INTERTECTAL COMMISSURAL PROJECTION IN THE LIZARD GALLOTIA-STEHLINI - ORIGIN AND MIDLINE TOPOGRAPHY

The retinotectal projection of reptiles is largely crossed. The intert ectal commissure is an important pathway that interconnects directly t he two sides of the optic tectum. The rostrocaudal topography of inter tectal commissural fibers at the dorsal midplane was examined by means of the in vitro horseradish peroxidase (HRP) labelling technique in t he lizard Gallotia stehlini. Unilateral large deposits of tracer in th e optic tectum as well as smaller deposits restricted to one quadrant were used to map the intertectal fibers anterogradely. Most commissura l axons reached the contralateral side grouped into a dense bundle at the transition between two structurally distinct parts of the midbrain dorsal midline. The smaller rostral zone relates laterally to the gri seum tectale, whereas the larger caudal zone relates to the tectum. Th e intertectal fibers seem to converge on the rostralmost part of the l atter midline region, even though they originate throughout the optic tectum. A rough rostrocaudal tectotopic order was detected at the midl ine. Retrogradely labelled neurons were best obtained by depositing HR P directly within the compact commissure at the midline. These belong to pyriform cells in the periventricular layers 3 and 5. Axons labelle d from the tectum did not enter the posterior commissure nor the inter vening commissural region related to the griseum tectale. (C) 1996 Wil ey-Liss, Inc.