The need for microphysiological systems

[The need for MPS]
The advances in drug discovery are severely impacted by limitations of workhorse in vitro model systems, such as immortalized cell lines / 2D culture. These and other study systems, such as PDx, are characterized by modest translational value, arising form to lack of 3D environment, cell-heterogeneity and matrix in the former, and interspecies differences in the latter.

The search for improved biological screening system has led to efforts to created microphysiological systems, which are more representative of human physiology and pathology. In addition, these systems promise greater standardization, lower costs, as well as reduced vivisection. Such model systems have included spheroids, organoids, and organ-on-a-chip systems among others. The latter had the advantage of microfluidic fluid exchange, while the latter have used are traditional static medica in multi-well plates.
Organoids are more sophisticated than spheroids, although they also 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 organization that resembles organs in certain key aspects. Epithelial organoids, such as for liver, lung or pancreas, are characterized by mesenchyme core (inner mass), and the early development resembles a blastocyst.

[Benefits of MPS]
Microphysiological systems (MPS) enabling organoid growth address a critical need. These systems not only facilitate the growth of organoids, their improve their culture conditions due to perfusion, in preference for shaking, resulting in better organoid growth, due to improved delivery of nutrients, and more consistent shear flow. Microfluidic culture also results in more uniform growth of organoids. This is an important feature, because variation of the differentiation of organoids grown in batch format cannot be controlled.

Microphysiological systems also can facilitate and improve optical imaging by distributing organoids in a more uniform fashion compare with gel matrix in a multi-well plate (which can result in high-content confocal imaging acquisition of z-stack with mostly empty space, slowing down acquisition of data, and wasting terabytes of storage).

[Specific applications]
MPS systems enable 3D cell culture with perfusion of media. This technology can enable organoid growth for:

  • Study organ development
  • Drug development platform (safety/ toxicity, efficacy)
  • Precision/personalized medicine (patient response prediction)
  • Regeneration technology

The application for drug development includes infectious and genetic diseases, as well as cancer. 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.
Instestinal 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 are well suited to simulate complex structures such as blood-brain barrier, skin, or gut. The advent of cell-based therapy (immuno-oncological treatments) requires even pre-clinical studies to involve complex, system-level model involving membranes, complex co-cultures, and other features. Successful development of microfluidic MPS systems to address those needs with depend on close collaboration of engineers and biologists.
 
Current limitations:
Current organoid technology still represents an imperfect version. Firstly, organoids only contain epithelial layer without tissue microenvironment, such as immune system and nervous system. Secondly, fully maturation to adult organs or tissues is a bottleneck required to be addressed. Thirdly, another limitation is the dependence on the extracellular matrix Matrigel or basement membrane extract of current organoids, which is produced from mouse tumor lines and thus might be unsuitable for human. Matrigel could also hamper drug penetration and be adverse to the potential of organoids in drug screens. Fourthly, culture medium needs to be further refined for long-term expansion of some organoids. Fifthly, growth
factors or molecular inhibitors in culture medium might have some effects on drug responses of organoids. Further efforts will be urgently exerted to solve these problems.