A Golgi-study of oculomotor neuroblasts migrating across the midline in chick embryos.

The Golgi-Stensaas impregnation technique was employed at appropriate stages of development to study the morphology of the oculomotor neuroblasts as these migrate across the midline. Data reported in previous publications were confirmed, such as the timing of the migration (occurring between the 4th and the 9th days of inoculation), the fact that the migrating cells carry their axons across the midline as trailing processes, and the absence of pre-existing fibrillar structures able to provide contact guidance for this migration. The most striking new fact discovered is that the leading processes of the oculomotor neuroblasts are often branched, and that all these branches are uniformly oriented towards the midline. This seems to indicate the existence of a non-random growth process. It is argued in the discussed that this can be explained through the presence of an orienting neurotropic influence. This one could have its source in certain non-oculomotor neuroblasts which were detected within the midline ventricular zone. On morphological grounds, this group of cells may be tentatively identified as the avian counterpart of the midventral mesencephalic proliferation described in several mammals. This proliferative zone is known to contain dopamine at early stages of development. A hypothetic casual mechanism of the oculomotor migration is therefore advanced, wherein dopamine, diffusing out of the non-oculomotor midline neuroblasts, induces at short range oriented outgrowth of oculomotor leading processes across the midline.