Glial cells stabilize axonal protoglomeruli in the developing olfactory lobe of the moth Manduca sexta

Odor information is processed in spherical structures called glomeruli, which in all animals with differentiated olfactory systems are sites of densely spaced synaptic contacts between olfactory sensory axons and target central nervous system (CNS) neurons. Glomerulus development in the antennal (olfactory) lobe of the moth brain, which is initiated by the arrival of antennal receptor axons, requires interaction among three elements: glial cells, receptor axons, and their targets, the antennal-lobe neurons. Receptor axons form an array of protoglomeruli that become surrounded by glia and serve as a template for mature glomeruli. Previous experiments showed that when the number of glial cells is sharply reduced during development either by irradiation or by an anti-mitotic agent, receptor axons form protoglomeruli, but in the mature lobes, glomeruli are absent and central neurons lack the characteristic glomerular tufting of their arbors. The current investigation was conducted to determine which cellular events in the process of glomerulus formation are disrupted by severe reduction in glial-cell number. The branching patterns of receptor axons and antennal-lobe neurons were examined in animals that had been irradiated to produce glia-deficient antennal lobes at stages during which glomeruli normally would develop. We found that the receptor axons did form protoglomeruli, but that the protoglomeruli quickly disintegrated in glia-deficient antennal lobes; the receptor axons branched diffusely, except where several neighboring glia survived irradiation and together formed a wall of processes that appeared to block the passage of neuronal processes. Multi-glomerular antennal-lobe neurons never developed tufted arbors even at early stages. These results suggest that maintenance of protoglomeruli depends on the border of glia that forms around each protoglomerulus and that the subsequent tufting of antennal-lobe neurons depends on maintenance of the protoglomerular template during the period of dendritic growth.