# Whole Organ Engineering: Approaches, Challenges, and Future Directions
## 4 Approaches to Engineering Whole Organs
### Building Organs from Cadaveric Organs (De-Cell/Re-Cell)  
While the shortage of donated organs is the primary reason why some individuals can remain on a transplant waitlist for years, compatibility between donor and recipient is a concern as well. Blood type and body size are the most apparent limiting factors [7]. Even when all other compatibility factors are satisfied, the fact that the DNA of the donated organ's cells are different from the recipient's DNA means there is risk of an immune response and organ rejection. However, top-down organ engineering methods can be used to overcome these limits. Cadaveric and potentially even animal organs (e.g., porcine) can be engineered to have the donor's cellular material removed while retaining the ECM structure and biochemical properties, then repopulated to house non-immunogenic cells. This not only removes concerns of blood type but also eliminates the concern of the organ recipient's immune system recognizing and attacking the donor's cells in the organ [47; 61; 62; 63; 64]. Removing the native cells, or decellularization, commonly involves perfusing the vasculature of an organ with surfactant and enzyme solutions to kill cells via lysis, then rinsing the vasculature with a saline solution which is often paired with mechanical stimulation to remove cells embedded in tissue [47; 61; 62; 63; 64]. This process can produce an organ construct with the structural properties and biochemical markers of the native organ while removing native DNA, and thus has the potential to mitigate the immune response to cadaveric human and even animal organs. Repopulating the acellular cadaveric construct, or recellularization, involves perfusing the vasculature of the organ with a cell solution and maintaining the construct in a bioreactor to promote cell adhesion and proliferation [65; 66; 67; 40; 62]. The decellularization will have removed cells but left the ECM with its structural and biochemical properties, maintaining a microenvironment conducive to cell adhesion and growth. If the cells used are sourced from the patient or from cells genetically engineered to be immune-compatible, then the cell-laden organ construct will not elicit an adaptive immune response upon implantation. Because of the potential for decellularization/recellularization procedures to remove the immunogenicity of tissue, using cadaveric human and animal organs as sources for ECM scaffold generation has gained prominence in whole organ engineering research [60; 61; 68; 69; 56].  
ECM and decellularization techniques are a promising method of personalized organ production. Early work in advancing decellularization approaches focused on maintenance of whole organ function after decellularization in animal models [70] as well as allogenic clinical studies [71] without seeding cells in vitro. For example, [PERSON] et al. investigated the mechanical properties and ECM composition of the rat heart, while [PERSON] et al. characterized clinical factors relevant towards allogeneic tracheal tissue implantation [37; 68]. Such results were limited in scope since they used mechanical tests to determine decellularized organ function rather than evaluating the regenerative capacity of the scaffolds. Further investigation by [PERSON] et al., tested the regenerative capacity of a rat heart in vitro by recellularizing the rat heart with a coronary perfusion bioreactor and then subjecting it to a periodic electrical stimulus [38]. They found that their approach resulted in the decellularized heart regaining roughly 2% of the pump function of native adult rat hearts and 25% of the pump function of a 16-week fetal human heart, and even responded to doses of phenylephrine with increased contractility as a native heart would [70]. Engineered whole human trachea allografts were analyzed before and 3 months after implantation, and it was found that physiological function was restored in the graft and that the grafting procedure resulted in the reversal of airway obstruction [71].