The development of artificial arteries

Your current set of investigations, in a way, brings together many of the threads we have been discussing. This is centred on the development of artificial arteries. Could you describe what these are?

The seed of this work developed in the early '80s, when Gordon and I were putting pieces of foreign bodies – for example, boiled blood clots or boiled egg whites, or gelatin, or even bits of glass or wood – into the peritoneal cavity to initiate an inflammatory response, to form a myofibroblast capsule around the outside. A myofibroblast, which causes wound contracture in the skin, was always thought to be halfway between a fibroblast and a smooth muscle cell. If you cut yourself, cells in the periphery, on the edges of the wound, become myofibroblasts which become contractile, and when they contract they bring the edges of the wound together. But there was some controversy on their cellular origin. So we were putting this foreign body in the peritoneal cavity – as other people were – to develop myofibroblasts and study their biology.

We noticed not only that the capsule that developed consisted of myofibroblasts but that on the outside there was a layer of mesothelium, the cells that line the peritoneal cavity and have properties very similar to endothelial cells that line blood vessels. They secrete prostacyclin and nitric oxide, supposedly so that they can cause a vasodilatation. They also form a frictionless surface in the peritoneal cavity so the guts can slide around and not stick, just as the endothelium provides a frictionless surface so the blood cells can slide down the lumen. So we had the foreign body, then a layer of these myofibroblasts, and the mesothelium on the outside.

When we did these studies we said, 'Gee, that looks like an artery, but with the cells that normally line the lumen on the outside. It's also a sphere, a solid body. Perhaps we could grow that in a tube structure and make an artery out of it.' Because we were doing so many other things, though, we just put it on the backburner. It wasn't till 10 years later, when yet another PhD student came to us and we were running out of PhD projects, that we thought, 'Hmm, why don't you put some tubes into the peritoneal cavity and see whether you can grow this myofibroblast capsule and mesothelium in the form of a tube?' And the student, Johnny Efendy, did so and found that was what happened.

We then harvested it from the peritoneal cavity and turned it inside out, removing the inner piece of tubing. What we got was a structure that had now the mesothelium, or pseudo-endothelium, lining the lumen of this tube of living tissue. Nothing else. And when we transplanted it into high-pressure arterial sites, we found that it differentiated further into an arterial structure.

We are now doing this in dogs. A piece of tubing 4.5 mm in diameter, which has been in a dog peritoneal cavity for three weeks, can have a capsule formed on the outside which is about 1½ mm to 2 mm thick. That's a pretty strong piece of tissue.

This is now big news, I gather, and it has led to your revolutionary lateral step.

Yes. Recently we have also proven that the myofibroblasts that we were studying years ago, wondering about their origin, are in fact derived from peritoneal macrophages. By using transgenic mice that you can get these days with specific labels for macrophages, we can trace their lineage. So, using new technologies, we have now come to solve a question that we were looking at in the '80s, and have been able to develop these artificial blood vessels.

Grow-your-own designer blood vessels

All this amazing stuff began with the observation that what was happening around those foreign objects looked a bit like a blood vessel. Are you the first people to have made this observation?

Yes, that we know of. People might have said it but not published it. We saw it, we published it, and we did something about it. That's what makes the difference.

Are there other ways of making artificial blood vessels?

A number of laboratories overseas have been trying to grow them in culture, but you have to sacrifice a healthy blood vessel to grow those smooth muscle cells and endothelial cells and then re-seed them into various biodegradable structures. We can grow ours in the peritoneal cavity or the pleural cavity of the person or animal that is going to get that transplant. So it is an autologous artificial blood vessel and there is no rejection.

When you take some healthy blood vessel out of the body and grow the cells in culture, the cells lose a lot of their antigenic properties. Then, if you put the artificial vessel into a bioscaffold and back into the animal or person, the host recognises that as a foreign body and can reject it. The fact that we are growing and transplanting it in the body means there is no rejection. Also, our tube of tissue grows from almost nothing, just cells floating in the peritoneal cavity, to this required structure within two to three weeks. Growing tissue-engineered blood vessels in vitro takes months. So we think we have done something a lot better.

We call these structures grow-your-own blood vessels, or grow-your-own designer arteries, and we can grow them very long. In fact, we have now developed a device whereby we grow the myofibroblast capsule inside an outer sheath that is adhesion resistant, so we don't get any problems, and has holes in it. The cells are attracted through holes into a biodegradable matrix around an inner polyethylene tube. We grow these in dogs to about 25 cm long.

The procedure is really quite non-invasive. We do a small incision, under general anaesthetic, in the linea alba and then a small incision in the peritoneal wall. We put the device in the peritoneal cavity, with a flange which sits flat against the peritoneal wall. We put purse-string sutures around the outside of that little incision, pull it tight so there is no leakage, and then just sew up the skin. The device, which has to be free-floating, just dangles free in the peritoneal cavity. Two to three weeks later, we come back and, under local anaesthetic, just do a very small incision, cut where we have sutured the flange down to hold it flat, pull it out, put a couple of sutures in to sew up the hole and then sew up the skin.

We can then transplant the new vessel as a vascular graft. In the dogs we have been transplanting it into the femoral artery as an interposition graft, and we have kept it there for many, many months. When it is transplanted into that high-pressure arterial site, it undergoes further development, further differentiation, such that it becomes identical to an adult vessel. If a sample of the blood vessel is stained with antibodies to smooth muscle myosin, you can see a media, an adventitia, even vasa vasorum, the small blood vessels in the adventitia. So it becomes almost exactly like a native blood vessel.

Focus questions

• What is the structure of an artery and how does this structure help it perform its functions?
• Why is the procedure used by Campbell to grow-your-own arteries an improvement on the other techniques used to make artificial arteries?