In recent years, organoid culture technology is developing rapidly. Organoids has been awarded 'Method of the Year 2017' by Nature Methods, for their immense potential as tools to study human biology in health and disease.

Previously, the term ‘organoid’ has been used to encompass all 3D organotypic cultures derived from primary tissues, pluripotent stem cells (PSCs), including embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs), established cell lines, as well as whole or segmented organs such as organ explants consisting of multiple tissue types. The ‘organoid’ was defined by Fatehullah et al. As an in vitro 3D cellular cluster derived exclusively from primary tissue, ESCs or iPSCs, capable of self-renewal and self-organization, and exhibiting similar organ function as the tissue of origin^[1-3].

Organoid technology is built upon the foundation of stem cell technologies, classical developmental biology and cell-mixing experiments. In the early 20th century, Wilson (1907) demonstrated that dissociated sponge cells can self-organize to regenerate a whole organism. Stem cell research began to thrive when murine ESCs (mESCs) were first isolated and established in 1981 ^[4-6]. In 2009, Hans Clevers lab established a long-term primary culture to generate the intestinal organoid culture system. It was an outstanding technological leap for the stem cell field^[5][7].

Lately, organoids have been successfully generated for an increasing variety of organs, including but not limited to gut, stomach, lung, kidney, liver, pancreas, mammary glands, prostate, thyroid, retina, inner ear, taste bud and brain ^[8].

Organoid model is a major technological breakthrough that acts as a valuable model for the study of tissue development, disease modeling, drug screening, personalized medicine and cell therapy^[1].

Recently, organoids have been widely used in many areas, including developmental biology, disease modeling, precision medicine, regenerative medicine, toxicology, drug discovery studies, host-microbiome interactions, gene editing, multi-omics, and phylogenetic studies^[5].

Organoids represent an important bridge between 2D cultures and in vivo mouse/human models. They are more physiologically relevant than monolayer culture models and are far more amenable to manipulation of niche components, signaling pathways and genome editing than in vivo models ^[1][10]. Some of the advantages of the organoid models are;

1) Compared to traditional two-dimensional (2D) cell culture, organoids are similar to primary tissue in both their composition and architecture, harboring small populations of genomically stable, self-renewing stem cells that give rise to fully differentiated progeny comprising all major cell lineages at frequencies similar to those in living tissues^[11].

2) Organoids can be expanded enormously, cryopreserved as biobanks, and easily manipulated using techniques similar to those established for 2D monolayer culture^[11].

3) Primary-tissue-derived organoids lack mesenchyme/stroma that provides a separate system for studying a specific tissue of interest without being influenced by the local microenvironment^[1].

Compared with the traditional patient-derived cancer cell line (PDC) and patient-derived xenograft (PDX) model, the PDO model has unparalleled advantages. In the screening of drugs for tumor therapy, tumor organoid models derived from patient tumors have higher sensitivity, heterogeneity, and stability and can restore the genuine attributes of tumors more effectively. In addition, tumor organoids can be preserved, resuscitated, passed infinitely, and mechanically cultured on a chip for drug screening^[13]. Therefore, organoid technology exerts enormous potential in evaluation of efficacy and toxicity of drugs, regenerative medicine, and precision medicine. Organoids have been established successfully for multiple cancer types, including but not limited to stomach cancer, colorectal cancer, liver cancer, pancreatic cancer^[14].

Many pathogenic viruses that infect humans display species specificity and animal models can’t be used to study those viral infections. Hence, studying viral biology and identifying potential treatments benefits by developing in vitro cell systems (organoids) that closely mimic human physiology. In the current COVID-19 pandemic, organoids have emerged as powerful tools for SARS-CoV-2 research, bridging the gap between cell lines and in vivo animal models^[16-17].

Organoids can be generated from tissue-resident adult stem cells (ASCs) or from PSCs. Under appropriate conditions, supplementation of proper culture medium, growth factors and small molecules, stem cells embedded in Matrigel can undergo continuous self-renewal and differentiation, and self-organize into 3D-structures. The methods of culturing different organoids are similar^[17].

1) Multiple sources:

1.1). ASCs-derived organoids: Primary tissue that is dissociated into functional sub-tissue units containing stem cells. These functional units are further digested into single cells and FACS-sorted to enrich for stem cells.

1.2). ESCs/iPSCs-derived organoids: stem cells undergo directed differentiation towards the desired germ lineage, eventually generating floating spheroids that are subsequently embedded in extracellular matrix (ECM) to initiate organoid culture^[1].

2) Manipulability of niche components:

Organoids are typically cultured in an ECM surrounded by culture media supplemented with specific niche factors (different from air-liquid interface (ALI) method which is introduced recently)^[18]. Organoids can either differentiate spontaneously or be induced to differentiate towards desired lineages or cell types by adding suitable differentiation factors and/or withdrawing factors that promote stemness. Common niche and ECM factors include R-spondin, EGF, Noggin, Activin A, and Collagen. Specific small molecules are added such as TGF-β inhibitor A-83-01, GSK3β inhibitor CHIR99021, and ROCK inhibitor Y27632^[1].

Stem cells are maintained and perpetuated in organoids, continually giving rise to differentiated progeny. In addition, organoids can be dissociated and plated onto membrane supports coated with Matrigel or Collagen to form 2D monolayer organoid models^[17].

MedChemExpress offers a variety of high-quality recombinant proteins and small molecules for organoid culture.