Author: Professor Sian Harding, 30th September 2019
Tissue engineering for cardiac repair has reached an advanced stage and engineered constructs are starting to be approved for first-in-human trials. This has been driven by positive factors such as the marked improvement in protocols for differentiating pluripotent stem cells (either embryonic or induced pluripotent) into beating cardiomyocytes as well as negative ones such as the high rate of loss when cardiomyocytes are injected as a suspension into the muscle of the heart.
Large numbers of cardiomyocytes (tens of millions to billions) can be produced with high purity even in an academic laboratory, and they readily form tissue engineered constructs with synchronous beating. The cardiomyocytes are rather immature, and this can have benefits as they are still able to expand their numbers, are resistant to low oxygen conditions and can integrate more readily into the host heart when grafted. On the other hand, their spontaneous beating and different characteristics from the host myocardium have resulted in dangerous disturbances of rhythm when they are directly injected into the interior of the myocardium. Tissue engineered constructs placed on the surface of the heart do not seem to produce this problem and so are now the preferred choice for cardiac grafting.
A roadblock to the use of engineered heart tissue has been the establishment of a blood supply after grafting. Vessels from the host heart have been observed to grow into the graft, but these are small and the flow is at least ten times lower than in normal myocardium. Many strategies have been tried, such as mixing in endothelial or other blood vessels cells with the cardiomyocytes and introducing tunnels in the construct with needles to encourage blood vessel formation. 3D printing is an obvious solution, but so far the complexity of producing non-collapsing tubes in a thick construct had not been achieved.
A recent paper by Noor et al. has overcome previous difficulties and produced a number of elegant solutions which have allowed close reproduction of human myocardium. First, they have both the cell source for pluripotent cardiomyocytes or endothelial cells and the “bioink” vehicle from the same patient. This bioink hydrogel is prepared from the extracellular matrix of the membrane covering the stomach, and the pluripotent cells are obtained by reprogramming the cells within it. This can potentially produce a tissue exactly matched to the patient and therefore resistant to immune rejection.
Another unique advance was the use of CT scan of the heart plus computer-aided design to recreate the exact architecture of the blood vessels in the heart of the patient. This could only be done for the larger vessels because of the resolution of the CT, so they used a computer model to develop the smaller vessel structure of the construct. To keep the open blood vessel structure, they used a second bioink – gelatin. This was used as a sacrificial scaffold, meaning it could be removed at the end by raising the temperature to body heat. The gelatin melted and was washed away, leaving large blood vessels structures which became lined with the endothelial cells, mimicking a normal vessel. In very important experiments, they were able to confirm the flow of liquid through these vessels. To maintain the structure without it collapsing in on itself, they added alginate microparticles to stiffen the gels. The cardiac patches were robust: they could be lifted out and handled well without losing their shape and the cardiomyocytes within them responded by elongating, aligning and maturing as in the normal heart. The images within this paper give the most convincing evidence to date that it is possible to build from scratch the complex architecture of the myocardium using materials completely compatible with the patient’s heart.
References
Noor, N., et al. (2019). “3D Printing of Personalized Thick and Perfusable Cardiac Patches and Hearts.“ Adv Sci (Weinh) 6(11): 1900344.