Neural Development in the Moth

The scientific work of many esteemed developmental neurobiologists has now firmly established that exposure to various types of sensory stimuli during early development is actually critical for proper wiring of the nervous system. As an undergraduate working in the developmental neurobiology laboratory of Dr. Leslie P. Tolbert, I have been working with Dr. Lynne Oland to understand the mechanisms by which sensory input affects the development of the brain. We use the moth Manduca sexta as a model to study development of the antennallobe, the part of the brain that receives olfactory input from the environment. Like its counterparts in the mammalian brain, the moth's antennallobe is organized into discrete compartments called glomeruli. In mammals and moths, the presence of these glomeruli in the adult depends entirely on the arrival of sensory input from olfactory receptors. The moth olfactory system, however, is much easier to manipulate experimentally.

During development, when olfactory axons reach the antennallobe, groups of them form a template for the future glomeruli. Glial cells surround the developing glomeruli, and previous work in the lab has shown that if you remove glial cells, stable glomeruli do not form.

My project addresses the specific role that glial cells play in the development of glomeruli. Taking advantage of the natural difference in the timing of birth of glial cells and neurons in moths, it is possible to remove a large proportion of the glial population either by drugs or radiation that selectively damage dividing glial cells, causing them to die, without affecting the neurons. After fluorescently staining the neurons that branch in the glomeruli at various stages of development, a laser scanning confocal microscope is used to examine the changes in branching patterns of both the olfactory axons and their targets, the antennal-lobe neurons, when they develop in the presence of too few glial cells.

Our initial results were completely unexpected. The branching patterns of olfactory axons in early glia-deficient antennallobes resembled those of axons in normal lobes at comparable stages of development, suggesting that axons do not require glial boundaries to organize into a template. However, the examination of older lobes showed that as development proceeds, the branches of the antennal-lobe neurons are unable to attain their characteristic glomerular branching pattern without sufficient numbers of glial cells. This paradox prompted us to question whether the axons actually remained organized into glomeruli throughout the development stages, as we had previously assumed, or whether they, too, required the stabilization provided by glia. Upon further investigation, we found that by later stages, the axons had indeed lost their organization.

We conclude that glial cells provide some sort of critical information to ingrowing sensory axons during the developmental cycle, making their presence in sufficient numbers imperative for proper development of this region of the brain. In the future, we hope to identify and characterize the signal that glial cells provide to growing axons enabling the axons to induce the formation of glomeruli.