Development of output connections from the inferior colliculus to the optic tectum in barn owls.

Abstract

We studied the development of the projection from the external nucleus of the inferior colliculus (ICX) to the optic tectum (OT) in the barn owl. The projection was labeled by tracer application in vitro to either the OT or the ICX, or by staining ICX cells intracellularly with biocytin. The axons of ICX neurons bifurcated into an ascending branch that projected toward the OT and a descending branch that coursed caudally to an unknown target in the brainstem. Axons of the ICX were observed to grow into the OT from embryonic day 16 (E16) on. From E22 on, side branches of the axonal projections could be found within the OT. At the day of hatching (E32), the projection displayed a dorsoventral topography comparable to the adult owl; however, atopically projecting cells remained. The complexity of the axonal arborization in the adult barn owl was found to be slightly increased compared with the hatchling. The terminal area of individual ICX cells in the OT of the adult barn owl was still broad, a finding that had not been expected from the sharply defined physiological response properties of the bimodal neurons in the space map of the OT. However, the width of the termination zone was in accordance with the large dendritic tree of the adult ICX cells, because both spanned comparable angles in their respective maps. Our data suggest that a coarse projection from the ICX to the OT can develop without coherent sensory input and may, therefore, be innately determined.