Patterns of myelination in the opossum superior colliculus with additional reference to the optic tract

Patterns of myelination have been studied in the optic tract and the superior colliculus (SC), with special reference to the optic layer, in the opossum Didelphis marsupialis. Myelination in the optic tract starts far in precedence of eye opening and follows a rostrocaudal gradient. Myelination in the SC presents the following features: it proceeds according to a general inside-out pattern and follows both rostro-caudal and latero-medial gradients in the optic layer, and it accelerates in the SC optic layer soon after systematic exposure to visual input. The data presented here, together with other available information, suggest that myelination in the opossum optic tract starts in parallel with the stabilization in the number of optic fibers, and advances in the rostro-caudal mode common to most eutherian mammals, and also that myelogenesis in the SC neither correlates necessarily with, nor recapitulates, the sequence of acquisition of GFAP-positive astrocytes in a given set of layers. Changes in the rate of myelination in the optic layer after exposure to visual input are regionally-selective, and seem compatible with the recruitment of thin axons into the myelogenetic cycle rather than with the thickening of pre-existing myelin sheaths. It is concluded that the SC is a favorable structure for the study of the differentiation of glial cells, particularly in species with an extended time course of maturation such as the opossum.