E14 Neurodevelopment

Topic

Developing nervous tissue undergoes five sequential processes: first, neurogenesis—specifically, the proliferation and death of neurons within the tissue; next, the cellular migration of these neurons, guided by molecular gradients within the tissue; then, an initial overproduction of synapses, followed by activity-dependent pruning; subsequently, the progressive myelination of axons; and finally, temporal windows of heightened plasticity known as critical periods, during which the nervous tissue is particularly sensitive to experience.

The net neuronal population of the nervous tissue results from the balance between two opposing processes: the logistic proliferation of neurons—limited by the tissue's capacity—which grows rapidly at first but levels off as the population approaches that maximum limit; and a rate of programmed cell death that steadily reduces the neuronal population. The closer the net neuronal population comes to the tissue's capacity, the slower the population grows via logistic proliferation, while programmed cell death continues to deplete the number of neurons.

Neuronal migration within the nervous tissue can be modeled using a reaction-diffusion equation—the same type of equation previously used to describe the propagation of electrical discharges in another model—but here it is biased by an attractive chemical gradient: migrating neurons preferentially move toward areas where this attractive chemical gradient is strongest. For its part, the synaptic density of nervous tissue follows a pattern of synaptic overproduction followed by pruning that depends on the correlation between presynaptic and postsynaptic activity at each synapse; this pruning follows a rule formally related to the synaptic plasticity previously described for synaptic weight in another model, yet it operates in the opposite direction: rather than strengthening the most highly correlated synapses, this pruning process specifically eliminates the least correlated ones.

Myelination in nervous tissue is modeled as a sigmoidal growth curve relative to age: it increases slowly during early development, accelerates during an intermediate period, and finally levels off as the nervous tissue approaches adulthood.

In contrast, the critical plasticity window of nervous tissue is modeled as a bell-shaped temporal profile centered on a peak age: plasticity is low at early ages, rises to a maximum at that peak age, and subsequently declines as the nervous tissue continues to mature. Ultimately, it is this very nervous tissue—with its net neuronal population, cellular migration, synaptic density, myelination, and critical plasticity window—that undergoes the entire process of nervous system development in this sequence.

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