In Vivo Engineering of Organs: the Bone Bioreactor
Molly M. Stevens, Robert P. Marin, Dirk Schaefer, Joshua Aronson, Robert Langer, and V. Prasad Shastri
It has long been the goal of tissue engineers to successfully utilize cells to create new tissues and organs so that they may be transplanted into patients in need. Over the years, some level of success has been achieved in simpler human tissues such as blood vessels and skin. It has proved to be harder to engineer organs and tissues with greater complexity, namely tissues with blood vessels running through them, such as bone tissue. Even if the tissue is cultured, the tissue needs to be successfully transplanted into the patient. At this point issues such as immune rejection and mechanically mismatched tissues need to be considered. To avoid such issues in culturing/transplanting bone tissue, this experiment focuses on exploring the regenerative properties of the bone’s periosteum cells to create an in-vivo bone bioreactor.
Periosteum cells are cells lining our bones that divide and differentiate upon wounding and fracture. It is hypothesized that the same healing response can be used to generate new tissue. To test this hypothesis, a saline solution was injected between the tibia and periosteum of rabbits. This was done to create a cavity that serves as a bioreactor for bone cells to grow. Alginate, a calcium-rich gel (which promotes bone cell formation), is then injected into this cavity to prevent it from collapsing when the saline solution is absorbed back into the body. Under these conditions, bone cells are allowed to grow and proliferate in this cavity over a period of a few weeks. The content (percentage of gel, percentage of saline, percentage of bone matrix) of the cavity is measured every week and data is compiled.
As predicted, within a few weeks, the cavities were filled with bone. The concentration of alginate gel and saline decreased over time as bone cells proliferated and filled up the cavities. By four weeks, more than 90% of the cavity was filled with new bone. This bone tissue was then removed and transplanted into damaged bone cites (defect was created in the rabbit’s bone) in the rabbit and the wounds healed seamlessly. As the transplantation was autologus, no immune rejection was observed. This result demonstrates not only the successful culturing of bone cells, but also the successful transplantation of bone tissue into a host with damaged bone. In conclusion, by utilizing this method of in-vivo bone regeneration, one can achieve desired periosteum differentiation and successful transplantation of new bone cells to other damaged bone cites within the same animal. Further research may allow for this method to benefit human patients with bone / spinal damage.
Not only was the hypothesis proven in this experiment, the methods utilized are clear and reproducible. The next step would be to conduct this experiment on human subjects. If this is successful, clinical applications would include fusing vertebrae in spinal fusions, bone repair for fractures/breakages, and cartilage replacement for arthritis. In addition, this process is much better than the current method of harvesting bone from a patient’s hip, which is very painful. Further research must be conducted to achieve this goal for humans, but the successful culturing/transplantation in rabbits has made this endeavor optimistic.
9 comments:
To get this straight...bone cell differentiation and proliferation is induced in vivo and eventually removed and transplanted into the bone region of damage? I know nothing of bone growth, so my question is: why can't we induce bone growth in and around the areas of bone damage in the first place? Why make the extra step in culturing in another area and then transplanting?
This is nice. Quite a bit has been done with bioreactors to tissue engineer bone. Bonzani, et al, for example, studied flow perfusion bioreactors to enhance nutrient delivery (http://ieeexplore.ieee.org/iel5/9104/28882/01300037.pdf). The in vivo version, though, is innovative. I would assume an in vivo “bioreactor” would experience even less complications regarding improper diffusion of nutrients and inadequate cell-cell interactions.
As Alvin stated, it is a bit confusing as to why they didn’t use the damaged site as a site for a wound-healing in vivo “bioreactor”. Perhaps it is easier to regulate cell growth in an alternate region?? I can’t think of any good reasons for implanting, explanting, and re-implanting tissue. It seems highly invasive, actually. But, I’m sure they had a reason. Did they mention anything about this in the article?? Or maybe Alvin and I are just interpreting it incorrectly…
When the saline/gel cavity filled with bone, was it periosteum tissue, osseous tissue, a combination, ect? Since the cavity was formed between the periosteum and tibia, do only periosteum cells grow? If you needed to replace osseous tissue in a patient would you need to make the cavity actually go into the tibia to have osseous tissue formed? Or can you just implant periosteum cells and rely on them to differentiate into osseous tissue? Also, are there negative affects on the original bone, as in does the removal of the grown tissue weaken the remaining bone and does the periosteum reattach after the removal?
So, the quality of the bone generated by the bioreactor is as good as the normal bone, right? That is so exciting! However, it seems that the quantity of bone that can be generated by this bioreactor is limited by the diffusion factor. It may not be able to produce enough amount of bone for an entire femur replacement. In my opinion, the first step to make the engineered bone useful may be plastic surgery, since a smaller amount of bone is required. Actually, I am always interested in gaining a couple more inches of extra height and this may be the way I can go with.
Using this technology to grow an extra couple of inches?? That is quite funny, even though it might be probable. I wonder how or if this "bioreactor" vary in its process/method in repairing bone fractures/breakages and cartilage replacement...
To Alvin and Angelee: That is a great question. The article itself did not mention reasons for growing the bone in another part of the body and then transplanting to the injured area; the focus was mainly on the ability to make a portion of the body into a in-vivo bioreactor. However, my best guess would be that for experimental purposes they wanted to actually see that bone cells are culturable IN ADDITION to being tranplantable. Because in this case, such autologous cells can be cultured in the bioreactor and then stored for banking purposes and used when needed. This also leaves room for future development for non-autologous transplantation when immune rejection issues can be resolved. Though I believe that it would be a good idea to attempt to created the bone bioreactor right at the site of injury; the only real-life issue here is that this process is slow, and from a clinical point of view, less efficient than pre-cultured cells that can be immediately tranplanted to the site of injury.
To Shannon: When the cavity began to fill with bone, the cavity was initially filled with periosteum tissue; within 2 weeks, these cells gradually differentiated into woven bone (osseous tissue). That is a great question, and I think the best answer is it depends on the time at which we are observing the cavity; it would make sense to see the presence of more structural tissue as time progresses. The end result is that the cavity would be filled with enough osseous tissue so that the bone would have simliar mechanical properties to the actual bone! To your second question, in the experiment the cavity did not need to go into the tibia to produce the desired osseous cells; implantation of periosteum cells was sufficient. The effects of cultured cell removal seem to have negligible effect on the mechanical strength of the remaining bone as the bone remains unchanged; a new portion of bone was cultured outside of this bone was was merely removed.
To Raymond: I believe you are much correct in saying so - the diffusion factor would definitely impact the size limit of the in-vivo cultured cells. But even then this is still a breakthrough as even a small portion of damage to bone can be extremely painful, and this method shows promise to fix this problem without damaging other parts of the body. I also believe this can be used in plastic surgery in terms of culturing cartilage - in the paper it was mentioned that they have also attempted the same experiment with cartilage. So even though entire bones will not be able to be grown with this method, it will still be extremely helpful if clinical trials succeed in the future. And I would be right there with you on the "gaining a few more inches of height" train, though that still doesn't sound super feasible, haha.
To Ben: There is a difference in culturing cells for bone replacement versus cartilage, as they are different types of cells (tweaks in growth factors, chemical cues, etc., would be needed). However, the overall picture is still the same, and results have proved to be promising for both cases.
Nice article, I know of a person who was born without jaw bone and doctors used a bone graft from the chest to reconstruct the jaw. This might be useful for small bone grafts where you do not require a large amount of bone mass to begin with. Another question I have is that doesn't creating a cavity weakens the host bone which contains the reactor, because healing is never scar free. I guess the benefits or advantages of getting an autologus bone graft is much greater than scarred bone.
How does the newly formed bone compare mechanically with native bone? I would imagine that if there are differences in microstructure, the interface between the new and old bone may be weak. Anyway, this is some very cool research but seems sort of painful if done in humans.
This is a very interesting paper. The methods do seem very invasive. It seems confusing to me that just saline solution injected in the right spot could cause bone formation. Does this procedure harm the area of the bioreactor?
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