Target recognition in neurogenesis: formation of the Mauthner axon cap.

To learn whether presence of a specific neuron, the Mauthner (M) cell, is required for the organization, during embryogenesis, of an associated synaptic neuropil, the M-axon cap, M-cell precursors were experimentally deleted in embryos of the zebra fish (Brachydanio rerio) and the axolotl (Ambystoma mexicanum). Examination of early larvae revealed that in about half of the cases M-axon caps were present in the absence of the corresponding M-cells. The locations and structures of such caps were approximately normal. We conclude that the M-cell is not a necessary target for normal development of the axon cap.