If you follow me on Twitter, you probably know I study adult
neurogenesis. Prior to working in a research lab I had no idea adult
neurogenesis was a thing and now it is my life and I love it. Adult
neurogenesis is the birth of new cells in the adult brain. It occurs in
songbirds, mice, rats, cats, non-human primates, wild mice, and humans. It
probably occurs in all mammals but we can't say for sure because no one has
gone out and done a systematic analysis of every mammal's brain looking for new
cells. The name, adult neurogenesis, implies the new cells being created are
neurons. That's true, most of them are (60-80%), but some of them are actually
support cells, called glia. Following is the story about the first time someone observed neurogenesis in the adult brain.
My favorite part about
science is seeing the results of an experiment and knowing that at that moment,
you're the only person in the world who knows the answer to the scientific
question you asked in your experiment. The ONLY person. Isn't that cool? So I
can only imagine how Joseph Altman and Gopal Das felt when they saw the first
evidence of adult-generated brain cells. Their experiment upset the dogma that
we create brain cells only during development. Sort of. I'll get to that.
The funny thing is that in their seminal 1962 letter in Science,
it doesn't even sound like the newborn neurons were the initial focus of the
experiment. It was a "simultaneous pilot study" alongside an
experiment to examine the kinetics of glial cell proliferation after brain
injury in rats. The injury was an electrolytic lesion, which is a fancy way of
saying they injected a needle into a specific brain area and applied current to
destroy some of the brain tissue. It sounds odd but lesion experiments are very
important in neuroscience.
In this case, they lesioned the brain and wanted to see how glia responded:
would they divide and travel to the lesioned area?
Molecular structure of DNA (Source) |
After the injection and lesion trauma, the rats were sacrificed
after 1 day, 1 week, 2 weeks, 1 month, and 2 months. This time course was to
help them determine the how glial cells act after injury: they divide and
migrate. After removing the brains, they cut them into thin slices called
sections and put them on slides. I
won't pretend to know exactly how H3 thymidine labeling works. The slides are
dipped in a photographic emulsion and then left to develop, in this case, for
THIRTEEN WEEKS. For their first experiment where they found adult neurogenesis
serendipitously that isn't too big of a deal but the follow-up experiments
where they knew what they were waiting for... I can't imagine the suspense!
Anyway, while they were examining the brain sections for glial
proliferation due to the lesion injury they noticed... some cells near the
ventricles and in the hippocampus were labeled with H3 thymidine. What the
what? Neurons can't divide! How freaking excited they must have been! I'll
admit just imagining being in their place makes me grin. So cool!
So they published this short letter in the journal Science and...
the scientific community was skeptical to say the least. People scoffed and
some even tried to discredit Altman and his findings (even after a more detailed study published in 2 papers in 1965). For decades, there was
little research on this phenomenon because scientists just couldn't believe it
was true. Then in the 1990s the field exploded! Citations for the term
"adult neurogenesis" in 1990 equaled exactly 2, according to a Pubmed
search. Both of those papers were studying songbirds, which isn't directly
applicable to humans because they aren't mammals (but were still very cool!). Now, there are hundreds of
papers being published each year regarding adult neurogenesis and two papers
have shown that it occurs in humans. It's a fun field to be in and I'm glad I
stumbled into it when I chose my grad school lab.
Reference
Altman, J (1962). Are new neurons created in the adult mammalian
brain? Science 135.
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