Ultrastructure of the crossed isthmotectal projection in Xenopus frogs.

The nucleus isthmi (NI) of frogs is a relay for input from the eye to the ipsilateral tectum; each NI receives retinotopic input from one tectum and sends retinotopic output to both tecta. The crossed isthmotectal projection in Xenopus displays tremendous plasticity during development. Physiological and anatomical studies have suggested that the location at which a developing isthmotectal axon will terminate is determined by the correlation of its visually evoked activity with the activity of nearby retinotectal terminals. What structures could mediate such communication? We have examined quantitatively the ultrastructural characteristics of crossed isthmotectal axons and synapses in order to determine whether retinotectal axons communicate directly with isthmotectal axons via axo-axonic synapses or whether the communication is indirect, e.g., via common postsynaptic dendrites. Our results support the conclusion that isthmotectal axons interact with retinotectal axons indirectly and that tectal cell dendrites are the critical site of interaction.