Organoids are simple tissue-engineered cell-based in vitro models that recapitulate many aspects of the complex structure and function of the corresponding in vivo tissue. They can be dissected and interrogated for fundamental mechanistic studies on development, regeneration and repair in human tissues, and can also be used in diagnostics, disease modelling, drug discovery and personalized medicine. Organoids are derived from either pluripotent or tissue-resident stem (embryonic or adult) or progenitor or differentiated cells from healthy or diseased tissues, such as tumours. To date, numerous organoid engineering strategies that support organoid culture and growth, proliferation, differentiation and maturation have been reported. This Primer highlights the rationale underlying the selection and development of these materials and methods to control the cellular/tissue niche; and therefore, the structure and function of the engineered organoid. We also discuss key considerations for generating robust organoids, such as those related to cell isolation and seeding, matrix and soluble factor selection, physical cues and integration. The general standards for data quality, reproducibility and deposition within the organoid community are also outlined. Lastly, we conclude by elaborating on the limitations of organoids in different applications, and the key priorities in organoid engineering for the coming years.
Experimentation
Cell sourceUnder defined physicochemical conditions, tissues such as small intestine 7 , colon 29,30 , stomach 31,32 , oesophagus 29 , tongue 33 , liver [34][35][36][37] , lung 14 , pancreas [38][39][40] , heart 41 , ear 42 and skin 43 have been obtained from iPSCs, adult or fetal cells and either stem/progenitor cells or differentiated cells. The starting cellular population for any given organoid is of prime importance, affecting not only the variability and heterogeneity in the structures obtained but also the function of the tissue they aim to model. To establish tissue-derived organoids or cancer organoids, tissue-resident stem/progenitor/differentiated cells or tumour cells, respectively, are obtained through an optimized tissue dissociation method. For iPSC-derived organoids, iPSC lines are established and fully characterized as the starting cells. Patient/tissue-derived stem cells are obtained through an optimized tissue dissociation method and then embedded into a 3D matrix mimicking stem cell niches. iPSCs can be maintained and expanded as undifferentiated clonal populations on feeder cells. To exemplify the generation of tissue-derived organoids we use intestinal organoids as an example (Fig. 2a), as this was the first tissue-derived organoid type established 7 . The small intestine and colon are opened longitudinally, washed and then cut into 2-4 mm fragments to increase the surface area for enzymatic digestion or further mechanical dissociation. EDTA treatment is used to chelate calcium, disrupting cell-cell adhesion and tissue integrity 44 . Larger tissue fragments and whole cells ...