Model systems drive biological research by recapitulating body processes and functions from the molecular to whole organism level. The human body is composed of both cellular and non-cellular material organized in a highly specialized manner. It is difficult to mimic all aspects of human biology with one in vitro model system. 3D cell culture models are a more accurate representation of the natural environment experienced by cells in the living organism as opposed to growing cells on 2D flat surfaces.
Organoids are in-vitro derived 3D cell aggregates derived from primary tissue or stem cells that are capable of self-renewal, self-organization and exhibit organ functionality.3 Organoids address the limitations of existing model systems by providing:
Figure 1.Mouse Intestinal Epithelial Organoids. 3D Organoids were generated from adult mouse intestinal tissue following the protocol outlined by Clevers et al. Science. 2013. Organoid cells begin to form lumens and bud structures at around day 3-5 in culture and form complex crypt-like structures around day 7-10. These crypt-like domains are functionally similar to those of the adult intestine, where dividing LGR5+ intestinal stem cells are intercalated with Paneth cells located at the crypt base.
Organoids and spheroids are both cells cultured in 3 dimensions. Spheroids are often formed from cancer cell lines or tumor biopsies as freely floating cell aggregates in ultra-low attachment plates whereas organoids are derived from tissue stem cells embedded within an ECM hydrogel matrix such as Matrigel. Organoids are highly complex and are more in vivo-like when compared to spheroids. Recently, tumor organoids have shown to predict how well patients respond to cancer drugs to aid in personalized medicine.
Figure 2.Organoids vs. Spheroids. Stem cell derived organoids have more in vivo-like phenotypes with higher order tissue complexity compared to tumor spheroids.
Organoids are generated either from primary tissues or pluripotent stem cells (induced pluripotent stem cells (iPSC) or embryonic stem cells (ESCs)) by providing appropriate physical and biochemical cues4.
Physical cues: Provide support for cell attachment and survival. Examples include collagen, fibronectin, entactin and laminin.
Biochemical cues: Modulate signaling pathways, thereby influencing proliferation, differentiation and self-renewal. Examples include EGF, FGF10, HGF, R-spondin, WNT3A, Retinoic acid, GSK3β inhibitors, TGF-β inhibitors, HDAC inhibitors, ROCK inhibitors, Noggin, Activin A, p38 inhibitors and Gastrin.
Organoids are physiologically relevant and amenable to molecular and cell biological analyses, holding great promise in both basic research and translational applications.
Developmental biology: Organoids derived from ESC, iPSCs retain features of their developmental stage and help in studying the process of embryonic development, lineage specification and tissue homeostasis. It also shed light on development of stem cells and their niche.
Disease pathology of infectious disease: Organoids represents all components of organ and are suited to study infectious diseases affecting specialized human cell types.
Regenerative medicine: Transplantation of organoids derived from the adult stem cells aid in replacing the damaged organ or tissue. In addition, feasibility for gene correction using CRISPR/Cas9 technology can be used in treating monogenic hereditary diseases.
Drug toxicity and efficacy testing: The possibility to test efficacy and toxicity of drugs against representative targets/organs (gut, liver and kidney) could potentially limit the ethical issues associated with animal usage.
Personalized medicine: Organoids derived from adult stem cell of individual patients allows ex-vivo testing of drug response.