Existing InVitro Pulmonary Cell and Tissue Culture Biological Models

The heterocellular nature and complex architecture of the alveoli presents a challenging problem for the development of physiologically relevant 3D in-vitro models. Traditionally, in-vitro models of alveolar tissue have consisted of 2D cultures of alveolar epithelial cells, either homotypic AE2 cells or heterotypic AE1/ AE2 cultures [142-144]. Early experiments utilized either pulmonary epithelial cell lines or primary cultures of isolated alveolar cells. Alveolar epithelial cell lines, such as A549 have been derived from a lung adenocarcinoma [145], or generated by immortalization [146]. Although A549 cells bear morphological similarities to AE2 cells, including the presence of lamellar bodies and microvilli, they lack several key characteristics of the differentiated in-vivo phenotype that is retained in cultures of primary isolates from normal tissues. While monotypic cultures of A549 cells can be used as a model for some of the type II cell functions, establishing appropriate heterocellular tissue models will require the use of primary or stem cell-derived AE2 cells [147-149].

The culture of primary AE2 cells in vitro often results in transdifferentiation of AE2 cells into AE1-like cells, as evidenced by morphological changes that include a loss of microvilli, cell spreading resulting in the formation of thin cytoplasmic attenuations [150], and reactivity to AE1 cell-specific membrane components [151].

Following the earliest reports of in-vitro 3D organotypic models of lung alveolar tissue [140, 152], only a few reports of such 3D organotypic models have since emerged, presumably due to the complexity of the system. For example, Sugihara et al. [153] reported the generation of differentiated 3D alveolus-like structures in vitro utilizing a type I collagen gel and growth factor- and hormone-defined medium. This report was promising, but limited in that only purified AE2 cells were used in the absence of the microvascular and connective tissue components. Successful engineering of proximal airway tissues in vitro have demonstrated the importance ofsustained cell-cell interactions between epithelial and mesenchymal tissue components [128-130]. Only recently have these principles been applied toward generating distal lung tissue constructs in vitro [133, 135].

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