A Phaseolus lectin anterograde tracing study of the tectorotundal projections in the domestic chick.

The projection from the chick optic tectum to the nucleus rotundus was investigated by iontophoresis of the anterograde tracer Phaseolus vulgaris lectin (PHA-L) into the optic tectum and by Golgi impregnation. PHA-L labelled fibres were observed within the ipsilateral nucleus rotundus and to course through the supraoptic decussation to the contralateral nucleus rotundus. They appeared similar to fibres evident in Golgi impregnated material. Before reaching the nucleus rotundus, PHA-L labelled fibres gave off short terminal branches in the caudoventral thalamic nucleus. Within the ipsilateral nucleus rotundus, the labelled fibres formed an extensive fibre net with a dense core and a less dense periphery. PHA-L labelled fibres in the contralateral nucleus rotundus exhibited a qualitatively similar but sparser distribution. There appeared to be a topographic representation of the optic tectum within the nucleus rotundus. Electron microscopy revealed that PHA-L labelled tectal fibres established asymmetric synapses with principal neurons in the nucleus rotundus.