Organoid is a buzz word today. Everyone is talking about organoids in the mass media and biomedical literature. But what are organoids? How can we define it? In simple terms organoid is miniature organ in vitro, but scientifically, definition could be more complex. I compiled a list of common properties, which can be used in defining of organoids.
- Organotypic culture. Organoid is cell/ tissue culture method, which is aimed to mimic organ structure and function. The other types of organotypic culture (different from organoids) may include organ slice, whole organ wit explant, primary cells (multiple cell populations mixed in suspension).
- 3D. Organoid is (at least) three-dimensional culture technique by definition. It could be 4D also, but not 2D.
- Extracellular matrix (ECM). Pretty much obligatory component of 3D culture is ECM. So, organoid culture includes ECM. The most frequent technique is embedding organoid into Matrigel.
- Complex multicellular constructs. Since organ includes multiple cell types, organoids are multicellular by definition.
- Recapitulation of developmental programs. If organoid generated from stem cells, it is usually recapitulating developmental organogenesis program. Since, it takes place in culture conditions, such developmental recapitulation could not always be the same (or complete) as it occurs in vivo.
- Self-organization. Organoid culture leverages ability of cells to self-assemble or/and self-organize into tissues.
Shamir and Ewald clearly define 2 types of organoids: tissue organoids and stem cell organoids. Tissue organoid implies stromal cell-free (or mesenchyme-free) culture. It mostly applies to epithelial tissues. Cell intrinsic ability to self-organize is not limited to stem/ progenitor cells withing tissues. Typical examples of tissue organoids culture – mammary and salivary glands, intestine and prostate. Stem cell organoids are generated from embryonic or adult stem cell and iPS cells.