Neuronal polarity: demarcation, growth and commitment

In a biological sense, polarity refers to the extremity of the main axis of an organelle, cell, or organism. In neurons, morphological polarity begins with the appearance of the first neurite from the cell body. In multipolar neurons, a second phase of polarization occurs when a single neurite initiates a phase of rapid growth to become the neuron's axon, while the others later differentiate as dendrites. Finally, during a third phase, axons and dendrites develop an elaborate architecture, acquiring special morphological and molecular features that commit them to their final identities. Mechanistically, each phase must be preceded by spatial restriction of growth activity. We will review recent work on the mechanisms underlying the polarized growth of neurons.

► Neuron polarization consists of three phases: demarcation, growth and commitment.
► Phase I involves the growth of one neurite from a site defined at the precursor stage.
► Regulation of growth-inhibitory and growth-promoting mechanisms controls axon specification.
► Both preexisting and de novo mechanisms determine final axon and dendrite identity.