Role of laminin in axonal extension from olfactory receptor cells

The role of laminin, an extracellular matrix molecule believed to be involved in axon extension, was explored in the outgrowth of olfactory receptor cells and therefore in the maintenance of organization in the olfactory pathway. First, immunocytochemistry was used to examine laminin expression in the olfactory nerve and bulb during development. Laminin immunoreactivity was high in the olfactory nerve and glomerular layers. Although it declined in intensity, laminin expression continued in the nerve and in single glomeruli of adults. Second, the influence of laminin on neurite outgrowth was examined in vitro using olfactory receptor cells harvested from E14 rat embryos. We developed an in vitro assay to quantify the substrate preference of outgrowing neurites. Cells were cultured for 48 h on coverslips coated with either poly-L-lysine alone, or poly-L-lysine overlaid with laminin. On laminin-coated regions of coverslips, the primary neurites of olfactory receptor cells were 52% longer than on the poly-L-lysine control substrates. In addition, the direction of the neurite outgrowth was influenced by laminin. Fifty-six percent of all receptor cells located in a defined area surrounding a laminin zone extended neurites onto laminin. In contrast, only 7% of all receptor cells located in the corresponding laminin zone extended a neurite onto poly-L-lysine. In summary, these data suggest that laminin provides a favorable substrate for the extension of the primary neurite from olfactory receptor cells and the direction of their extension. Therefore, laminin may be a factor underlying continuous olfactory receptor cell axon outgrowth and its pathfinding in the olfactory system.