78 Chapter 4 resemblance of the physiological architecture and multicellularity of the lung. These models represent the status of the lung at a given time of harvest, thus providing excellent tools to study different stages of disease progression. Overcoming the current restrictions, combined with facilitating immune cell recruitment, would aid in unlocking the true potential of PCLS. ORGANOIDS Organoid cultures are defined as 3D cultures grown from pluripotent stem cells (PSCs) or adult progenitor cells that self-organise to form structures histologically similar to human organs [29, 30]. The requirements for 3D cultures to be considered as an organoid are self-renewing capacity and the replication of some function specific to the mimicked organ (such as the mucociliary function in the case of bronchial organoids) [29, 30]. In contrast to organoids, spheroids are typically cell aggregates grown scaffold-free from a single or a mix of multiple cell types and have a lower complexity while recapitulating native tissue [31]. While organoids modelling bronchial epithelium have been rapidly instituted, those for alveoli have been more challenging to establish [32]. Early efforts employed flow cytometry sorting of murine alveolar type II epithelial cells co-cultured with mesenchymal cells [33]. To date, the minimal composition of lung organoids built from adult progenitor cells comprises bronchial epithelial cells that are embedded in a 3D ECM-based hydrogel with a complex combination of growth factors [34]. This gives rise to spherical organoids often filled with secretions, that are classified into tracheospheres [35], bronchospheres [36], or alveolospheres [33, 37], depending on the source of epithelial cells. More recently, isolation (often with selection of EpCAM+ cells, via flow cytometry [38] or magnetic bead-based selection [39]) of epithelial cells, coupled with culture with or without mitomycin restricted feeder fibroblasts has been described for alveolar organoid culture [40, 41]. Moreover, generating alveolar organoids without the presence of a feeder cell population has also recently become possible [42, 43]. Research using organoids has progressed exponentially during the last decade, and results obtained have been reviewed in detail elsewhere [44, 45]. Relatively simple to obtain, human airway organoids derived from epithelial cells and adult stem cells have been successfully used to study important processes such as respiratory virus infections including respiratory syncytial virus (RSV) [46] and influenza viruses [47] respectively. Donor-specific organoids can be generated from progenitor cells purified from human lung tissue to model the healthy state and various respiratory diseases (see Table 1). In contrast, PSC-derived lung organoids rely on a complex manipulation of developmental signalling pathways, to form a lung bud
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