Organoids: Modeling Development and the Stem Cell Niche in a Dish Kai Kretzschmar, Hans Clevers  Developmental Cell  Volume 38, Issue 6, Pages 590-600 (September 2016) DOI: 10.1016/j.devcel.2016.08.014 Copyright © 2016 Elsevier Inc. Terms and Conditions

Figure 1 Organoid Cultures Can Be Generated from Pluripotent Stem Cells, Minced Tissue Pieces, or Adult Stem Cells When organoids are initiated from pluripotent stem cells (PSCs) such as embryonic stem cells (ECSs) or induced PSCs, for instance, reprogrammed skin fibroblasts, cultures are differentiated toward the germ layer from which desired organoids are derived. In the case of intestinal organoids, PSCs are treated with activin A to activate transforming growth factor β signaling to trigger endodermal differentiation. Developing cell aggregates (spheroids) are then embedded into Matrigel and cultured in the presence of niche factors, for example, epidermal growth factor, Noggin, and R-spondin-1, to generate organoid cultures containing both epithelial and mesenchymal cells. Organoids derived by this approach often recapitulate organogenesis, differentiating from premature tissues into near-native organotypic cultures resembling their fetal counterparts. Adult tissue can be sourced to generate organoid cultures from either minced tissue pieces or specific compartments such as intestinal crypts, or purified single cells such as Lgr5+ stem cells. Minced tissue pieces are usually first transferred to derive similar spheroid cultures as described above before proceeding with Matrigel-based epithelial and mesenchymal organoid cultures, while tissue compartments or single cells are directly embedded into Matrigel to generate epithelial organoid cultures without a cellular mesenchymal niche in the presence of niche factors. Organoids generated by this approach most closely resemble the adult organ of origin containing fully differentiated cell types. Developmental Cell 2016 38, 590-600DOI: (10.1016/j.devcel.2016.08.014) Copyright © 2016 Elsevier Inc. Terms and Conditions

Figure 2 Organoid Cultures Have Been Established for Different Organs and Tissues (A) Organoid cultures have been generated from the several adult epithelial tissues such as esophagus, fallopian tube, small and large intestines, liver, lung, pancreas, prostate, salivary gland, stomach, and taste buds. Epithelial organoid cultures have also been established from fetal intestine. Minced tissue pieces of small intestine and colon of neonatal, juvenile, and adult mice containing both epithelial and mesenchymal cells were also used to generate organoids. (B) PSC-derived organoid cultures have been established for different organs such as inner ear, intestine, kidney, liver, lung, pituitary gland, and stomach. The most remarkable examples are organoids mimicking brain structures and the eye retina. Developmental Cell 2016 38, 590-600DOI: (10.1016/j.devcel.2016.08.014) Copyright © 2016 Elsevier Inc. Terms and Conditions

Figure 3 Organoid Cultures Are Used to Model Different Aspects of Development and Stem Cell Biology (A) Organoid cultures are exploited to model organogenesis and tissue morphogenesis. For example, intestinal cultures are used to study crypt-villus compartmentalization when cystic spheroids start budding and build the stem cell-containing intestinal crypt. (B) Stem cell lineage selection and differentiation as well as tissue plasticity is heavily studied using organoid cultures, for instance by modulating key cellular signaling pathways (i.e., using growth factors) that specify for mature cell types or maintain stem cells in an undifferentiated state. (C) A reductionist approach can be utilized to model the stem cell niche using organoid cultures. Epithelial organoids are particularly suitable model systems for co-culture experiments with components of the tissue microenvironment such as fibroblasts, immune cells, endothelial cells, bacteria, or viruses to study cell-cell interactions and bidirectional signaling of stem cells and their niche. Developmental Cell 2016 38, 590-600DOI: (10.1016/j.devcel.2016.08.014) Copyright © 2016 Elsevier Inc. Terms and Conditions