Errors in lamina growth of primary olfactory axons in the rat and mouse olfactory bulb

In the adult olfactory nerve pathway of rodents, each primary olfactory axo n forms a terminal arbor in a single glomerulus in the olfactory bulb. Duri ng development, axons are believed to project directly to and terminate pre cisely within a glomerulus without any exuberant growth or mistargeting. To gain insight into mechanisms underlying this process, the trajectories of primary olfactory axons during glomerular formation were studied in the neo natal period. Histochemical staining of mouse olfactory bulb sections with the lectin Dolichos biflorus-agglutinin revealed that many olfactory axons overshoot the glomerular layer and course into the deeper laminae of the bu lb in the early postnatal period. Single primary olfactory axons were anter ogradely labelled either with the lipophilic carbocyanine dye, 1,1'-dioctod ecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI), or with horse radish peroxidase (HRP) by localized microinjections into the nerve fiber l ayer of the rat olfactory bulb. Five distinct trajectories of primary olfac tory axons were observed in DLI-labelled preparations at postnatal day 1.5 (P1.5). Axons either coursed directly to and terminated specifically within a glomerulus, branched before terminating in a glomerulus, bypassed glomer uli and entered the underlying external plexiform layer, passed through the glomerular layer with side branches into glomeruli, or branched into more than one glomerulus. HRP-labelled axon arbors from eight postnatal ages wer e reconstructed by camera lucida and were used to determine arbor length, a rbor area, and arbor branch number. Whereas primary olfactory axons display errors in laminar targeting in the mammalian olfactory bulb, axon arbors t ypically achieve their adult morphology without exuberant growth. Many olfa ctory axons appear not to recognize appropriate cues to terminate within th e glomerular layer during the early postnatal period. However, primary olfa ctory axons exhibit precise targeting in the glomerular layer after P5.5, i ndicating temporal differences in either the presence of guidance cues or t he ability of axons to respond to these cues. (C) 1999 Wiley-Liss, Inc.