The lungs are comprised of a complex branching system, lined with highly specialized cells. These cells facilitate several crucial functions, such as oxygen exchange, but also immune functions to protect this vulnerable part of the body from infection.
Recent advances have discovered that it is possible to grow lung-derived epithelial cells in a 3D culture, without the structural supporting cells they usually grow in within the lungs. In this setting, the cells form self-forming structures referred to as “organoids”.
Cell cultures have largely been grown in a monolayer which lacks more complex in vivo interactions between cells and cell types. Growing cells in 3D organoids offers a more complex and realistic insight because organoids fulfill three important aspects of in vivo organs: more than one cell type can be grown together, these cell types can structure themselves approximately like they do in the native organ and can, thanks to this, retain some innate functioning.
The production of organoids relies on the same processes that take place during organ production in normal development. These processes include specific adhesion properties which give rise to self-organization of cells, and differentiation of progenitor cells which is limited to certain areas. This can be done using embryonic tissue progenitor cells, which will organize into structures characteristic of early organogenesis. Mature cells from adult organ tissue can also be grown into organoids, but to a lesser extent.
The respiratory system originates as buds on the endoderm, which become airways and alveoli through a series of branching processes. Various cell specifications take place during organogenesis and the formation of an organoid.. Human lung organoids have been developed from pluripotent stem cells, which can be transplanted into mice to allow growth to continue into the airways and alveoli of the lungs. After six months of culturing, the lungs were equivalent to human lungs in the second trimester of gestation. This could also be achieved when the lung bud organoids remained in culture.
Research and Applications
Organoids provide a realistic, animal testing-free method to study tissue and organ development. The lung organoids can also be derived from different tissues, such as stem cells and basal cells. In lungs, basal cells make up part of the epithelial cells lining the airway and are crucial for maintaining the function of the barrier. Normally, it is the basal cells that change behavior and proliferation in response to cell damage to restore the epithelial layer. However, when damage is induced to the basal cells themselves, certain secretory cells undergo changes which result in them becoming functional basal cells, thereby working as stem cells. Lung organoids can be used here to elucidate more about the regeneration mechanisms of the epithelial barrier and to screen for drugs regulating plasticity and lineage.
Diseases have also been modeled using lung organoids. Respiratory syncytial virus (RSV) affects newborns, and currently has no licensed effective treatment. Previously there was no model which could reproduce the RSV infection. However, subsequent research which saw lung organoids being infected with RSV, found the epithelial cells of the organoids shed into the lumen of the branching structures. This pathology has previously been seen when studying infection of RSV and is in concurrence with the causes of the virus’ symptoms. This opens up opportunities for lung organoids to be used as viable models to study pulmonary diseases.
In addition to strictly viral infections, the potential to use lung organoids in genetic conditions has also been investigated. Hermansky-Pudlak Syndrome (HPS) is, in some forms, associated with pulmonary fibrosis. Using CRISPR, HSP associated pulmonary fibrosis was partially induced in lung organoids. The resulting effect was less sharp branching structures and an accumulation of mesenchymal cells. This research also found evidence to support the theory that certain forms of pulmonary fibrosis are caused by injury to epithelial tissue.
Therefore, it seems possible that lung organoids are so functionally similar to lung organs that they can be used to model some forms of pulmonary fibrosis. Several challenges still face lung organoids and their use in research. As mentioned, a lung organoid matching the developmental stage of the second trimester was grown in six months. This indicates that organoid maturation matches the pace of normal development. Therefore, reaching full maturation stage remains a challenge. Another current limitation is that the branching process that takes place appears to be random. While this in congruence with a theory stating the branching process will take place in a space-filling manner, this theory is not proven and the randomness can present potential difficulties.
Thirdly, the exact mechanism of the patterning and general nature of the organoid mesenchymal cells is unknown. However, lung organoids still hold much potential as models for lung functioning, disease, and drug screening purposes.