Spanish Translation also available here: Neuro Cartilla: Neurodesarrollo
How does your brain become a brain?
Neurodevelopment is the crucial process through which every brain is formed. This process facilitates the maturation of an embryo into distinct brain cells that will subserve some of humanity’s most complex functions. Notably, development of the human brain follows the same principles as the development of most animal brains. Each rung of neurodevelopment is highly regulated by a diverse symphony of molecules and proteins orchestrated specifically to promote typical development. Many steps of neurodevelopment occur well before birth!
“How do cells choose whether they will become skin tissue or brain tissue?”
Early in development, the animal embryo becomes a neurula, a layered mass of cells that serves as the very first phase of the nervous system. The neurula’s layers of cells proceed to fold into a neural tube which contains cells that are the basis for the brain, spinal cord… and skin. This presents cells with a choice. How do cells choose whether they will become skin tissue or brain tissue? Diffusible molecules called morphogens regulate cell choices, guiding cells in their vicinity to become specific cell types. (For example, a morphogen curiously named Sonic hedgehog aids cells in becoming eventual interneurons and motor neurons!)
Once cells have chosen the path towards the nervous system, they must migrate to occupy the correct locations within the brain. The cortex is made of six layers that build from the inside-out similar to the way rock layer formations start at the bottom. Neuronal migration entails the movements of newborn neurons along a gradient from the “bottom” of the cortex up to the outer surface of the brain. At the surface, a stop signal called Reelin cues the cells to cease migration and begin elaborating.
“Akin to dating, growth cones must establish not just any connection with a target, but the right connection.”
Neuronal elaboration holds up to its name: If we think of elaboration as the addition of more detail, we can think of neuronal elaboration as the addition of more parts and processes to immature neurons. Specifically, these elaborations include structures known as axons and dendrites which are required for proper neuronal function. Both axons and dendrites stem from growth cones, extensions of the immature neurons that seek distinct targets in the cortical environment. Akin to dating, growth cones must establish not just any connection with a target, but the right connection. Also like a series of dates, the surrounding chemicals can be either attractive or repulsive, causing growth cones to partner up only when the connection lends to typical brain development. These growth cones will form direct connections called synapses, leaving the brain with even more cells and connections than necessary.
The final stage of development is the fight for neurotrophic support. Because development yields so many neurons and synapses, only the strongest connections will survive and flourish into maturity. Neurotrophins–small, basic secretory proteins that aid in cell survival–are present in the environment and promote the survival of some neurons over others. The neurons lacking support undergo apoptosis, programmed cell death. The result of this well-controlled process is a brain capable of learning and changing with experience immediately after birth and throughout the lifetime.
Well after birth, brain development continues through synaptic growth and pruning, especially during infancy and childhood. Experiences during childhood, especially learning, strengthen the existing synapses and shape brain growth.
What part of brain development surprises you the most? Tell us in the comments!
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