Terminal arborizations of retinotectal axons in the bullfrog.

Abstract

AbstractIn the optic tectum of Rana catesbeiana four laminae of myelinated fibers in the superficial zone of the optic tectum (laminae B, D, F, and G: Potter, ′69) are identified as retinal axons on evidence from patterns of degeneration following contralateral eye removal. After survival times of 5 to 22 days Cajal's block method II shows either fragmentation or abnormal beading of the axons in the four laminae and the paraffin‐Nauta method shows coarse granules, representing axonal debris, in these laminae.Golgi impregnations of terminal arborizations arising from fibers in laminae B, D, F, and G show three major types of branching patterns. The arbors are horizontally flattened and are elongated in the same direction as the parent fibers. Densely branched (DB) arbors have stem fibers in laminae B and D and are distributed within the same and adjacent laminae. Widely branched (WB) and thin, straight (TB) arbors have stem fibers in laminae F and G and are distributed mainly in the two laminae of origin with some overlap into adjacent layers of cells and fibers. The horizontal dimensions of DB arbors are 30 μ–70 μ by 100 μ–200 μ, whereas the WB arbors are more variable in overall dimensions. The TB arbors consist of long thin stem axons that give off infrequent terminal specializations in the form of forked or spiny appendages. Numerous terminal forks are present on the WB arbors but are rare on the DB arbors. Swellings and varicosities, that probably represent synaptic specializations, are particularly numerous on the long tertiary branches of DB arbors.