Of all the organs in your body, the brain is arguably the most important -- sorry to all the cardiologists out there. Most people understand the importance of the brain, so I won’t highlight all of its wonderful functions here. Rather, I’ll summarize by saying that you can’t live without a brain (duh), so making sure we have all the tools to study and understand how the brain works, as well as how it can become dysfunctional, is an extremely important medical and scientific endeavor.
How we usually study the human brain: Cells in a dish, brains from an animal
To study diseases of the brain (think Alzheimer’s, Parkinson’s, Autism, depression, bipolar disorder), neuroscientists use a variety of methods. The methods used in any research setting really depend on the type of scientific question being asked, but generally, neuroscientists study the brain using two methods: In vitro experimentation (cells in a dish) and in vivo experimentation (brains from model organisms such as mice, rats, or non-human primates).
In vitro experimentation employs the use of single cells to answer research questions. For example, if you were interested in how neurons play a role in Alzheimer’s Disease, you might acquire human neurons (usually from an aborted fetus), genetically modify them to display some of the same mutations observed in Alzheimer’s, then study those single cells under different conditions. This allows you to measure things such as morphology (how the neurons look), proliferation (how the neurons grow), gene and protein expression, and cell signaling (how proteins within the cell interact with one another). Also, these experiments allow for great experimental control and predictable timelines. Asking cell-specific questions is the greatest strength of in vitro work…yet it is also its greatest weakness!
While it is important to ask cell-specific questions, cells tend to act differently in flasks than they do in your body. For example, any given neuron in your brain is currently interacting with hundreds of other cells at once. This influences the health and overall function of the neuron, but unfortunately, this isn’t something that is easily recreated in a petri dish.
Here lies the limitations of in vitro experimentation: biological relevance, or how accurately the experiment will represent what is actually going on inside our bodies.
This is where in vivo experimentation comes in. In vivo experimentation gives scientists the ability to do most of the things that can be done using cells in a dish, but adds a degree of biological relevance. In these cases, scientists are studying actual brains.
Going back to our example, we’d be studying neurons within the brain of a mouse that has Alzheimer’s Disease. This would be a better representation of how neurons not only interact with each other, but also how they interact with other cells types and how those interactions may alter their function, all in the context of Alzheimer’s Disease.
It seems pretty straight forward that in vivo would be preferred over in vitro, right? Studying an actual brain has to be better than simply studying a bunch of cells in a dish. This is true – but animal models have their own drawbacks. In vivo work is often very time consuming, expensive, and it may be difficult to ask cell-specific questions in some contexts.
Further, brains from mice, rats, and non-human primates are different than human brains. For example, mice are used for many neuroscience studies, but there is about 75 million years of evolution between us and mice, which has resulted in our brains being quite different than mouse brains. Although mouse models are effective in studying some aspects of the brain, they are less useful in other areas and often fail to model the complexity of our brain. For these reasons, scientists, such as myself, are in search of a more perfect model system to study the most influential entity on this planet: the human brain.
What is a mini-brain?
What if scientists could design a system that combines the benefits of studying cells in a dish with the benefits of studying whole brains from an animal? That’s exactly what mini-brains, or cerebral organoids, aim to do!
A cerebral organoid is generated using human-derived stem cells, which are just really immature cells from a human that haven’t yet decided what type of specialized cell they will become. In the body, stem cells receive different types of signals from molecules in their local environment that tells them what type of cell to become. For example, in the brain there are stem cells will receive signals that tell them to become neurons. Same thing with stem cells that reside in the liver; they’ll eventually become liver cells, or hepatocytes. The local environment will ultimately determine the type of cell that any given stem cell will grow into – and this is the exact biological phenomenon we can leverage to make mini-brains.
Imagine a bunch of stem cells sitting in a petri dish. Remember, their environment determines what cells they grow into. So, if we want these stem cells to eventually mature into brain cells, we need to figure out what specific molecules in the brain environment tell the stem cells to grow into brain cells, then we need to add those molecules to the petri dish.
After we add those molecules, the stem cells’ fate has been determined – they will now become brain cells! With a little experimental help, the stem cells begin to grow into a spherical clump of brain cells, and this accurately models formation of the brain during pregnancy. Crazy, right?
After a few months, and a few other steps to keep the mini-brain healthy and help retain its spherical shape, you have a ~4-millimeter cluster of different types of brain cells that interact intimately with each other. It contains neurons that fire, astrocytes that help maintain neuronal health, and can even contain immune cells called microglia (part of my project is creating a mini-brain that contains microglia). All of these cells interface, talk to one another, and form what looks like a small brain (see photo from my project!). What’s amazing about these mini-brains is that, other than telling them which cell types they should become, these cells are left to their own devices to form a 3D structure, similar to how a real brain would develop.
In addition to a diverse cell population, these mini-brains contain distinct cell layers resultant of cell migration which occurs during fetal development (also see photo). For this reason, mini-brains have primarily been used to answer questions that pertain to the development of the human brain or developmental diseases.
Mini-brains have more recently been used to study other types of diseases such as Zika Virus (mostly due to its potential to cause microcephaly) and Alzheimer’s disease. Taking this a step further, I aim to study HIV-1 infection of the central nervous system using mini-brains! Inherently it is more challenging to study these types of diseases using mini-brains because of their immature developmental stage, but the in vitro-like benefits combined with its biological relevance make mini-brains an extremely attractive model for modeling brain disease in general, especially since human brain tissue is hard to come by.
Mini-brains are still fairly complicated to generate and the “correct” way of making them has yet to be established – right now, the models are usually generated a bit differently between labs, which is not ideal for reproducibility. But it’s all part of an on-going pursuit across science to develop a more feasible and biologically-relevant model for studying the function, dysfunction, and complexity of the magnificent human brain.