Cytoarchitectural fields and retinal termination: an axonal transport study of laminar organization in the avian optic tectum.

The cytoarchitecture in the retinoreceptive zone of the pigeon optic tectum has been studied in Nissl-stained sections taken in four planes. As suggested by a previous study, two cytoarchitectural fields are present. Reconstructed views of the tectum show that the fields are separated by a narrow transition zone approximating to the tectal representation of the retina's horizontal meridian. In field 1 (which is upper and rostral), sublayer IIb is wide, IIc wide and trilaminate, IId narrow and IIe continuous; in field 2, IIb and c are narrow, IId wide and IIe discontinuous. The distribution of retinal terminals was investigated by the anterograde axonal transport of [3H]proline or horseradish peroxidase from intravitreal injections. The depth distribution of grains or reaction product throughout the entire tectum was quantified by scanning with a microdensitometer. Both autoradiography and horseradish peroxidase transport show two patterns of lamination separated by a narrow transition zone and these two terminal fields correspond closely to the cytoarchitectural fields. In field 1 optic terminals are concentrated in sublayer IIb, superficial c, d, and to a lesser extent in f; in field 2 concentrations are present at the IIb/c boundary, across deep IIc and d, and a small concentration is found IIf. The patterns of retinal termination with depth in the tectum found by axonal transport are compatible with those found by electron microscopy, and are discussed in relation to the optic termination found by other techniques. Study of the time course of axonal transport shows that both radioactive material and horseradish peroxidase are fast transported to all the bands of optic terminals at about 150 mm/day. Horseradish peroxidase gradually accumulates in the retinoreceptive zone, filling clusters of terminals and horizontal processes. At 12 days, it has begun to disappear from the zone and a few diffusely filled profiles, that may be transcellularly labelled, are present. Electron microscope autoradiography of fast transported material shows clusters of grains over optic terminals and preterminals and a percentage density analysis confirms that these profiles are specifically labelled. The two tectal fields each contain the projection from specialized areas of the retina, suggesting functional specialization in the tectum for the processing of different kinds of visual information.