In recent years, researchers have used decellularization to strip cells from rat hearts, leaving behind the intact and intricate extracellular matrix structure, and then rebuilt a functional heart by seeding it with new cells. This approach has the potential to create patient-matched donor organs with minimal issues of immune rejection when transplanted, but there is still work to be done in order to reliably carry out the process in human hearts:
Researchers have taken some initial steps toward the creation of bioengineered human hearts using donor hearts stripped of components that would generate an immune response and cardiac muscle cells generated from induced pluripotent stem cells (iPSCs), which could come from a potential recipient. Using a scaled-up version of the process originally developed in rat hearts, the team decellularized 73 hearts from both brain-dead donors and from those who had undergone cardiac death. Detailed characterization of the remaining cardiac scaffolds confirmed a high retention of matrix proteins and structure free of cardiac cells, the preservation of coronary vascular and microvascular structures, as well as freedom from human leukocyte antigens that could induce rejection. There was little difference between the reactions of organs from the two donor groups to the complex decellularization process.
Instead of using genetic manipulation to generate iPSCs from adult cells, the team used a newer method to reprogram skin cells with messenger RNA factors, which should be both more efficient and less likely to run into regulatory hurdles. They then induced the pluripotent cells to differentiate into cardiac muscle cells or cardiomyocytes, documenting patterns of gene expression that reflected developmental milestones and generating cells in sufficient quantity for possible clinical application. Cardiomyocytes were then reseeded into three-dimensional matrix tissue, first into thin matrix slices and then into 15 mm fibers, which developed into spontaneously contracting tissue after several days in culture.
The last step reflected the first regeneration of human heart muscle from pluripotent stem cells within a cell-free, human whole-heart matrix. The team delivered about 500 million iPSC-derived cardiomyocytes into the left ventricular wall of decellularized hearts. The organs were mounted for 14 days in an automated bioreactor system that both perfused the organ with nutrient solution and applied environmental stressors such as ventricular pressure to reproduce conditions within a living heart. Analysis of the regenerated tissue found dense regions of iPSC-derived cells that had the appearance of immature cardiac muscle tissue and demonstrated functional contraction in response to electrical stimulation. "Regenerating a whole heart is most certainly a long-term goal that is several years away, so we are currently working on engineering a functional myocardial patch that could replace cardiac tissue damaged due a heart attack or heart failure. Among the next steps that we are pursuing are improving methods to generate even more cardiac cells - recellularizing a whole heart would take tens of billions - optimizing bioreactor-based culture techniques to improve the maturation and function of engineered cardiac tissue, and electronically integrating regenerated tissue to function within the recipient's heart."