Electron microscopy of experimental degeneration in the avian optic tectum.

TLDR: In this paper, the authors used the simplified classification of the tectal layers used by Cragg, Evans & Hamlyn (1954) modified from van Gehuchten (1892) to study the effects of axons and their terminals.

Abstract: One of the major problems of electron microscopy of the central nervous system is to locate with precision the region involved in the degenerative process in grey or white matter. For this reason, the avian optic tectum was chosen to studythe effects of degeneration of axons and their terminals. Axon section is accomplished simply by unilateral removal of the eye so that the optic nerve afferent fibres to the contralateral hemisphere undergo degeneration (Evans & Hamlyn, 1956). Their trunks are easily located by electron microscopy since they enter the tectum as a superficial layer of myelinated fibres and the terminal ramifications and presynaptic processes are also easily located by reference to a discrete double layer of neuronal perikarya marking the deepest limits of their distribution (Evans & Hamlyn, unpublished; Cowan, Adamson & Powell, 1961). This present work follows naturally from the light-microscopic degeneration studies of Evans & Hamlyn (1956) with the Glees (1946) and Nauta-Gygax (1954) methods. The time courses of the two methods are quite distinct, the Glees method showing rings and clubs, absent from normal tectum, at the 7-11-day stage. At 28-30 days the Glees method is negative, but the Nauta-Gygax picture is fully established. Here electron microscopy has been used to follow these changes in the axons and their presynaptic processes in order to relate them to the different mechanisms of the Glees and Nauta techniques. Few attempts have been made so far to study central nervous degeneration with the electron microscope. De Robertis (1956) has described experimental changes in the ventral acoustic nucleus. Bunge, Bunge & Ris (1960, 1961) have reported on experimental demyelination and remyelination in the spinal cord, the axons remaining apparently unaffected. In this account the simplified classification of the tectal layers used by Cragg, Evans & Hamlyn (1954) modified from van Gehuchten (1892) will be used.