# Application of Conditioned Medium for In Vitro Modeling and Repair of Respiratory Tissue
###### Abstract  
Background: The idea of exploring respiratory therapy in vitro predominantly guided by cell-secreted substances has gained ground in recent years. A conditioned medium (CM) consists of protein milieu that contains a diverse spectrum of cytokines, chemokines, angiogenic agents, and growth factors. This review evaluated the efficacy of using CM collected in an in vitro respiratory epithelial model. Methods: Twenty-six papers were included in this review: twenty-one cellular response studies on respiratory secretone application and five studies involving animal research. Results: The CM produced by differentiated cells from respiratory and non-respiratory systems, such as mesenchymal stem cells (MSC), exhibited the similar overall effect of improving proliferation and regeneration. Not only could differentiated cells from respiratory tissues increase proliferation, migration, and attachment, but the CM was also able to protect the respiratory epithelium against cytotoxicity. Most non-respiratory tissue CM was used as a treatment model to determine the effects of the therapy, while only one study used particle-based CM and reported decreased epithelial cell tight junctions, which harmed the epithelial barrier. Conclusion: As it resolves the challenges related to cell development and wound healing while simultaneously generally reducing the danger of immunological compatibility and tumorigenicity, CM might be a potential regenerative therapy in numerous respiratory illnesses. However, additional research is required to justify using CM in respiratory epithelium clinical practice. Keywords: airway epithelium; airway epithelial regeneration; conditioned medium; secretone; tissue engineering; regenerative medicine; preclinical study; mesenchymal stem cell +
Footnote †: journal: Medical  
that are in immediate contact with the external environment and protect the underlying tissue. In humans, different cell types, including mucus-producing goblet cells, ciliated cells, and non-ciliated cells, are arranged into a protective pseudostratified epithelium, depending on the airway segment [10; 11]. Lung lining fluid, which consists of mucus in the upper airways and surfactant in the alveoli, protects the epithelia that line the respiratory tract through mechanical, antibacterial, and antioxidant activities [12; 13]. As the airways are continually exposed to foreign agents and infections, airborne toxicants can potentially translocate through an intact or compromised epithelial barrier, generating possible systemic effects [12; 14; 15]. The epithelial damage can even result in the loss of surface epithelial integrity and partial or total epithelium shedding [16; 17]. Consequently, developing and validating in vitro models to assess the biological responses to disruptions of this first-line upper airway defense is critical [18; 19; 20]. Tissue engineering models strive to mimic the natural components and interactions of tissues by utilizing biological and biochemical advances to achieve the closest feasible physiological similarity [21]. Understanding cellular activity at the molecular level is therefore critical to preserving the cell quality and regenerative potential to produce better-quality epithelium for in vitro uses. Tissue alternatives, such as 3D cell cultures and organoids, are being developed for a range of fundamental and practical reasons, including therapeutic applications and in vitro testing, and the development of human cell- and tissue-based in vitro screening methods has accelerated [22; 23; 24]. However, the use of cells is accompanied by disadvantages that include dedifferentiation after implantation, the need for in vitro expansion to obtain more cells for in vivo regeneration, and donor site morbidity due to cell isolation, particularly in patients debilitated by age and disease. Furthermore, cell proliferation and regeneration capacity decrease as people age and illness severity increases [25; 26; 27; 28].