Neuronal microtubule-associated proteins in the embryonic avian spinal cord

We have used monoclonal antibodies to study the distribution of three developmentally regulated microtubule-associated proteins-MAP2, MAP5, and tau-during the morphogenesis of the thoracic spinal cord and peripheral nervous system in the quail. MAP5 is the only one of the three that is present in growing motor neuron processes in the day 3 embryo. The low-molecular weight form of MAP2, MAP2c, is found in motor neuron cell bodies at embryonic day 3. At later stages MAP2c appears in axons and in glia; it decreases in abundance between embryonic days 5 and 7. High-molecular weight MAP2 appears in motor neuron cell bodies and spinal cord gray matter at embryonic day 4, and is never encountered in axons. Tau is found in axons, but only at embryonic day 3.5, after they have commenced active extension. The molecular form and patterns of intracellular compartmentalization of each of the microtubule-associated proteins studied is conserved in mammalian and avian neurons. We conclude that MAP5 may be involved in the active growth of neuronal processes, whereas MAP2 and tau are not, and that high-molecular weight MAP2 and tau may stabilize dendritic and axonal processes, respectively.