Progenitor and stem cells are involved in respiratory tissue regeneration and homeostasis in the respiratory system in cases of injury or disease. Various cell types can be affected by majority of lung diseases, and therefore different progenitor cell types would be involved in restoration and repopulation of the trachea and lung. Apart from epithelial cells, there are also interstitial fibroblasts, vascular, lymphatic and neuronal cells that might be damaged by respiratory disease s, with ECM also being affected. There are endogenous and exogenous sources of stem cells for the lung [26]. Among endogenous epithelial stem cells there are basal epithelial cells, variant club cells, bronchoalveolar stem cells, and AEC-II, that are hypothesized to be precursors of most lung epithelial cells [27]. Recent studies indicate that endogenous progenitors also include side population (SP) cells with mesenchymal (chondrocyte, osteocyte, and adipocyte), endothelial, and potentially epithelial differentiation capacity [28–30]. Isolation of endogenous cells from donors for therapeutic and bioengineering purposes is rather complicated since they are not easily accessible; therefore leading to the consideration of exogenous stem-cell sources as an alternative.

Prospective exogenous stem-cell treatments have to target regeneration of ECM and the multiple lung cell types (including epithelial, mesenchymal, and endothelial) that can be affected in respiratory disease s. There are several donor tissues e.g., bone marrow (BM), peripheral blood (PB), amniotic fluid (AF), or umbilical cord blood (UCB) that could serve as sources of exogenous stem and progenitor cells , including endothelial and mesenchymal progenitor cells [26]. Bone marrow, PB, AF, and UCB were reported to contain progenitor cells for blood, endothelium, and mesenchyme, which can contribute differently to the respiratory tract regeneration (Table 17.1) [31–36]. AF-derived stem cells were demonstrated to possess epithelial differentiation potential [37–39]. Cells from all these sources exhibit reparative properties in various lung disease models [37, 38, 40–44]. They can be involved in the regeneration process directly (proliferating in vivo, differentiating towards the targeted cell type at the injury site, or modulating ECM, see Table 17.1) or indirectly (contributing to lung homeostasis via epithelial–mesenchymal crosstalk) [45]. This involvement suggests that donor-derived stem cells can be a suitable tool for understanding the mechanism of injury repair [46–48], and offer possibilities in therapeutic application.

Synthetic scaffolds can be made from biodegradable or non-degradable materials, and engineering procedures range from moulding to electrospinning, 3-D printing, and others. Because synthetic scaffolds are not derived from donor tissue, they do not pose a contamination risk during development , and can be rapidly produced and reproduced using relatively economical engineering processes. On the other hand, biological scaffolds hold significant bioactive information and cues that are important for the correct engraftment, proliferation, and differentiation of seeded cells. Because these cues are entirely lacking in synthetic scaffolds, this complicates their use in vivo or in clinical translation. New strategies and concepts are required to overcome these hurdles. The design of synthetic matrices using nanotechnology approaches may open a new direction in this so-far dead-end street of synthetic scaffolds.

Disorders affecting more than 50 % of the entire length of the adult trachea (or 1/3 in children) are inoperable due to the lack of reconstructive tissue. During the last decade, various surgical techniques and experimental methods have been investigated [49]. All of them have eventually failed due to time consuming protocols, heavy immunosuppression, or extremely difficult surgical procedures. Tissue engineering may provide a potential solution to this challenging situation. Based on experimental in vitro and in vivo animal data, the first-in-man transplantations of tissue-engineered tracheae have recently been performed, using both decellularized and synthetic scaffolds [50, 51]. The early and intermediate success, recently published in our 5-year-follow-up demonstrates the feasibility of this methodology [52]. Long-term data and clinical trials will provide significant data that may prove the real clinical meaning of this novel, currently rather conceptual strategy for tracheal replacement and reconstruction. Currently, a number of clinical as well as experimental research groups are investigating the use of both biological decellularized and synthetic tracheal scaffolds [50, 51, 53, 54]. A report from the recent expert group meeting [55] came to the conclusion that at present there is no consensus with regard to treatment methods for patients that suffer from complex tracheal disorders (Table 17.2). Even though the trachea appears to be a simple tube, its reconstruction or replacement remains challenging. Thus, research regarding novel methods of tracheal tissue regeneration , cell–surface interactions, and in vivo transfers are necessary to advance clinical frontiers in this challenging field.

Bioengineering of the lungs is complex, since this organ contains an intricate structure and a plethora of functions. There are over 60 different pulmonary cell types with multifaceted functions, an intricate alveolus-capillary interface to maintain gas exchange, and a fine network of microvasculature that must resist thrombosis and preserve elasticity for lung compliance to be adaptable for pressure–volume changes [56, 57]. One of the major challenges for bioengineering a three-dimensional lung structure is preserving or re-creating a delicate ECM with elasticity required for alveolar expansion, and thinness for efficient gas exchange.

The ECM microenvironment has a prominent role in identifying putative lung progenitor cells and driving cellular fate to maintain tissue integrity [58–60]. In turn, the composition of ECM is also highly dependent on, and can be manipulated by, the surrounding cell types. The non-alveolar lung tissue consists of approximately 50 % ECM proteins [58]. The predominant ECM proteins, collagens, and elastin fibers provide structural and mechanical strength while laminin, fibronectin, and glycosaminoglycans (GAGs) are essential for cell attachment, proliferation, and differentiation [61–63]. However, the composition of ECM is highly dependent on the applied decellularization method [64–66]. Aside from preserving ECM composition, an ideal decellularization protocol should also preserve the gross native architecture of the lung (Fig. 17.2) that allows the scaffold to be physiologically ventilated and perfused [65]. Early experimental in vivo data do suggest that even complex structures such as the lung can be engineered. However, these experiments were performed in immunodeficient rodent models that cannot be immediately translated into a clinical setting because they do not address the most relevant clinical questions and concerns about graft rejection and the need for immunosuppression. Nonetheless, this type of experiment using decellularized lung tissues can be used to investigate and further understand various pulmonary cell–surface interactions, physiological changes, cell fate, and organ development . Besides, TE technologies can be applied to investigate lung disease models, such as elastase or bleomycin-induced lung injuries and the underlying pathohistological changes in the ECM [59, 67].

In vitro assays using decellularized lung scaffolds [68, 69], 2-D monolayer culture systems [70], or 3-D matrices such as matrigel [71], or porcine skin gelatin (Gelfoam) [72] have been used to assess the reparative and regenerative capacity of candidate endogenous lung stem cells and exogenous stem cells. Huh and colleagues [73] have developed a lung-on-a-chip, to create a combination of different cell types and a mechanical force interface to simulate living organs. Biomimetic microsystems of this type are useful to determine if implanted cells can release therapeutic and immunomodulatory substances at an adaptive dynamic rate, as judged by the cellular feedback mechanism. This can also be an accurate alternative to “replace, reduce, and refine” animal studies for predicting lung responses [73].

Numerous research groups have successfully utilized three-dimensional models (trachea and en bloc lung ) to characterize biocompatibility and biomechanics after tissue decellularization and recellularization. These models can potentially be used as tools for researching disease models, cell therapy , cell differentiation and fate, and drug application. The choice of stem cells for recellularizing tissues or organs has been a major focus of investigation in TE [74]. Cell types such as embryonic [75, 76] and induced pluripotent stem cells [68], amniotic fluid stem cells, mesenchymal stromal cells [77] or peripheral blood and bone-marrow-derived mononuclear cells are currently under investigation for their potential uses in different models using mouse, rat, porcine, non-human primate and human cadaveric donors. Furthermore, these ex vivo systems enable one to create simple tools to examine microenvironmental variables such as scaffold stiffness, mechanical ventilation, growth factors , pH, etc. and influence cell adhesion, proliferation, differentiation, or transdifferentiation [60, 78–80] (Fig. 17.3). These systems will also help to identify those factors that are important for providing the complex structural and biophysical cues required for tissue remodeling and regeneration .

In some circumstances, patients who opt for potential therapeutic options utilising cell therapies or tissue-engineered scaffolds (e.g., tissue-engineered tracheal replacements) could suffer from recurring tumours. According to the cancer stem-cell paradigm it is believed that dysfunctional resident stem-like cells undergo oncogenic transformation giving rise to tumor-initiating cells which generate benign or malignant tumors [81]. Activated cancer-associated fibroblasts are also an integral component of the tumor microenvironment , influencing tumor growth and progression (e.g., by secreting growth factors , altering tissue metabolism and remodeling ECM) [82, 83]. Experimental models for the analysis of the interaction of airway tumor initiating cells with biomatrix scaffolds are essential in order to devise optimal therapeutic protocols employing tissue engineering technologies in lung cancer patients. Although various mouse models and assays have been used to identify adult lung stem and progenitor cells , and analyze their behavior in the normal and diseased lung [45, 84–86], the identity and properties of human airway stem/progenitor cells and tumor initiating cells are less clear.