Neurogenesis Basics

Posted: December 18th, 2008 | Author: Matt | Filed under: The Neurogenesis Experiment | Tags: , , | No Comments »

As I wrote Sunday, neurogenesis is the process by which neurons are created. So let’s start with those.

Neurons and Glia

Rat Neurons (Image: ewedistrict@flickr)

Rat Neurons (Image: Dmitry Sarkisov@flickr)

A neuron is basically a cell that can conduct an electric signal, and therefore information. There are about 1011 (100 billion) neurons in the average human brain1, each of which can have thousands of connections to other neurons. While a single neuron’s activity is simple (it either fires its action potential or not), the connections are not. It might require activation by two or more different incoming neurons with a certain timing for a neuron to fire. Neurons can also depress each others activity, so if neuron A fires, neuron B will remain silent, even if it is excited. These connections are though to be the critical factor in allowing the complexity we see in human and even animal behavior to emerge from a rather simple basic building block, the neuron.

There is another type of cell in your brain that deserves mention, called glia. Outnumbering neurons 10 to 12, glia cells have traditionally been thought of as the brain’s maintenance brigade, providing structural support, nutrition and immune defenses. In recent years, there is growing support for assigning glia a more important role in the CNS, as interactions between glia and neurons have been found3

Now to neurogenesis. Obviously a large amount of neurons (a complete brain) are created even before birth. I might revisit pre-natal neurogenesis at a later point4, but for now, when I use the term neurogenesis, I’m talking about adult neurogenesis.

Adult Neurogenesis

Adult Neurogenesis in humans is, like continental drift and epigenetics, a relatively new concept. Consensus until 1998 was a fixed amount of neurons that could only be lost, never gained. Though it is a relatively new proposition, adult neurogenesis is now widely accepted and the evidence is strong. When my previous article hit Hacker News5 a user called a-priori was skeptical:

Let me be the first to say, “sources please”. Adult neurogenesis in humans is largely speculative, which leads me to think you’re confusing it with neuroplasticity.[..]6

It is important to continually challenge scientific theories, and it’s even more important not to take “but the majority believes it” as definite proof. So let’s take a look at the evidence. The standard method to detect and measure neurogenesis uses a substance called Bromodeoxyuridine

BrdU-Labeled Cells in the Human Brain (Image: Peter S. Eriksson)

BrdU-Labeled Cells in the Human Brain (Image: Peter S. Eriksson)

or BrdU. It’s a molecule that closely resembles Thymidine, the T in the ACGT alphabet DNA is written in. When cells divide, they copy their DNA. If there’s BrdU around (i. e. if an evil scientist has injected it), it will be incorporated into the newly synthesized DNA. When the animal dies, new cells contain BrdU that can be colored so they show up clearly under a microscope. The image shows a slice of human brain tissue with three BrdU-labeled cells. These cells must have been created after the BrdU injections.

While the decades-old dogma that adult neurogenesis does not exist in humans was overturned should caution us not to blindly accept the new, reversed dogma, this evidence is much stronger than before (“I cannot find my wallet” does not proof that my wallet does not exist, while “I have my wallet in my hand” is pretty good evidence that the wallet does exist).

Adult neurogenesis was first shown in canaries by Goldman and Nottebohm7 It took until 1998 before it was shown in humans8 . So adult neurogenesis in humans exists. The next questions will be where in the brain it is located, what goes on in those regions and what effects neurogenesis actually has.

  1. Wikipedia: Brain [↩]
  2. Society for Neuroscience [↩]
  3. Haydon P: Glia: listening and taking to the synapse, Nature Reviews Neuroscience 2001 [↩]
  4. at least some of the ideas I’ll be talking about should also be applicable in pre-natal neurogenesis, such as anything regarding substance abuse [↩]
  5. http://news.ycombinator.com/item?id=397537 [↩]
  6. http://news.ycombinator.com/item?id=397628 [↩]
  7. Goldman et al. … Production, Migration, and Differentiation in a Vocal Control Nucleus of the Adult Female Canary …. Proceedings of the National Academy of Sciences (1983) [↩]
  8. Eriksson et al. Neurogenesis in the adult human hippocampus. Nature Medicine (1998) [↩]