The basis of polarity in neurons.

It has been recognized since the very early studies on the cytology of vertebrate nervous systems that neurons produce two fundamentally different types of neurite, the axon and the dendrite. Contemporary studies using electron microscopy have defined in detail the many structural differences between axons and dendrites. Perhaps the best known of these differences concerns ribosomes and Golgi elements, which are present in dendrites, but are absent from axons. In this article, we present a possible explanation for this compartmentalization which is based on current understanding of organelle transport in cells and our recent demonstration of a fundamental difference in the organization of the microtubule-based transport systems that convey organelles into the axon or into the dendrite.