The telencephalic vesicles are innervated by olfactory placode-derived cells: A possible mechanism to induce neocortical development

Abstract

During early embryonic development, the olfactory placode is the source of different cell types migrating toward the telencephalic vesicle. Among these cell types are the ensheathing cells, the luteinizing hormone-releasing hormone-producing cells and the olfactory marker protein-immunoreactive cells. We have identified a novel group of olfactory placode-derived migratory cells using an antibody against beta-tubulin to label neurons and acetylcholinesterase histochemistry to label postmitotic cells. In this paper we describe the morphology, migration and fate of this novel group of cells. The first neurons detected in the rostral prosencephalon with acetylcholinesterase and anti-beta-tubulin antibody are localized in the olfactory placodes at embryonic day 11 in the rat. At embryonic day 12, anti-beta-tubulin antibody-positive cells were observed in the mesenchymal tissue between the olfactory pit and the rostral pole of the telencephalic vesicle. Anti-beta-tubulin antibody-positive cells were seen running superficially over the pial (dorsal) side of the telencephalic vesicle at embryonic day 13. The majority of these cells have a bipolar profile with short leading and trailing processes, suggesting that they are migratory elements. However, some of these cells showed elaborate processes extending for quite long distances, overlying the pial surface of the telencephalic vesicle. A mass of cells extending over the telencephalic vesicle from the developing olfactory epithelium were observed at embryonic day 13 using acetylcholinesterase histochemistry. Some of these acetylcholinesterase-positive cells were identified as neurons with the specific neuronal marker anti-beta-tubulin antibody. On embryonic day 12, neurons from the olfactory epithelium send axonal fibers toward the telencephalic vesicles. Most of these fibers spread over the anteroventral pole of the vesicles but others entered deep into the telencephalon, reaching the germinal ventricular zone. We also show that fibers run rostrocaudally over the surface of the telencephalic vesicles. We suggest that these cells and fibers, apparently originating in the olfactory placode and migrating through non-conventional routes, might play a significant role in the earliest stages of telencephalic vesicle development.