Microtubules and Neuronal Polarity: Lessons from Mitosis

Neurons are the principle signaling cells of the nervous system. In order to transmit signals, neurons stop dividing early in development and direct their efforts instead toward the elaboration of elongated cellular processes. A typical vertebrate neuron extends a single axon and many dendrites. Axons are long slender processes that are specialized to transmit information, while dendrites are short tapering processes that are specialized to receive and process information. Axons and dendrites differ from one another in many aspects of structure and function, and these differences are collectively referred to as the polarity of the neuron. Several fundamental questions regarding the generation of neuronal polarity have now been identified. How is one developing process selected to become the axon, while the others are selected to become dendrites? What accounts for the unique morphological and compositional features of axons and dendrites? Why are axons long and dendrites short? How do environmental factors affect the differentiation of axons and dendrites? How do the growth cones at the tips of developing axons find their appropriate target tissues? Several lines of evidence suggest that the cytoskeletal elements known as microtubules may be central to all of these issues.