The need for microphysiological systems
Overview of the Need for MPS in Research
Advances in drug discovery are limited by in vitro models such as immortalized cell lines and 2D cell cultures. These and other models including PDx are characterized by modest translational value that arises from a lack of 3D extracellular matrix and the interspecies differences caused by cell-heterogeneity.
The search for an improved biological screening system has led to efforts to create microphysiological systems (MPS) which are more representative of human physiology and pathology. Additionally, these systems promise greater standardization, lower costs and reduced vivisection. Such models include spheroids, organoids and organ-on-a-chip. The former comes with microfluidic fluid exchange while the latter has traditional static media in multi-well plates.
Organoids are more sophisticated than spheroids but generally have a roughly spherical shape. The latter are characterized by the presence of stem cells, or pluripotent cells, which differentiate and propagate, growing into a mass of cells with an organization that resembles organs in certain aspects. Epithelial organoids, such as those that model the liver, lung or pancreas, are characterized by mesenchyme core (inner mass) and their early development stages resemble a blastocyst.
Benefits of Using MPS
MPS enables organoid growth to address critical needs in research. These systems not only facilitate the growth of organoids but also improve culturing conditions due to perfusion. Perfusion results in better organoid growth since nutrients are delivered more efficiently and there is a more consistent shear flow of material. Microfluidic cultures also show more uniform growth of organoids. This is an important feature since variations in the differentiation of organoids grown in batch cannot be controlled.
MPS also facilitate and improve optical imaging by distributing organoids in a more uniform fashion compared to gel matrix models in multi-well plates. Gel matrix models can result in high-content confocal imaging acquisition of z-stack with mostly empty space, slowing down the acquisition of data and wasting terabytes of storage.
Specific Applications of MPS
MPS systems enable 3D cell culture with perfusion of media. This technology can enable organoid growth for:
- Studying organ development
- Drug development platforms (safety/ toxicity, efficacy)
- Precision/personalized medicine (patient response prediction)
- Regeneration technology
Drug development platforms include infectious and genetic diseases as well as cancers. Examples of established organoid cultures for study of healthy and disease tissue in various organs include:
| Cancer Type | Validation | Notes |
| Gastric |
Architecture, cancer markers: morphology, transcriptomics, “mutational landscape” |
Seidlitz et al Gut. 2018. |
|
Intestinal |
Architecture, transcriptomics |
Vlachogiannis et al. Science. 2018;359:920–6. |
| Liver |
Architecture, transcriptomics |
Broutier et al. Nat Med. 2017; 23:1424–35. |
|
Pancreas |
Gene (expression?) alterations, tumoroid formation/morphology |
Seino et al Cell Stem Cell. 2018;22(454–67):e6 |
| Prostate |
Histological patterns in PDx models, similar genetic diversity in tumoroids to PDx |
Gao et al Cell.2014;159:176–87. |
| Bladder |
Mutational profile similarity in vitro and in vivo |
Lee et al Cell. 2018;173(515–28):e17. |
| Breast |
Morphology, histopathology and gene profiles |
Sachs et al Cell. 2018;172(373–86):e10. |
Additionally, these microfluidic systems simulate complex structures such as the blood-brain barrier, skin and gut. The advent of cell-based therapies (immuno-oncological treatments) requires even pre-clinical studies to involve complex, system-level models involving membranes, complex co-cultures and other features. The successful development of microfluidic MPS systems to address those needs will depend on close collaboration between engineers and biologists.
Current Limitations of MPS
The current organoid technology available still represents an imperfect version. Firstly, organoids only contain epithelial layers without a tissue microenvironment such as the immune and nervous systems. Secondly, full maturation to adult organs or tissues is a bottleneck that must be addressed. Thirdly, organoids depend on the extracellular matrix Matrigel or basement membrane extract of current organoids. This is produced from mouse tumor lines and thus might be unsuitable for humans. Matrigel could also hamper drug penetration and be adverse to the potential use of organoids in drug screening. Fourthly, the organoid culture mediums need to be refined for long-term expansion of organoids. Fifthly, growth might have some effects on drug responses of organoids. Further efforts will be urgently exerted to solve these problems in the use of organoids in various biological applications.