Does Your Brain Make New Cells?

When I tell people that I’m a neuroscientist, one of the most common questions they ask me is: “Can the brain make new cells?”

This is a pretty reasonable question, especially since many of us have heard that we lose thousands of brain cells per day. That’s mostly a myth, by the way. Neurodegenerative diseases like Alzheimer’s do kill brain cells, called neurons, but normal aging doesn’t seem to be a downhill spiral of neuronal death.1

But back to the topic at hand: can you make new brain cells as an adult?

Other parts of your body make new cells all the time. Cells in your skin, gut, and bone marrow are constantly replicating themselves. Cells in your liver and kidney have the ability to multiply when necessary.

But for the most part, your brain isn’t making new cells. (Unless you’re a fetus, in which case I’m impressed by your reading abilities.) Almost all of the 100 billion cells in your brain were there before you were born. If you lose a bunch of them, like through an injury, disease, or stroke, you’re not getting them back.

New neurons in the hippocampus

There are a couple exceptions, though. The main exception is in the hippocampus, the part of the brain that’s super important for learning and memory. Your hippocampus actually does create new brain cells during adulthood—about 1400 neurons per day.2 Scientists first noticed this in the 1960s, but the idea that the adult brain could make new neurons (called neurogenesis) was controversial for decades and wasn’t widely accepted until the 1990s.

So now that we finally all agree that the brain can make new neurons, are those new neurons actually good for anything?

Some studies have suggested that newly born neurons in the hippocampus play an important role in certain types of learning and memory.3 New neurons may also regulate emotional processes such as anxiety and depression.4 In fact, many antidepressants stimulate neurogenesis in the hippocampus, and the birth of new neurons is critical for the antidepressant effects of these drugs.

adult neurogenesis

Adult neurogenesis in the hippocampus (credit and image details by J. Snyder here)

1400 new neurons per day is nothing to sneeze at, but it’s a minuscule number compared to the billions of cells our brains already have. Why is this handful of new neurons so important for our brain function?

Neuroscientists are actively addressing addressing this question. One thing we know is that newly born neurons in the hippocampus behave differently from old neurons.5 The young neurons are hyperexcitable and respond in all sorts of situations, just like bouncy kids who are constantly distracted. In contrast, mature neurons are more restrained in their responses. Like weary parents sick of their kids bouncing off the walls, the mature neurons sit around languidly, only responding when they really need to.

New neurons for smell

There’s a second area besides the hippocampus that produces new neurons. This region is called the subventricular zone, and in most mammals the neurons born there migrate to the olfactory system. That’s right—of the two sole brain areas capable of creating new neurons, one area dispatches its precious new neurons not to help with thinking or learning or decision-making, but with sensing smell!

SVZ neurogenesis

Side view of a rodent brain showing new neurons being produced in the subventricular zone (SVZ) and migrating via the rostral migratory stream (RMS) to the olfactory bulb (OB), where they rejuvenate the olfactory system. (from Abrous et al., 2005, Physiol. Rev.)

Now I like smelling stuff as much as the next person (and probably more), but to me this seems like a waste. I mean, how come the olfactory system needs new neurons when the rest of our brain, some of which does way more complicated stuff, gets by just fine with the neurons it started with?

Scientists still haven’t figured out why the olfactory system is so needy. But they recently discovered that we humans actually don’t have new neurons in our olfactory systems, making us fairly unique among our mammalian comrades such as rodents and monkeys.6 This might be because we rely less on our sense of smell than other mammals, so we don’t need to keep rejuvenating our underused olfactory system.

It turns out that we humans still make new neurons in our subventricular zone, but the neurons born there aren’t destined for the same smelly fate as they would meet in other mammals. In humans, these neurons migrate to the striatum, a region that’s best known for motor control and learning.7 The function of these new neurons is still unknown.

It’s hard to fit in: integrating new neurons

Adding new brain cells is actually a pretty crazy idea, if you think about it. It takes months for our brains to develop properly the first time, with our 100 billion neurons settling in the right place and forming the right connections to each other and tuning those connections in exactly the right way. Now that we finally have a beautiful network that works perfectly, we’re going to mess with it by adding in new cells?

Adding new neurons is like adding new code into the middle of a computer program, or (for you non-techies) adding a new character into your novel. You can’t just randomly insert new code or characters without carefully deciding how they’re going to fit in with the existing code or story. In fact, failing to incorporate the new elements properly might break the whole system, as programmers know all too well. So researchers are super interested in studying how new neurons are assimilated into existing brain circuits.

neuron assimilatedMiracle-gro for your brain?

Once I tell people that some parts of the brain do produce new neurons, they usually have the same follow-up question: “How can I get my brain to make more new neurons?”

mice running

Mice that exercise by running on wheels show more adult neurogenesis (credit: Med Associates Inc.)

It’s actually not that hard. There are lots of things that enhance adult neurogenesis in animal models, such as exercise, learning new things, and living in an enriched, stimulating environment. Conversely, negative conditions such as stress, injury, poor diet, and plain old aging can suppress neuron production.

But seriously guys, don’t get too hung up on whether your life is stimulating enough to make new neurons or whether the stress of your job is hampering this process. Making new neurons is just one of many ways that your adult brain can adapt and change, which we call “plasticity”.

Most plasticity doesn’t occur by creating new neurons, but instead involves changes in how neurons are connected to each other or how they function. Even after serious injuries like a stroke, which can kill two million neurons per minute, the brain is often able to adapt and recover much of its previous function. New connections form and the surviving neurons take on more work.

These other forms of plasticity probably explain why adult neurogenesis is so limited—the brain has other ways of adapting. So you don’t need to be bummed that you’re not making tons of new brain cells all the time.

Personally, I kind of like the idea that most of my neurons are as old as I am. They’ve been there for the long haul, loyally sticking around for the good times and the bad. Just like me, they’ve been changed by experience and adapted to new situations. Knowing that they’ll never be replaced, that quitting isn’t an option, they continue to chug along and do the best they can.

 

References:

1. For example, this study showed that the number of neurons in a particular brain area remained constant from age 60 to 90:

Gómez-Isla T, Price JL, McKeel DW Jr, Morris JC, Growdon JH, Hyman BT. Profound loss of layer II entorhinal cortex neurons occurs in very mild Alzheimer’s disease. J Neurosci 16:4491-500 (1996).

2. Spalding KL, Bergmann O, Alkass K, Bernard S, Salehpour M, Huttner HB, Boström E, Westerlund I, Vial C, Buchholz BA, Possnert G, Mash DC, Druid H, Frisén J. Dynamics of hippocampal neurogenesis in adult humans. Cell 153:1219-1227 (2013).

3. Deng W, Aimone JB, Gage FH. New neurons and new memories: how does adult hippocampal neurogenesis affect learning and memory? Nat Rev Neurosci 11:339-350 (2010).

4. Sahay A, Hen R. Adult hippocampal neurogenesis in depression. Nat Neurosci 10:1110-1115 (2007).

5. Christian KM, Song H, Ming GL. Functions and dysfunctions of adult hippocampal neurogenesis. Annu Rev Neurosci 37:243-262 (2014).

6. Bergmann O, Liebl J, Bernard S, Alkass K, Yeung MS, Steier P, Kutschera W, Johnson L, Landén M, Druid H, Spalding KL, Frisén J. The age of olfactory bulb neurons in humans. Neuron 74:634-639 (2012).

7. Ernst A, Alkass K, Bernard S, Salehpour M, Perl S, Tisdale J, Possnert G, Druid H, Frisén J. Neurogenesis in the striatum of the adult human brain. Cell 156:1072-1083 (2014).


Leave a Reply

Your email address will not be published.