The retinotectal projections in the pigeon. An experimental optical and electron microscope study

Abstract

Optical and electron microscope studies have been made of the termination pattern of the retinotectal projection in the pigeon. The use of Golgi impregnation and orthograde transport of peroxidase concurred to show that: (i) the majority of retinotectal preterminal segments descend obliquely rather than radially through the superficial layers of the tectum opticum; (ii) terminals in Cajal's layers 2–3 and 7 arborize tangentially, terminals in layer 4 arborize diffusely in a loose way, and, in layer 5, terminal arborizations are arranged in radial columns in which terminal ramifications are tightly packed. Radioautographic labeling of the retinotectal terminals confirmed that they do not extend beyond layer 7. It demonstrated that the total thickness of the superficial tectum opticum tends to decrease from rostral to caudal, and that the density of retinotectal terminals is higher in the plexiform 3rd and 5th layers than in the 4th and 7th layers. Fink-Heimer staining of the terminal degeneration consecutive to an enucleation confirmed the results of radioautographic studies. It showed that the evolution of the silver impregnation differs from one layer to the next. This evolution is roughly parallel in the 3rd and 5th layers on the one hand, in the 4th and 7th layers on the other hand. The electron microscope study disclosed three successive stages of terminal degeneration over the whole projection area. They respectively present a dilatation of synaptic vesicles, a hyperplasia of the neurofibrillar system, and a darkening of the axoplasm. The three stages do not appear simultaneously in the 3rd and 5th layers and in the 4th and 7th layers. Comparison between the evolution of the terminal degeneration at the electron microscopic level and that of Fink-Heimer staining at the optical microscope level shows that the Fink-Heimer technique impregnates the earliest stage of degeneration (with vesicle dilatation), but not that with neurofibrillar hyperplasia. Following the resorption of retinotectal boutons, most of the postsynaptic differentiations disappear. However, some persist and become partially reoccupied by small terminals. The possible origin of these new synaptic relations is discussed.