Construction of a protoglomerular template by olfactory axons initiates the formation of olfactory glomeruli in the insect brain

Olfactory glomeruli in insects share many features of organization with their vertebrate counterparts, and yet offer distinct advantages for study of neuronal development. Previous studies have revealed that the olfactory lobes of the brain of the moth Manduca sexta arise postembryonically and that glomeruli in the lobe are induced by olfactory afferent axons (Hildebrand et al., 1979; Oland and Tolbert, 1987). In the present study, we have used the Golgi method, intracellular labeling of neurons with Lucifer yellow, and electron microscopy to follow neuronal development in the antennal lobe through the period when glomeruli develop. Our results, taken together with other results from our laboratory, suggest that olfactory sensory axons have the intrinsic ability to form protoglomeruli, and that an interaction between these axons and glial cells (but not the majority of the neurons of the antennal lobe) causes the glial cells to surround the protoglomeruli. Ingrowth of the neurites of most antennal-lobe neurons into the protoglomeruli occurs after a small delay and appears to be constrained to glomerular units by the presence of the glial boundaries. Synapses, initially not detected in the protoglomeruli, begin to appear as soon as the neuntes of antennal-lobe neurons appear in the glomeruli. Thus, antennal axons, instead of immediately seeking out postsynaptic targets, first form the template for organization of future glomeruli. The neuntes of most of the neurons of the antennal lobe grow outward to meet the olfactory sensory axons, and in doing so, join with these axons to form glomeruli. Preliminary results concerning the development of a second class of neuron in the lobe, the projection neurons, indicate that at least some of these neurons may arborize in the region of the protoglomeruli very early and therefore participate with the afferent axons in laying the foundation for glomeruli.