BRAIN on a CHIP
Putting a brain on a chip? A group of researchers didn’t just grow neurons on a chip, they put whole organoids on chips. Here’s how they did it…
“Organs-on-chips” are an effort to bring cells grown in the lab closer to mimicking actual organs and tissues found in the human body. For example, neurons growing in a single layer on a plastic petri dish is very different from what’s going on in your body. In your brain, there are multiple different types of cells. Also, the cellular microenvironment, or mini-environment directly surrounding the cells, isn’t plastic. In the body, cells are exposed to lots of proteins and fibers secreted by other cells, mechanical forces like stretching and squeezing, and flows of fluid and diffusion that takes place. Another consideration is how these cells can be reached by medications. For example, if we wanted to test a candidate medication to combat neurodegeneration, it wouldn’t just get poured directly on the neurons like would happen in the lab. Rather, the drug might have to pass through membranes in the body separating the actual cells from nutrient-rich fluids, like the blood brain barrier.
Organ-on-a-chip research uses tiny devices (chips) containing different compartments, with different channels and membranes that fluids can flow and diffuse through. The chips can also accommodate multiple types of cells. However, both the petri dish and these chips are generally flat. Your brain and other organs are complex 3D structures. A single layer of neurons don’t have anywhere near the structural complexity of your brain. For example, this could greatly affect the response of the cells to a candidate medication being researched. That’s where organoids come in.
Organoids are 3D clusters of different cell types commonly found in a given organ. Organoids can be thought of as miniature versions of organs that share some features in common with actual organs: different types of cells, basic functions, and more complex structures. Of course, organoids are currently not anywhere near as complex and advanced as an actual organ, but still a huge step up from a single layer of cells growing in a flat plastic dish. However, organoids alone don’t experience the complex microenvironment present in the body: they’re just floating in their growth liquid. Organoid tissues generally can’t mimic the fluid flows, physical forces, and diffusion of drugs that cells grown on chips can. But researchers set out to combine the best of both worlds by putting brain organoids on chips.
The researchers designed a tiny chip device containing five parallel, interconnected channels. Such devices that distribute tiny volumes of fluid to cells are called microfluidics. (Think micro=small and fluidics=fluid.) Two of the channels were used to grow the organoids. They did this by filling the channels with a jelly like substance called Matrigel that clusters of cells were embedded in as they grew. The other three channels were used to deliver nutrient-rich fluids. These fluids that are used in the lab to grow cells are called media. To make the organoids, the researchers used human induced pluripotent stem cells, which are capable of turning into nearly any type of cell in the body. They then made clusters of these cells, which triggered them to start changing into different cell types, or differentiating. This differentiation process caused the groups of cells to turn into cohesive tissues called embryoid bodies.
Embryoid bodies contain three types of cells: endoderm, mesoderm, and ectoderm. In the body, these types of cells go on to form organs and specialized cells like gut, heart, and brain. Here, the researchers didn’t want to look at embryoid bodies on a chip, they wanted to study brain organoids on a chip. So they needed to continue the differentiation process beyond embryoid bodies. External signals trigger cells to change and differentiate. So by altering the chemical composition of the media the cells are growing in, and embedding them in matrigel, which is a different physical environment, the researchers could guide the differentiation of the embryoid bodies to contain cells of the brain, forming brain organoids. But this process takes some time, and some work. On their way to becoming brain organoids, the embryoid bodies first differentiate into neuroectoderm: the cells that are produced in early stages of nervous system development. The researchers put the mixture of neuroectoderm tissues and Matrigel into the two channels of their device. The tissues continued to differentiate within the chip device into brain organoids.
As the organoids formed, growth media was pumped through the central channel of the device. This allowed nutrients to diffuse through the matrigel to reach the organoids. After about a month, the neuroectoderm tissues had differentiated into much larger and more complex brain organoids, up to three millimeters in diameter. The scientists found that this perfusion system appeared to improve organoid growth. In organoids grown traditionally, without this device, it is difficult for nutrients to reach cells within the dense center of the organoids, causing them to die. The fluid perfusion seemed to help nutrients reach the cells better and decrease cellular death in the center of the organoids.
But how did the researchers know that these clumps of tissue were really made up of cells found in the brain?
They observed that genes associated with neurons and other cells of the brain were turned on in their organoids. Intriguingly, they also observed that genes associated with forebrain and hindbrain were active in some regions. They also observed changes in shape or “morphology” of the tissues, like fluid filled cavities for example. Of course, brain organoids aren't the same as growing real, “mini brains”. The human brain and associated consciousness is an exceedingly complex system. But brain organoids show similarities to early developing tissues of the brain both in terms of the genes they have activated and the structures observed. The scientists felt that the device they designed greatly improved the brain organoids, by decreasing the number of cells that died and promoting the development of the organoids into structures and cells reminiscent of those observed in early brain formation.
The researchers felt their work was a proof-of-concept that microfluidics could improve brain organoid culture, with the potential to improve the setup even further. Still, what about the ethical issues of growing human cells in the lab into brain-like structures? Should there be boundaries?
Reference for the paper discussed in this article:
Wang, Yaqing, et al. "Engineering stem cell-derived 3D brain organoids in a perfusable organ-on-a-chip system." RSC advances 8.3 (2018): 1677-1685.
