Human embryos form a network of neurons from radial glial cells, which are stem cells. Radial glia appear in the 7th week of gestation, as the embryotic brain sprouts and grows.
Cell division creates a neuron connected to its glial mother cell. Initial transmitter expression in the womb is calcium dependent.
After a neural net is properly placed throughout the body, the final trimester of pregnancy furthers hooks up the neural connections to the brain. Astrocytes grow and fill niches near support neurons.
Astrocyte proliferation coincides with birth. Radial glia stop their neural network from creating divisions and turn themselves into astrocytes.
The explosive growth of the human brain in the 1st year after birth owes to astrocyte propagation. Meantime, nerve cell growth is fractional.
Humans begin to experience dreams and can retain long-term memories by around age 4, after glia grow and establish themselves postnatally. If neurons held memories, humans could recall being in the womb.
Nerve cells predominate in the cortex, which is gray matter. Cortex development increases to about age 8, then the brain becomes more streamlined. An adult cortex is considerably smaller than that of an 8-year-old.
Learning results in a temporary increase in neurons in the affected area. But as the learning takes hold and becomes rote, the neural pathways streamline: neurons atrophy and lessen in number. Meanwhile, more glia grow and remain robust with learning.
The cortex thinning that occurs from childhood is mostly apoptotic neuron loss, as streamlining of neuron-astrocyte synaptic contacts.
Impairments that we see in autism seem to be partly due to different parts of the brain talking too much to each other. You need to lose connections in order to develop a fine-tuned system of brain networks, because if all parts of the brain talk to all parts of the brain, all you get is noise. ~ neurobiologist Ralph-Axel Müller
Autism arises with a failure to prune neurons.
More is not better when it comes to synapses, for sure, and pruning is absolutely essential. ~ American molecular biologist Lisa Boulanger
Smarter children experience accelerated neural thinning. What neurobiologists believed to be neuronal plasticity is actually glial growth and signaling refinement via nerve cell pruning.