Microglia in tadpoles of Xenopus laevis: normal distribution and the response to optic nerve injury.

We have studied the distribution of microglia in normal Xenopus tadpoles and after an optic nerve lesion, using a monoclonal antibody (5F4) raised against Xenopus retinas of which the optic nerves had been cut 10 days previously. The antibody 5F4 selectively recognizes macrophages and microglia in Xenopus. In normal animals microglia are sparsely but widely distributed throughout the retina, optic nerve, diencephalon and mesencephalon (other regions were not examined). After crush or cut of an optic nerve, or eye removal, there occurs an extensive microglial response along the affected optic pathway. Within 18 h an increase in the number of microglial cells in the optic tract and tectum can be detected. This response increases to peak at around 5 days after the lesion. At this time the nerve distal to the lesion contains many microglial cells; the entire optic tract is outlined by microglia, extended along the degenerating fibres; and the affected tectum shows a heavy concentration of microglia. This microglial response thereafter decreases and has mostly gone by 34 days. We conclude that the microglial response to optic nerve injury in Xenopus tadpoles starts early, peaks just before the regenerating optic nerve axons enter the brain, and is much diminished by the time the retinotectal projection is re-established. The timing is such that the microglial response could play a major role in facilitating regeneration.