Development of a glia-rich axon-sorting zone in the olfactory pathway of the moth Manduca sexta

Olfactory receptor cells (ORCs) of a particular odor tuning are dispersed i n the olfactory epithelium, but their axons converge on distinct glomeruli in primary olfactory centers. As a consequence, axon associations must chan ge to bring axons of ORCs with the same odor specificity together. Studies in Manduca sexta have indicated that just before they enter the antennal lo be (AL), ORC axons undergo extreme reorganization, finally entering the AL in fascicles destined for subsets of glomeruli. This axon-sorting zone is h eavily populated by glial cells, and ORC axon growth cones often are in clo se physical contact with the glia. In moths rendered glia deficient, ORC ax ons fail to fasciculate in this region. Using propidium iodide to label nuc lei and 5-bromo-2'-deoxyuridine to monitor proliferation, we found that the glia in the sorting zone arise from the AL, appearing shortly after the fi rst ORC axons arrive. Experimental removal of some or all of the sensory in nervation revealed that proliferation of sorting-zone glia is triggered by ORC axons. A second set of glia arises in the antenna and migrates along th e antennal nerve toward the brain, populating the nerve after the establish ment of the sorting zone. Development of this type of glial cell is indepen dent of contact of the ORC axons with their central targets. We conclude th at the sorting zone arises from CNS glia in response to ingrowth of ORC axo ns, and a critical number of glia must be present in the sorting zone for a xons to correctly establish new neighbor-neighbor associations.