Developmental expression of a neuron-specific beta-tubulin in frog (Xenopus laevis): a marker for growing axons during the embryonic period.

In mammals, there are seven classes of beta-tubulin genes, one of which, class III, is neuron specific. Using class-specific monoclonal antibodies, class III beta-tubulin protein could not be detected in frog embryos or in adults with either Western blotting or immunohistochemical techniques. In contrast, the class II beta-tubulin protein, which is predominant in mammalian brain but is also expressed in other tissues, is expressed only in neurons in frog embryos. Protein was detected only in neurons from late stages of neural tube closure through premetamorphic stages. At stages 21-28, the pioneering axons of Rohon-Beard, commissural, primary motor, and trigeminal ganglion neurons were distinctly stained in the axon scaffolds that they formed in the embryonic brain and the peripheral mesenchyme. Nonneuronal cells, both outside the nervous system and within it (e.g., radial glia, Müller glia, roof plate, and floor plate cells) were not immunoreactive. Throughout swimming and premetamorphic stages, neuronal cells in all brain regions became immunoreactive as they differentiated and extended axons. Whereas many embryonic neurons became postmitotic during gastrulation stages, neurons expressed detectable levels of class II beta-tubulin protein only beginning at the onset of overt axon outgrowth. These observations demonstrate that the neuron-specific beta-tubulin in frog is a different gene from that in mammals, and its protein product is detectable at the time of axonogenesis rather than neurogenesis.