Importance of timing of olfactory receptor-axon outgrowth for glomerulus development in Manduca sexta

In the moth Manduca sexta, development of glomeruli in the antennal (olfact ory) lobes (ALs) follows a precise timetable and involves interactions of o lfactory receptor cell (ORC) axons with AL glial cells and neurons. To stud y the importance of timing for these intercellular interactions, we experim entally desynchronized the development of the ALs and the ORCs by altering the temperature of the developing antenna and brain for defined periods of time during development. Selective cooling of the antenna relative to the b ody resulted in a delay of ORC-axon outgrowth, and slightly warming the ant enna while cooling the body caused precocious ingrowth of axons into the AL . Whereas cooling of the antenna for 24 hours caused only a delay in the fo rmation of glomeruli, cooling for 48 hours led to significant disruption of glomerular development. Glial cells did not form normal glomerular borders , and glomeruli were shaped abnormally. Axons of pheromone-specific ORCs pr ojected to their correct target, but terminal branches within the macroglom erular complex (MGC) were not clearly segregated. The results suggest that proper formation of glial glomerular borders requires interaction of ORC ax ons and glial cells within a sensitive period, whereas targeting of ORC axo ns appears to be effective over extended periods in development. Precocious ingrowth of ORC axons after warming the antenna and cooling the body for 4 8 hours resulted in enlarged protoglomeruli. Glial borders formed normally, but a subpopulation of MGC-specific ORC axons grew past the MGC. The decre ased accuracy of targeting in these cases suggests that targeting mechanism s are not fully developed before the time when ORC axons normally would ent er the brain. J. Comp. Neurol. 425:233-243, 2000. (C) 2000 Wiley-Liss, Inc.