# Whole Organ Engineering: Approaches, Challenges, and Future Directions
## 5 Recent Developments and Accomplishments
### Lungs  
Proper respiratory function is heavily dependent on lung tissue mechanics, due to the need for an ECM structure that promotes gas exchange and elasticity. Replication of native lung tissue mechanics has been accomplished via various decellularization approaches. The first experiments in which a whole bioengineered lung was created using decellularization and recellularization were published in 2010 by [PERSON] et al. and [PERSON] et al. [48, 116]. Due to the dependence of respiratory function on the compliance of lung tissue, it is crucial that engineered whole lungs retain the native lung tissue mechanics as much as possible after decellularization, as the process can damage native ECM in addition to removing cellular material. Because of this, decellularization protocols must be developed to be as harmless as possible to lung ECM. Various perfusion methods using detergent solutions such as SDS, Triton X-100, and CHAPS, as well as enzymatic solutions, have been studied in various animal and human models to determine effective decellularization methodologies [117, 118, 119, 120]. However, decellularization protocols are not well standardized, and the optimal decellularization method is undefined. Early lung recellularization procedures were done via injection of primary pulmonary and endothelial cells, and related cell lines, into the trachea and pulmonary artery [48, 116]. Restoring the ability of the lungs to perform their most primary function, gas exchange, requires reconstruction of the microstructures comprising the airways and blood vessels of a recellularized lung repopulated with epithelial and endothelial cells, respectively. To that end, recellularization has been approached via supplying endothelial cells and perivascular cells through the pulmonary artery and the pulmonary vein, while the airway is epithelialized via the bronchial system [48, 116, 117, 118, 119, 120]. In 2018, [PERSON] et al., published their results of a large animal study in which they recellularized acellular porcine lung scaffolds with human airway progenitor cells and HUVECs [117]. The recellularized lung constructs were maintained in a bioreactor for 6 days to allow the seeded cells to mature. The humanized porcine lungs were then transplanted into pigs and perfused using the host's pulmonary circulation for 1 h. It should be noted that the bioengineered lungs displayed reduced capabilities of gas exchange and mechanical compliance when compared to native lung tissue. Various groups have developed bioreactors for maintaining recellularized grafts in vitro, in which cellular differentiation or maturation can be facilitated via steady vascular perfusion and bronchial ventilation. These bioreactors are intended to mimic the physiological conditions native lung tissue is exposed to and thus enable cells to create an organ in vitro that resembles the native lung. For recellularization of acellular lung scaffolds, pulmonary cells are often derived from iPSCs because iPSCs can be autologously sourced and then expanded to the requisite cell population size needed to repopulate whole lungs [48, 116, 117, 118, 119, 120]. Functional endothelialized pulmonary vasculature must be able to maintain a fluid-tight seal while still permitting the exchange of oxygen and carbon dioxide. Dr. [PERSON]'s group developed a successful protocol for seeding decellularized lung scaffolds with endothelial cells by perfusing cells into the lung microvasculature [116]. However, this method was not able to successfully generate an endothelial layer with barrier functionality and was susceptible to thrombosis. In a 2019 study, [PERSON]'s group expanded upon their work in [PERSON] et al., 2019 by incorporating various small, bioactive molecules into the recellularization of decellularized lungs to determine if this would result in improved endothelial function [121].