There is a famous myth that the brain doesn’t finish developing until the age of 25.

Or rather, there is a famous incorrect belief, since the fact it’s a myth is, sadly, not so well-known as it could be.

The reality is that the 2006 study the “brain doesn’t finish developing until 25” people are referring to examined the development of the brain up to the age of 25, and that was where the study ended, because the study was about the adolescent brain, and in any case all studies have to stop somewhere or else nothing would get published.

This is the equivalent of saying “it didn’t stop raining until four o’clock” when the reality is that four o’clock is simply when you gave up on checking.

The study didn’t misrepresent this, by the way, but the popular press did!

Another 2012 study looked at various metrics of brain development, and found:

• Synapse overproduction into the teens
• Cortex pruning into the late 20s
• Prefrontal pruning into middle age at least (that’s where they stopped looking)
• Myelination beyond middle age (that’s where they stopped looking)

Source: Experience and the developing prefrontal cortex ← check out figure 1, and make sure you’re looking at the human data not the rat data

You can read much more about the original study, here: The Brain As A Work-In-Progress

There’s more (there’s always more)

Those latter bulletpoints listed there are about pruning and myelination (that is, encasing neurons in protective sheathes of myelin), so what about actual brain growth?

It was long believed that brain growth could not occur later in life, due to expending our innate stock of pluripotent stem cells. However, this was mostly based on rodent studies.

Rodent studies are often used for brain research, because it’s difficult to find human volunteers willing to have their brains sliced thinly (so that the cells can be viewed under a microscope) at the end of the study.

However, in 2018 and 2019, there was a flurry of studies that (using brain tissue samples from the autopsies of formerly healthy humans) proved, disproved*, and then re-proved, that neurogenesis (creation of new brain cells) occurs in adult humans:

*This middle one was a mistake; in an effort to disprove the prior work of Dr. Maura Boldrini et al., the next research team (Dr. Shawn Sorrels et al.) accidentally destroyed the evidence they were looking for, and then proclaimed “look, it’s not there”. It then took a follow-up study by Dr. Elena Moreno-Jiménez et al. to fix the error. If you’ll pardon the pun, because actually the accident in the middle study was due to the fixing process they used (in the sense of chemical tissue fixation to preserve them for study).

You can read those studies in order, here:

• Human Hippocampal Neurogenesis Persists throughout Aging – Dr. Maura Boldrini et al.
• Human hippocampal neurogenesis drops sharply in children to undetectable levels in adults – Dr. Shawn Sorrels et al.
• Adult hippocampal neurogenesis is abundant in neurologically healthy subjects and drops sharply in patients with Alzheimer’s disease – Dr. Elena Moreno-Jiménez et al.

That third study corrected the mistake made in the second study, by using a shorter fixation time for the cell samples they wanted to look at, and found that there were tens of thousands of newly-made brain cells in samples from adults ranging from 43 to 87.

There was still room for doubt

Since those studies, it’s been generally considered no longer contentious that humans do, indeed, do neurogenesis throughout life—at least in the hippocampi, which is where the tissue that was tested came from, and the hippocampus is a focal point for a lot of such research as it’s almost entirely responsible for memory.

Thus, to get philosophical for a moment, it could be argued that that’s the part of the brain that’s the most uniquely “us” (since it contains more or less our entire conscious life experience), and other parts of the brain serve as processing apparatus of various kinds. So, it’s an important bit of gray matter.

However, scientists (Dr. Marta Paterlini et al.) wanted to check that those newly-formed neurons really were newly-formed. The reason this was in question was that it wasn’t known whether the predecessors of these new neurons, neural progenitor cells, were still able to proliferate.

The alternative explanation (if it turned out they weren’t) would be that those “new” neurons found in aged brains were, in fact, old neurons in a state of arrested development (because they had been considered “new” on account of their cellular features that mark them as in an early stage of neural development).

Good news: the study confirmed the existence and division of those precursor cells that generate new neurons in adults.

Specifically, they examined brain tissue from human brains aged 0 to 78 using a lot of very hi-tech methods including:

• Flow cytometry ← this is a lot fancier than it sounds like, and essentially involves rapidly laser-scanning tens of thousands of cells one at a time. And you thought photocopying a workbook was arduous!
• Single nucleus RNA sequencing ← this one is what it sounds like
• RNAscope ← think of this as a “noise-cancelling” microscope for looking at RNA in the context of in situ hybridization
• Xenium ← when sci-fi shows have a fuzzy image and someone says “enhance” and now it has details, that’s what this software does, but for looking at RNA
• Carbon-dating ← yes, really! It may seem funny to use carbon-dating to tell the age of a brain cell (with an undertone of “are we really that old now?”, but measuring C₁₄ decay was a reliable way to know definitively when a given cell was formed (because, being carbon-based life as we are, our cells can be carbon-dated just like any carbon-containing sample from an archeological site).

Using these technologies, they were able to detect not just various stages of neuron development, but also many actively dividing cells, and the progenitor cells of which we spoke above.

Notably, some adults had many neural progenitor cells while others had very few, suggesting differences in brain regenerative potential. Almost certainly this is linked to overall brain health, but identifying the cause(s) of the difference was not part of this study, so we can’t say for sure.

You can find the paper here: Identification of proliferating neural progenitors in the adult human hippocampus

What to do with this information

The practical take-away here is that our brain, at any age, is a developing thing and will continue to rejuvenate itself given the chance. So, we have to give it that chance.

This means looking after our brains with such things as:

• Good exercise: specifically, exercise that gets your blood pumping. As well as helping with clearing away waste products, exercise is one of the biggest things we can do to boost Brain-Derived Neurotrophic Factor, or BDNF. Here be science: Brain-Derived Neurotrophic Factor, Depression, and Physical Activity: Making the Neuroplastic Connection
• Good nutrition: considering such things as: Brain Food? The Eyes Have It!
• Good supplementation: you may remember, for example, that Astaxanthin: Super-Antioxidant & Neuroprotectant also specifically promotes adult neurogenesis
• Good sleep: bearing in mind One More Reason To Prioritize Sleep To Fight Cognitive Decline

And of course: don’t smoke, and don’t drink alcohol. They are terrible for everything, and brain health is near the top of the list for each of them.

See also: What Happens To Your Body When You Stop Drinking Alcohol ← for a timeline of physical recovery, including repairing the damage done to the brain by alcohol

Want to learn more?

You might like this book we reviewed a little while ago:

The Brain’s Way of Healing: Remarkable Discoveries and Recoveries from the Frontiers of Neuroplasticity – by Dr. Norman Doidge

Take care!