Scientists seek to create model systems that accurately recapitulate conditions in living organisms so that they can study everything from basic cellular functions to the effects of drug therapies. In the race to find the best model, the field of organoids has rapidly matured. To discover more about how (or even if) cell scientists are using organoids and the challenges they face, the American Society for Cell Biology’s Public Policy Committee has created an Organoid Taskforce that is conducting a survey on organoids. The survey results, which will help guide the task force as it identifies areas of focus and the Organoid Taskforce report will be issued at the 2018 ASCB|EMBO Meeting in San Diego this December.
ASCB member Orly Reiner and colleagues from the Department of Molecular Genetics at the Weizmann Institute of Science in Rehovot Israel have created organoids and recently published “Human brain organoids on a chip reveal the physics of folding” in the journal Nature Physics. Reiner and lead author Eyal Karzbrun responded to a few questions regarding their innovative work, which only hints at the potential of this exciting new model system.
What are the fundamental differences between an organoid and a lab-on-a-chip?
Eyal: By definition, a lab-on-a-chip is a device that integrates laboratory functions on a single chip (e.g., silicon or glass surface), which is relatively small in size—a few millimeters to a few square centimeters. The advantages of a lab-on-a-chip approach are (that they are useful for) a high-throughput study, controlled conditions, and compatibility with live imaging. An organoid is a miniaturized and simplified version of an organ produced in vitro in three dimensions enabling realistic micro-anatomy. Due to their size, organoids are challenging to study using live imaging. In our work, we managed to combine the two methodologies, by growing organoids in micro-fabricated compartments on a glass surface (chip). This required confining one dimension of growth and opened up several new possibilities such as live imaging of the developmental process.
What does your organoid look like? And why were this particular shape and size important to this study?
Orly: Our organoids are ovaloid in shape. They grow in the z-axis in a confined space of 150 microns but in x- and y- dimensions they reach one millimeter in size. The cells are polarized and their apical side is toward a very thin lumen in the middle of the organoid. The confinement of the z-axis enables live imaging in the whole volume and an unlimited supply of nutrients by diffusion. Therefore, there is no cell death in the center, as is observed in other organoid systems.
What’s the difference between “folding” of the brain during development and “wrinkling” and how is that important with respect to disease? It seems like purposeful folding would be desired but wrinkling (in response to physical forces that might not be present during normal healthy brain development) would be undesired and detrimental.
Orly: In our system, we see early folding. This recapitulates some aspects of brain folding, which occurs later in development when there are already many neurons in place. We do not claim that our system is identical to brain folding in the embryonic brain but it may exhibit many common features and could be used as a model to study the basic principles involved.
Tell me about the forces you examined and why the directions of those forces are important to study.
Eyal: Buckling or wrinkling can occur when a material is under compression or when different parts of the same tissue are growing at different rates (differential growth). One example of differential growth is when the inner part of a tissue is contracting (“negative growth”) and the outer part is expanding. In the organoid, we observed two phenomena that may lead to wrinkling. First, the cytoskeleton on the inner apical surface is actively contracting. Second, the cell’s nuclei on the organoid perimeter are growing faster than the nuclei in the inner parts of the organoid. Thus, overall, the organoid is growing on the perimeter but contracting on its inner surface. If the entire organoid was growing homogeneously, no wrinkles would appear. Indeed, when we reduced the cytoskeletal contraction by low concentrations of blebbistatin, practically no folds developed. In case of the LIS1 mutation, the nuclei did not spend most of the time in the periphery, and therefore the differential growth force was eliminated. This resulted in an overall reduced curvature. To watch a video of the organoid growing, click here.
What do you plan to do with what you’ve learned?
Orly: We plan to use the system for several purposes. As this is a new system, the possibilities are unlimited. Here I will mention a few obvious ones:
1. Model other types of brain malformations.
2. Use the model to study genes that may play a role in brain folding and test them in the system.
3. Continue to develop our system in other directions; generate neuronal layers, generate different brain regions, etc.
Watch the video: Developing wild-type organoid. Fluorescence time-lapse images showing nuclear motion and cell division during cell-cycle. Green: Lifeact-GFP, RED: H2B-mCherry. Film duration 36 hours, time steps 10 minutes.