymer. It was a natural polymer collagen, so sort of like that collagen gel that I talked about several slides ago that we suspended the endothelial cells in, only this time it's made into the form of a tube.

The idea was to take those endothelial cells and coat the inside of this tube with endothelial cells. Make a tube that's mechanically strong that will stay like a tube, but coat it with the natural cells that form the lining inside your blood vessels. Allow it to develop and culture, to the point where you have a natural endothelial cell lining, and then implant this back into an animal in place of its own artery. What you hope is that blood will flow through here, and it should flow because there's an opening, and that the blood won't clot, because now instead of an artificial material you have a lining of blood vessel cells on the inside.

This is a picture from studies that were done in the early 1980s on this approach and there was some success. You could make them work for some period of time, but they always eventually failed, and I won't go through the whole history of this but there's a long history of people trying different materials, different ways of putting cells on different sources of cells. All the things that you could imagine changing, and all of them working for a little while and then failing. You can imagine it's a hard engineering problem, a difficult engineering problem, because you need to make this material strong enough that it serves as a vessel. It's got to withstand blood pressure, not burst. It's got to be strong enough to put a stitch into because you've got to sew this into the natural artery. It's got to have good mechanical properties on its own but still allow the formation of a natural tissue around it. All those criteria turn out to be difficult to meet.

I want to talk in detail about an advance that was made by someone who's now here at Yale, a woman named Lauren Niklason. She's in our department of Biomedical Engineering and Anesthesiology. When she started looking at this problem, she thought, 'Well, everything that's done is working kind of but it's not quite working well enough.' The vessels when you put them in the body they don't develop the way that normal vessels do, they don't develop a normal structure. She thought maybe that's because we're not exposing them to the kinds of environments that arteries normally develop under. When our arteries developed during embryonic development they developed as the heart developed. While they were developing and turning from just collections of cells into arterial tissue, they were exposed to flowing blood because the embryonic heart was already beating, there's already an immature circulatory system. These arteries developed in the presence of a beating heart.

What if you simulated that in cell culture and took these tissue engineered structures that are polymer scaffolds filled with the right kinds of cells. You hook them up to--on both ends to a tube where you can expose them to blood flow and blood pressure and build a reactor around these blood vessels, so that as they're developing outside the body. You can control the conditions that they're grown under so that it's close to the conditions that a natural artery would experience when it's developing inside an embryo. This shows some of the things that she did in order to create this environment. Here's a picture of a bioreactor, here is an artificial blood vessel that she made in the laboratory. Again, this is a polymer scaffold, the same kind of scaffold I showed you several slides ago of a material, a network, an unwoven fiber mesh, and formed into a tube. Smooth muscle cells or the internal cells of the artery are cultured inside, and then it's placed in this reactor. You can see it's hooked up to two tubes here, so blood is constantly flowing through the inside. It's flowing in a pulsatile manner just like our blood is flowing through our coronary arteries or our artery.

She found that if she did this they developed into tissues that looked much, much more like real arteries and that you could take these and implant them into animals. One is shown here, in the carotid, this is in the neck of a large animal, I think either a sheep or a pig I'm not sure which one. It's stitched into place, the animal then leaves the operating room with a new blood vessel which you can study by using imaging techniques. You can follow this blood vessel. They've done this for many, many months now, even years in some cases. You can form vessels that are smaller than any that have been transplanted in this way and they maintain their function in culture.

I show this because it's an exciting example of tissue engineering. It's also, I think, a good illustration of one of the advantages of tissue engineering, in that you can create a tissue outside by arranging things together. You can maintain it under controlled conditions in the laboratory until its ready to be transplanted. Then, you can transplant it at the right time. All of these things are under control so it could be optimized to produce the best quality tissue. In this case, they've cultured the vessels for eight weeks in order to get the kind of function that they want. I think you can imagine from these examples I've shown you, lots of different ways that you could build outside the body neo-tissues or structures that will develop into, hopefully, real tissues.

This illustration is just to sort of show you all of the kinds of tissues that, at this point in time, people have tried to make using tissue engineering approaches. Many of you might know Erin Lavik in our Biomedical Engineering Department, who's been working and has some fabulous results on tissue engineering to create spinal cords, to treat injured spinal cords. There's many examples like this. There's already tissue engineered skin that's available for physicians to use. The other thing I tried to show you with this diagram is that what you're trying to achieve while you might be using the same approaches that I've shown here, you're trying to achieve different things in different tissues.

Some things you're trying to replace structure, mainly. If I'm trying to use this approach to make new bone I'm trying to make new structural material. What I care about is the mechanical properties of the tissue that I create; at least I care about that a lot. A bone that cannot form--perform the mechanical functions of bone is not a very good bone. It has to have the right elastic modulus, which we talked about earlier. In other tissues you're trying to replace metabolism or the cellular biochemistry that goes on inside the cells. In the liver, you don't care so much about the mechanical properties of the liver, you care that all the cells are performing all the metabolic properties that a liver ordinarily performs; that table of things I showed you last time.

In the pancreas, in the same way, you're trying to create a tissue that forms a metabolic function. In the pancreas, one of the most important metabolic functions is that it senses glucose and secretes insulin in response to glucose. You don't care about the structure you care about the metabolism. There are some things like blood vessels where you're in between. A blood vessel needs to perform the mechanical function, needs to serve as a physical plumbing to carry blood from one place to another. It also performs a biological function, it carries the blood and it anti-coagulates at the same time, it stops coagulation. There are many other biochemical functions that blood vessels perform.

Chapter 5. Tissue Engineering in Control of Drug Delivery [00:32:19]

The next aspect of tissue engineering I want to focus on is that tissue engineering often uses cellular processes to control drug delivery. This provides a new way to do drug delivery to supplement the kinds of engineering approaches we talked about a few weeks ago. Now how might you do that? Well, let's imagine, just look on this side of the slide here, and imagine you have a hollow fiber. A hollow fiber is a fiber that has an outside diameter, and then it's hollow on the inside, made of some kind of synthetic material. It might look like a straw, like a straw that you drink through, only the straw has special properties. In general it's much, much thinner than a drinking straw would be. This might be a millimeter across instead of several millimeters like a drinking straw would be.

You fill this up with cells, you fill up the hollow fiber with cells. The fiber is a special material in that: 1) it's biocompatible so you can implant it in the body and the body doesn't respond adversely to it, and 2) it has a controlled porosity so that molecules can pass in and out. It's porous, it allows molecules to pass in and out but it doesn't allow cells to pass in and out. It's a filter, it's a porous filter that allows small molecules to go in and out, but doesn't allow large molecules to go in and out.

Now, if this is filled up with cells. I show this cut away so that you can see inside. Here's little clusters of cells growing inside, but it's a full cylinder in reality. The ends are sealed and I implant this into the body. Let's say I put it into the abdominal cavity or I put it under the skin. What you would like, then, is for these cells to be able to communicate with the body through molecules but not directly. They're isolated from the body because this hollow fiber is surrounding them everywhere. What if these were cells from the pancreas? Then, maybe glucose, blood sugar, could diffuse or move through the wall of this hollow fiber, exposing these cells to glucose. The cells would sense the glucose is there because they're cells from the pancreas and they're functioning normally. They would secrete insulin in response to glucose. The insulin would flow out, and then into blood vessels outside here.

Now why go to all this trouble? Why not just inject these cells right into the body? Why surround them by this hollow fiber? What do you think would happen if you just injected them under the skin or you injected them into the abdominal cavity? Caitlin?

Student: There would be an infection.

Professor Mark Saltzman: They would be recognized as foreign by the immune system, unless they were your cells. You need this treatment because your cells aren't working properly so you don't have any pancreas cells to contribute. The cells either have to come from somebody else, an allograph, foreign to you. It could even be a zenograph; it might be cells from another species. They might be cells from a pig, for example. It turns out that the insulin that pigs make is very similar to human insulin. Not exactly the same but very similar. So, for decades and decades, until we discovered ways to make recombinant human insulin in the 1980s, pig insulin was the primary source of insulin to treat diabetics.

Why not use cells from a species where cells are readily available and put them inside of a capsule like this so that they're separated from your immune system but still able to function as a pancreas? In a sense, it's functioning as a drug delivery system. The drug here is insulin. It's a smart drug delivery system, and it has the intelligence of the cells that are encapsulated inside. It's smart in what way? It's smart in effect that it can detect the presence of glucose and secrete insulin only when glucose is present and insulin is needed. If this was used to treat Diabetes, if you were a diabetic you wouldn't have to be checking your glucose levels and adjusting how much you inject because you'd have a system inside your body that's doing it automatically in the way, hopefully, that a normal pancreas is.

What I show on the left hand side of the graph here is some different configurations that people have now made using this same concept. It doesn't have to be a hollow fiber, but you can make a micro-capsule like this. Remember when you made the alginate particles in the section meeting? We mentioned that not only could you put food coloring inside these particles but maybe you could put cells inside these--and you can. You can put cells inside these particles and make little capsules that have cells on the inside but are surrounded by a barrier that keeps them away from the immune system. There are a variety of other ways of doing this as well.

This next slide, just shows you the advantage of this strategy, here the dashed line is the membrane whether it's formed by a hollow fiber or an alginate--or a hydrogel seal, engineered cells are on the inside. Now, I talked about putting cells from an animal inside but maybe you could genetically engineer beta-cells so that they function for a long time.

Maybe you could take a patient's own diseased cells and genetically engineer them so they work properly and then give them back. Maybe you could take cells from the pancreas of an individual donor, genetically engineer them so that they grow in culture, and then they'd be available to treat lots of patients. The idea is that nutrients, oxygen, glucose can come inside, insulin can come back out, but that the immune system won't recognize it as foreign.

Now, this is an idea that's also been around for a long time. There's been a lot of people that have worked on it, great progress has been made. It turns out to be a tremendously challenging engineering problem because you need to engineer a material that's compatible with the body that doesn't--that allows only the passage of certain molecules and not other molecules. It has to allow insulin to pass but not antibodies. It has to do that very reliably without allowing any immune system components to come inside, and it has to do it at a high enough rate. You have to not only allow insulin to go through; you have to allow it to go through in large quantities so that you can provide enough insulin in order to treat Diabetes without having a huge device. It needs to be small and produce a lot of insulin. These turn out to be challenges that we haven't quite achieved yet.

Chapter 6. Summary and Conclusion [00:39:54]

Let me just summarize what we've talked about. In the field of tissue engineering, which I think you're going to see make a lot of progress during your lifetime. A logical extension of conventional medical and surgical practices, it involves the control or regulation of normal healing. We talked about tissue engineering approaches that use cells and tissue engineering approaches that don't use cells, where the material or some other aspect of what you're using controls the healing process. We talked about tissue engineering to supplement the cellular component, and even to guide the ways that cells form into new tissues to control drug delivery.

This last aspect, which I think you can probably see more clearly now then when I mentioned it last time, is that now you could use these new tissues that are created in the laboratory to understand physiology better. You could take Dr. Niklason's tissue engineered blood vessels and use them to understand more about how blood vessels develop during embryonic development, because she's created a system where she can control many of the key variable. You could add cells at different time and see how this developing tissue responds to that. You could add signaling molecules at different times and see how this developing tissue remodels or responds to these signals that you added. These turn out to get very powerful tools, not only for creating new therapies but for understanding human physiology.

I'm going to stop there but there's some time for questions. It could be questions about this or last minute questions about your papers that are due tomorrow. Any questions about that? I know I've answered quite a few by email, it seemed like everybody was on the right track. No, everybody's good? Good, so I'll see you in section this afternoon.

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