Ingrowth of optic nerve fibers and onset of myelin ensheathment in the optic tectum of the trout (Salmo gairdneri).

The early differentiation of the optic pathway of the trout was studied by means of autoradiography, silver impregnation and electron microscopy. Ingrowth of the optic nerve fibers into the optic tectum was consistently shown by tracer application and Golgi studies to occur at stage 28, about one week before hatching. Fibers being arranged in discrete bundles were rapidly growing through the longitudinal axis of tectum and at stage 33 reached its posterior end. Cross sections of these fiber bundles at different positions revealed myelin ensheatment to be initiated at the end of stage 34 at the anterior pole of the tectum. Since in the optic nerve of the trout the onset of myelination occurred even earlier (stage 33), it is assumed that this differentiation process follows a rostro-caudal gradient during development of the optic pathway.