Structural characteristics of cell and fiber populations in the optic tectum of the frog ( Rana catesbeiana )

In the optic tectum Cajal's method II for neurofibrils reveals two major fiber systems in which the fibers are oriented perpendicular to one another. The outer fiber system , composed of seven laminae, lies in the superficial zone with its fibers oriented in the anteroventral to posterodorsal direction; Golgi preparations show that optic tract fibers enter the outer system laminae at the tectal margin, run within single laminae, and end in arborizations that are elongated in the dominant fiber direction. The intermediate fiber system is composed of bundles of fibers in lamina 7 of Ramón and the superficial zone; its fibers are oriented in the anterodorsal to posteroventral direction. A third deep fiber system , forms fibrous laminae in the region between lamina 7 and the ependyma. In Golgi impregnations the deep cells are of two major classes: pyramidal cells with relatively narrow dendritic arbors in the superficial zone and candelabra cells with broad arbors. Many pyamidal cell dendrites have lamellar branching patterns such that terminal bushes occur within one or another of the outer system laminae. The multipolar cells within lamina 6 and lamina 7 of Ramón send large, unbranched axons directly into the deep medullary lamina and appear to be the major efferent neurons. Near the pia the perikarya of superficial granule cells give rise to single processes that branch repeatedly within the superficial zone.