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 (olfactory) lobes (ALs) follows a precise timetable and involves interactions of olfactory receptor cell (ORC) axons with AL glial cells and neurons. To study the importance of timing for these intercellular interactions, we experimentally 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 body resulted in a delay of ORC-axon outgrowth, and slightly warming the antenna 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 formation 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 projected to their correct target, but terminal branches within the macroglomerular complex (MGC) were not clearly segregated. The results suggest that proper formation of glial glomerular borders requires interaction of ORC axons and glial cells within a sensitive period, whereas targeting of ORC axons appears to be effective over extended periods in development. Precocious ingrowth of ORC axons after warming the antenna and cooling the body for 48 hours resulted in enlarged protoglomeruli. Glial borders formed normally, but a subpopulation of MGC-specific ORC axons grew past the MGC. The decreased accuracy of targeting in these cases suggests that targeting mechanisms are not fully developed before the time when ORC axons normally would enter the brain.