What a question! How the heck did researchers find out where stem cells reside in the brain? Well, there is a great deal of background information on where these cells reside based on emryonic development, and how our nervous systems develop as babies. Scientists have used this evidence as clues as to where stem cells may lie in the adult brain.
After four weeks of development, the neural tube of the human embryo establishes a series of interconnected chambers called ventricles. We can see those partitions in the picture up there. Ventricles are chambers filled with cerebrospinal fluid that is necessary as a protective shock absorbing medium. This fluid also exists between the brain and the dura mater. During development, the cells surrounding the ventricles, sometimes referred to as founder cells, go through rapid cell division, making more and more founder cells in a process called symmetric division. About three weeks later, in the seventh week of development, these founder cells go through assymetric division where each founder cell produces one other founder cell and one neuron that travels outward into the cerebral cortex. This process is guided by growth factors and other cell signals, which will be discussed later in the section about differentiation.
So by examining human nervous system development, we have a clue as to where stem cells might be in the adult brain.... near the ventricles. And this is exactly the case. Only now, we have a new term for that region: the subventricular zone. The region under the lateral ventricles, sort of where that "Foramen of Monroe" label is pointing, is where this region is. Research on cells in this region has given us confirmation that this is indeed the home of our adult stem cells. How did scientists find this out? Well, they took cells from this region and plated them in different growth factors and found that subependymal cells had properties of multipotentiality as well as being repeatedly passageable. What this means is that the cells had the same properties as founder cells in that assymetric division stage. They could give rise to neurons, astrocytes, and oligodendrocytes, as well as producing another stem cell which were also found to be multipotent and repeatedly passageable.
Another region of the brain where adult neural stem cells exist is in
the dentate gyrus of the hippocampus. This region is particularly important
in learning new information. Research done primarily on songbirds who acquire
new songs every mating season has been invaluable. It primarily gave scientists
the idea of adult neurogenesis after decades of believing that the brain
did not regenerate itself after development. Psychobiology is rooted in
the assumption that transformations in thought and behavior must be accompanied
by transformations in the brain. Previous assumptions were that changes
in the cell signals were solely responsible for new learning, but we now
understand that new "wiring" can and does occur in the learning
process. This is generally referred to as the
plasticity of the
brain. Here's the section on memory and plasticity.
Research has also found general paths of migration from these regions. The stem cells from the subventricular zone provide a source of new olfactory neurons that are renewed throughout life. Tube-like structures made of glial cells provide a pathway called the rostral migratory stream (RMS) that leads to the olfactory bulb. The olfactory bulb is the part of the brain that is responsible for information from your nose. An interesting find is that stem cells taken from different sections of this pathway have different properties from one another in that cells in the subventricular zone generate more quickly than those in the rostral extension.
Now we know where these stem cells live in our brains. So, it seems like it should be pretty easy to go stem cell mining now that we know where the mother lode is, right? Nope. Not quite, but I like your spunk! Because, you see, there are a lot of other types of cells that surround the stem cells, and we have to be able to tell which ones are which, before we start trying to culture anything. Sound research depends on the best cell samples, and if you click here, we can check out how scientists identify neural stem cells.