A piece of myocardium
Every fourth death in the world is associated with heart disease, about 3,500 patients in the United States are waiting for a heart transplant. Many of them will wait for more than six months, and for some the time will run out before the transplant queue comes up. These statistics illustrate the urgent need to create effective strategies for the replacement of cardiac tissue.
Unlike other organs, which can recover to varying degrees after damage, the heart has practically no regenerative ability. Heart cells that have died as a result of chronic diseases or myocardial infarction are replaced by a fibrous scar that disrupts the normal contraction of the heart. Modern technologies make it possible to produce patient-specific cardiomyocytes from stem cells, but copying the highly structured architecture and complex functionality of the myocardium remains a serious task.
The left ventricle of the heart pumps blood through a large circle of blood circulation, contracting in a “twisting” motion. This reduction is provided by layers of cardiomyocytes oriented in the same direction inside each of the layers of the myocardium. These layers are stacked on top of each other across the wall of the heart muscle with a thickness of 1 cm, each of which is oriented at an angle with respect to neighboring layers. Despite the fact that each cardiomyocyte is compressed in one direction, the different location of each layer of cardiomyocytes causes the ventricle to twist, squeezing out blood and causing it to flow throughout the body. Tissue engineers have developed various methods of aligning heart cells in separate layers, but have not been able to recreate a myocardium of sufficient thickness for use in regenerative therapy of heart damage.
In a new study, a group of Jennifer Lewis from the Wyss Institute of Biological Engineering at Harvard University and the Harvard John A. Paulson School of Engineering and Applied Sciences developed a set of technologies that allowed recreating the multilayer architecture of the contractile elements of the heart. Using bioprinting with tightly packed contractile blocks (organ building blocks, OBB) consisting of cardiomyocytes grown from human induced pluripotent stem cells, they were able to create aligned layers of cardiac tissue with a complex architecture. These sheets have the same organization and functionality as a natural heart muscle.
Step by step
The study is based on the SWIFT 3D bioprinting platform, which allows you to create cardiac tissue structures with a high cellular density characteristic of normal cardiac tissue. This approach allows us to solve an important problem of tissue engineering – the introduction of a vascular network. However, the resulting tissue structures did not reproduce the complex arrangement of the layers of the human myocardium.
To control the direction of contractility in the constructed layers of cardiac tissue, the researchers first developed a programming strategy for parallel alignment of stem cells. To do this, they created a platform with 1050 separate wells, each of which contains two micro-columns. Stem cells mixed with fibroblasts and collagen of the extracellular matrix, which are necessary for the development of the myocardium, were sown into the wells.
When the cells filled the extracellular matrix, they formed a dense micro-tissue in which cardiomyocytes and their contractile mechanisms are oriented along the axis connecting the micro-columns. Anisotropic OBB (aOBB), that is, those that are reduced in one direction, are then removed from micro-columns and used as raw materials for the manufacture of dense biochernils. The high-performance approach allowed generating an unprecedented number of aOBB contractile blocks.
The second stage of alignment is the printing process itself. The mechanical shear forces generated by the printer's printhead affect the aOBB, giving them direction. The group has previously shown that using 3D printing, it is possible to orient anisotropic soft materials in space. Now she has demonstrated that this principle applies to cardiac micro-tissues as well. To emphasize the versatility of the bioprinting process, the researchers printed sheets of cardiac tissue with linear, spiral and angular geometries in which contractile aobbs demonstrated the necessary alignment.
The group also measured the contractility of cardiac structures created from aOBB. To do this, long macrofilaments connecting two columns were printed, similar to the OBB generation stage using a micro-column platform, only on a larger scale. By evaluating the deflections of the columns, the researchers could determine the contractile forces created by macrofilaments.
The researchers found that the strength and speed of contraction increased over seven days, proving that the threads continue to mature into real muscle fibers. The next step the group plans to use this method to create a larger number of physiological tissues.
Not only transplantation
The ultimate goal of tissue engineering efforts is the transplantation of an entire organ, but the new approach can also be used in smaller-scale tasks, for example, to model diseases or create highly organized patches on the myocardium that will restore the heart after a myocardial infarction or close the holes in the partitions of newborns with heart defects.
Article by J.Ahrens et al. Programming Cellular Alignment in Engineered Cardiac Tissue via Bioprinting Anisotropic Organ Building Blocks is published in the journal Advanced Materials.
Aminat Adzhieva, portal "Eternal Youth" http://vechnayamolodost.ru based on the materials of the Wyss Institute: Getting to the heart of engineering a heart.