The rapid developments in organoid technology, and the wide usage of the term organoid for a variety of both in vitro and in vivo structures, led Lancaster and Knoblich to suggest a basic definition for organoids. They defined organoid as:
According to Lancaster and Knoblich, an organoid should possess several important features characteristic to the respective organs: “ it must contain more than one cell type of the organ it models;  it should exhibit some function specific to that organ;  the cells should be organized similarly to the organ itself”
In 2009, Hans Clevers and Toshiro Sato used adult stem cells from mouse intestine to create the first mini-gut organoids from murine cells  and later extended their method to human epithelial organoids . These organoids were expected to allow researchers to gain new insights into the biology of gut health and disease, including colorectal cancer.
This method inspired many other scientists to create a variety of organoids from mouse and human tissues. These clumps of cells are small enough to survive without blood supply, yet large and complex enough to teach us something about tissue and whole-organ development and physiology.
A typical organoid protocol starts with isolated embryonic or pluripotent stem cells, which are then cultured in a supporting scaffold (such as Matrigel) that enables three-dimensional growth. Organoids are comprised of multiple differentiated cell types that are found in the relevant organ in vivo. For example, all cell types of the intestinal epithelium are represented in the Matrigel-based model described by Sato et al. . The signaling pathways governing organoid formation were found to be identical to those used during in vivo organ development and homeostasis; thus, cytokines, growth factors and small molecules were also included in the culture medium in order to activate or inhibit specific signaling pathways. Even tissues that are closely related, such as the small intestine and colon, require different combinations of signaling molecules in the process of organoid formation .
There are different ways to obtain an organoid culture, and some of these are just beginning to be explored. On the following pages, you will find some examples of various organoid models that have been developed, with an emphasis on the cytokines, growth factors and small molecules that were used.
For more details, please download PeproTech booklet “Organs in Dishes” and the poster “Organoids Model” below.
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2. Lancaster MA, and Knoblich JA, Organogenesis in a dish: modelling development and disease using organoid technologies.
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3. Sato T, Vries RG, Snippert HJ et al. Single Lgr5 stem cells build crypt-villus structures in vitro without a mesenchymal niche.
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4. Sato T, Stange DE, Ferrante M et al. Long-term expansion of epithelial organoids from human colon, adenoma, adenocarcinoma, and Barrett’s epithelium. Gastroenterology 2011; 141: 1762–1772.