PLASTICITY OF CONNECTIONS OF THE OLFACTORY SENSORY NEURON - REGENERATION INTO THE FOREBRAIN FOLLOWING BULBECTOMY IN THE NEONATAL MOUSE

Neonatal mice underwent unilateral bulbectomy, which included the main and accessory olfactory bulbs. From 5 days of survival onward, there was a marked anterior displacement of the frontal cortex into the cavity previously occupied by the bulb. As a result of the bulbectomy and consequent damage to olfactory axons, the olfactory perikarya underwent retrograde degeneration. New neurons were then reconstituted from stem cells within the olfactory neuroepithelium. By 20 postoperative days the new olfactory axons had reached the level of the lamina cribrosa and by 30 days the fibers had penetrated into the telencephalon and had formed typical glomerular structures within the host tissue. Fibers were directed to either the paleo- or neocortex where they were observed in close proximity to large cortical neurons. The formation of glomeruli persisted over the course of the study (180 days) and showed an expansion within the cortical tissue up to 60 days of survival. The identification of these fibers and glomerular structures as olfactory was confirmed by immunohistochemical techniques using antisera to the specific olfactory protein. Ultrastructural observations clearly indicated the typical glomerular pattern of the structures and demonstrated synaptic contacts between the sensory terminals and dendritic processes, as yet unidentified, originating from the surrounding cerebral matrix. Our observations thus demonstrate that following bulbectomy and retrograde degeneration of olfactory neurons, the cells can regenerate in the absence of their normal target. Furthermore, the newly formed axons can penetrate a 'foreign' environment, the cerebral cortex, and form typical glomerular structures and corresponding sensory synapses. The findings suggest a heretofore unsuspected degree of plasticity in the olfactory system as well as in the cerebral cortex. © 1979.