Subplate neurons -- once thought to die after directing the wiring of the cerebral cortex or gray matter-- remain in the white matter of the adult brain in small numbers and maintain activity, communicating with other neurons in the brain said researchers from Baylor College of Medicine and the University of Alabama at Birmingham in an article that appears in the Journal of Neuroscience.
The finding -- that approximately 10 percent of the cells survive and have functional connections -- opens the door to new ways of thinking about fixing injured brains, said Dr. Michael Friedlander, chair of the department of neuroscience at BCM, and senior author of the paper. "Since those cells are critical elements that guided the wiring of the brain's cerebral cortex in the first place, maybe we could tap into that ability later on."
However, he emphasized that this just a hypothesis and has yet to be proven. Friedlander credits an M.D./Ph.D. student of his, Dr. Juan Torres-Reveron, with coming up with the notion that the surviving subplate neurons are electrically active and in chemical communication with their neighbors and then proving it in the laboratory. Torres-Reveron received his M.D. from Baylor College of Medicine and his Ph.D. from UAB. He is now a neurosurgical resident at Yale University School of Medicine.
The finding challenges dogma about the elusive cells that serve as guidepost for the nerve fibers growing into the cerebral cortex during embryonic development. The cerebral cortex forms a mantle of a few millimeters in the brain composed of "gray matter," the classical six layers of nerve cells, and the underlying "white matter," composed primarily of the nerve fibers connecting the cortex to other areas of the brain and their insulating layers of myelin derived from supporting glial cells.
The cortex governs higher-order functions like visual perception, language and planning actions. Scientists have long known that when the brain is developing in a fetus, a group of cells underneath the cortex -- in what is called the subplate -- serve as a kind of transitory guidepost for nerve fibers, leading them to go into the right places to "wire up" the brain, said Friedlander.
"Lots of subplate cells die through a normal process of programmed cell death during early brain development, but about 10 percent of them stay and persist into adulthood," said Friedlander. "Although their role as transient guide targets for ingrowing nerve fibers in the fetal brain has been well characterized previously, the functional properties of these cells in the postnatal brain have remained elusive."
Torres-Reveron thought he saw the cells in the white matter of more mature postnatal brains. He found a way to visualize these cells in isolated sections of the brains of animals and target them with a micropipette under a microscope for analysis of their functional properties.
When he found one of the cells, he had to guide his micropipette to it and then record the cell's electrical activity and chemical inputs from neighboring brain areas in order to analyze the cell's properties.
He found that the surviving subplate cells generated electrical signals, just like other nerve cells, and they receive input from other cells. Their inputs also undergo plasticity, effectively changing the strength of inputs from other neurons based on experience and activity. This process is widely thought to be a critical component for the formation of memory, said Friedlander.
"Here we have a cell that at one stage of life is performing one function to help assemble the brain and then takes on another function later in life," said Friedlander.
Funding for this study came from the National Eye Institute of the National Institutes of Health.
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