Conclusions
In recent years, remarkable progress has been made in the development of in vitro culture system for epithelial stem cells. The realization of the long-term culture of epithelial stem cells allows us not only to reproduce physiological events in vitro but also enables the development of therapeutic platforms based on cell transplantation. An increasing number of studies of epithelial stem cells clearly indicated that understanding the basic biology of these cells will be closely linked with clinical studies of human disease pathology, such as cancer and inflammation. The interactions of biological networks during tissue development and disease progression are complex at the cellular and molecular levels. Building an in vitro epithelial structure model can simplify this complexity and provide comprehensive views of epithelial physiology and pathophysiology. Moreover, in vitro epithelial models can easily be combined with genomic and epigenetic approaches and single-cell analyses. In addition, genome editing, e.g., the CRISPR-Cas9 system, can also be readily incorporated into the model. One drawback of in vitro epithelial structure models derived from the stem cells is that epithelial structures lack stromal populations. Although a simplified system provides direct insight into epithelial physiology in most cases, the interaction between different cell types is important for reproducing a genuine phenotype because all tissues consist of multiple cell types, such as epithelial cells, endothelial cells, mesothelial cells, fibroblasts, and hematopoietic cells. One possible solution to this problem is a self-organizing method, in which several cell types are mixed in vitro and spontaneously form actual organ-like structures. Although improvements are required to recapitulate the in vivo behavior of human organs, the ability to expand epithelial stem cells and generate a 3D structure model holds great promise for both basic and clinical research.