outside surface. If you think of the ventricle pools as the body of a porcupine, the radial glia are its quills. Radial glial cells appear around gestation week seven as the part of the embryo’s brain sprouts and grows. Shortly after radial glia appear, they begin dividing at the ventricle. The cycle of cell division starts with the radial glial cell body migrating up its long and thin processes that stick up from the ventricle; the porcupine quill reach up like a blind man’s cane to test its environment. This phenomena is demonstrated in Figure 7.1 . As the cell decides to divide, it begins to replicate its DNA as it migrates down the porcupine quill back to the ventricle. It then becomes two cells, one of which migrates back up its long radial glial cell mother and becomes a neuron.
FIGURE 7.1 A neuron climbing like a squirrel up its radial glia mother tree
Reprinted from Brain Research Reviews, Vol. 55, Issue 2, Rakic, P., “The Radial Edifice of Cortical Architecture: from Neuronal Silhouettes to Genetic Engineering,” p. 208, Copyright (2007) with permission from Elsevier.
The radial glial cell is also a mother in the sense that it guides its progeny to their places in the world. In this case, the world is our brain. After each new cell migrates up the radial glia to its new position in space, it morphs into a neuron. When mapping the cortex in the late nineteenth century, Cajal did so based on connections neurons make when bringing information from the sensory and sending to the motor environments. Based on neuronal connectivity, the cortical layers are numbered 1–6. Layer 1 is closer to the outside surface of the brain; layer 6 is deeper inside the brain.
Layer 6 forms first in development. Radial glial extend short processes near the ventricle, and bulbs of cells are laid down like seeds. At this point, as the fetus is bathed in embryonic fluid, the ventricular fluid takes up almost the entire space of the peanut-sized brain.
For the next 100 days, radial glia extend further out of the brain. They push through already established layers, through its continual neuronal offspring to what will be the outside of the brain. After the last layer is formed (layer 1, which is at the extreme outside of the brain), the neuronal support infrastructure of the cortex has been laid down like a map. In the second month of pregnancy, at the peak of proliferation, it is estimated that 200,000 neurons bud every minute.
The explosion of cell proliferation in primates compared to other animals is believed to be the reason for our unique folds of the cortical surface when we observe a brain removed from its skull. In a rat, the cortex has no folds and is simply an outer sheath covering the basal areas of the brain. However, in humans, the cortex folds and refolds to fit all the cell bodies in the cortex. It can be compared to shoving a crumpled blanket in a box that is too small. It won’t fit if you lay it flat, but if you fold it up or mash it in the box, it fits nicely. Because of our unique brain structure, the massive amount of cells in the cortex is one of the reasons the cortex is believed to be where higher thought is processed.
The cortex of the human is only about twice as thick as the rodent. Most mammal brains have long white tracks of axons from neurons extending out of or in to the cortex from our sensory and motor functions. However, if you flattened out all the folds and spread out the cortex like a cookie sheet, the human cortex would take up about 400–500 times more area than a rodent cortex. Of course, this isn’t as impressive when one considers the size of an elephant brain. Complex cortical folds exist in whales, elephants, and dolphins. For all we know, an elephant might have finally solved many of the problems of quantum theory. Although the cortical area from an elephant brain would probably take up more area, the human brain has a significantly higher glia-neuron ratio in the cortex.
After
FIGURE 7.1 A neuron climbing like a squirrel up its radial glia mother tree
Reprinted from Brain Research Reviews, Vol. 55, Issue 2, Rakic, P., “The Radial Edifice of Cortical Architecture: from Neuronal Silhouettes to Genetic Engineering,” p. 208, Copyright (2007) with permission from Elsevier.
The radial glial cell is also a mother in the sense that it guides its progeny to their places in the world. In this case, the world is our brain. After each new cell migrates up the radial glia to its new position in space, it morphs into a neuron. When mapping the cortex in the late nineteenth century, Cajal did so based on connections neurons make when bringing information from the sensory and sending to the motor environments. Based on neuronal connectivity, the cortical layers are numbered 1–6. Layer 1 is closer to the outside surface of the brain; layer 6 is deeper inside the brain.
Layer 6 forms first in development. Radial glial extend short processes near the ventricle, and bulbs of cells are laid down like seeds. At this point, as the fetus is bathed in embryonic fluid, the ventricular fluid takes up almost the entire space of the peanut-sized brain.
For the next 100 days, radial glia extend further out of the brain. They push through already established layers, through its continual neuronal offspring to what will be the outside of the brain. After the last layer is formed (layer 1, which is at the extreme outside of the brain), the neuronal support infrastructure of the cortex has been laid down like a map. In the second month of pregnancy, at the peak of proliferation, it is estimated that 200,000 neurons bud every minute.
The explosion of cell proliferation in primates compared to other animals is believed to be the reason for our unique folds of the cortical surface when we observe a brain removed from its skull. In a rat, the cortex has no folds and is simply an outer sheath covering the basal areas of the brain. However, in humans, the cortex folds and refolds to fit all the cell bodies in the cortex. It can be compared to shoving a crumpled blanket in a box that is too small. It won’t fit if you lay it flat, but if you fold it up or mash it in the box, it fits nicely. Because of our unique brain structure, the massive amount of cells in the cortex is one of the reasons the cortex is believed to be where higher thought is processed.
The cortex of the human is only about twice as thick as the rodent. Most mammal brains have long white tracks of axons from neurons extending out of or in to the cortex from our sensory and motor functions. However, if you flattened out all the folds and spread out the cortex like a cookie sheet, the human cortex would take up about 400–500 times more area than a rodent cortex. Of course, this isn’t as impressive when one considers the size of an elephant brain. Complex cortical folds exist in whales, elephants, and dolphins. For all we know, an elephant might have finally solved many of the problems of quantum theory. Although the cortical area from an elephant brain would probably take up more area, the human brain has a significantly higher glia-neuron ratio in the cortex.
After
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