In the developing brain, the subplate sits below the cortical plate, a precursor to the cortex. During some stages of development, the subplate is the largest layer of the brain—making its ultimate disappearance all the more confounding. “The understanding about the subplate was that it expands and then the cells of the subplate just die out. But we hypothesized: What if these subplate cells are not dying? What if they’re just moving to a different level of the cortex—becoming part of the cortex?” says Brivanlou.The researchers tried to duplicate the situation in vitro, and found that the subplate cells didn't die. They migrated outward after they had performed their main function of generating lots of other neurons. For clarity: the idea of migration is astonishing, and NOT part of conventional wisdom, but it is well-known in serious neurology. This research isn't claiming to discover the migration, only trying to see what happens after the migration. Here's the migration as currently understood. The subplate forms from the brainstem. It first builds the radial glia (the tracks) extending to the outer rim of the brain (the cortex). Then the subplate generates a few billion fresh neurons, each equipped to ride the tracks. These neurons ride to the frontier where they claim their homesteads. Previously it was believed that the subplate simply faded out after building the rails and sending the trains. This new finding indicates that the subplate's cells hop on the tracks and join the migration. Do they still have a special leadership role in the new territory? The article includes a little video of the (in vitro) subplate neurons migrating in open territory without rails. They wander in various directions, but all are traveling axon-first, perhaps showing that each neuron is seeking an appropriate destination for its signals rather than an appropriate source as you might assume**. In other words, each neuron knows its PURPOSE, its intended JOB, rather than its desired CONSUMPTION. = = = = = ** Later: Why is this a default assumption? I had to stop and think about it. When animals wander around, and when plant roots explore the soil, both are putting their sensors first. They're looking for food. These neurons are NOT leading with their sensing end (dendrites). Instead, they're pushing the purpose end (axon) out front. Later 2: How do the neurons wander? In the video they don't seem to be waving or whipping their axons. Are the dendrites acting as fins? Cilia on the sides of the soma? Prestin in the cell walls? Rocket reaction like squids? Magnetic or electrostatic gradients? The latter wouid be a direct use of a neuron's primary skill.
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