What are mini brains? - Madeline Lancaster
This pencil-eraser-sized mass of cells
is something called a brain organoid.
It’s a collection of lab-grown neurons
and other brain tissue
that scientists can use to
learn about full-grown human brains.
And it can be grown from
a sample of your skin cells.
Why would we need such a thing?
Neuroscientists face a challenge:
shielded by our thick skulls and
swaddled in layers of protective tissue,
the human brain is
extremely difficult to observe in action.
For centuries, scientists have tried to
understand them using autopsies,
animal models,
and, in recent years, imaging techniques.
We’ve learned a lot through
all these methods,
but they have limitations.
Conditions like Alzheimer’s
and schizophrenia,
and the effect on
the human brain of diseases like Zika,
continue to hide beyond our view,
and our understanding.
Enter brain organoids,
which function like human brains
but aren’t part of an organism.
Each one comes from
an undifferentiated stem cell,
which is a cell that
can develop into any tissue in the body,
from bone to brain.
Scientists can make undifferentiated
stem cells from skin cells.
That means they can take a skin sample
from a person with a particular condition
and generate brain organoids
from that person.
The hardest part of growing
a brain organoid,
which stumped scientists for years,
was finding the perfect combination of
sugars, proteins, vitamins, and minerals
that would induce the stem cell
to develop a neural identity.
That was only discovered recently,
in 2013.
The rest of the process
is surprisingly easy.
A neural stem cell essentially
grows itself,
similar to how a seed grows into a plant,
all it needs are the brain’s equivalents
of soil, water, and sunlight.
A special gel to simulate
embryonic tissue,
a warm incubator set at body temperature,
and a bit of motion to mimic blood flow.
The stem cell grows into
a very small version
of an early-developing human brain,
complete with neurons that can connect
to one another
and make simplified neural networks.
As mini brains grow, they follow
all the steps of fetal brain development.
By observing this process,
we can learn how our neurons develop,
as well as how we end up with
so many more of them in our cortex,
the part responsible for higher cognition
like logic and reasoning,
than other species.
Being able to grow brains in the lab,
even tiny ones,
raises ethical questions, like:
Can they think for themselves,
or develop consciousness?
And the answer is no, for several reasons.
A brain organoid has the same tissue types
as a full-sized brain,
but isn’t organized the same way.
The organoid is similar to an airplane
that’s been taken apart
and reassembled at random;
you could still study the wings,
the engine, and other parts,
but the plane could never fly.
Similarly, a brain organoid allows us
to study different types of brain tissue,
but can’t think.
And even if mini brains were organized
like a real brain,
they still wouldn’t be able
to reason or develop consciousness.
A big part of what makes our brains so
smart is their size,
and mini brains have only
about 100,000 neurons
compared to the 86 billion
in a full-sized brain.
Scientists aren’t likely to grow larger
brain organoids anytime soon.
Without blood vessels to feed them,
their size is limited
to one centimeter at most.
Finally, mini brains aren’t able
to interact with the outside world.
We learn by interacting
with our environments: receiving inputs
through our eyes, ears, and other
sensory organs, and reacting in turn.
The complex neural networks that underlie
conscious thoughts and actions
develop from this feedback loop.
Without it, the organoids
can never form a functional network.
There’s nothing quite like
the actual human brain,
but mini brains are an unprecedented tool
for studying everything
from development to disease.
With luck, these humble
organoids can help us discover
what makes the human brain unique,
and maybe bring us closer
to answering the age-old question:
what makes us human?
