The role of laminin, an extracellular matrix molecule believed to be i
nvolved 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 lam
inin expression in the olfactory nerve and bulb during development. La
minin immunoreactivity was high in the olfactory nerve and glomerular
layers. Although it declined in intensity, laminin expression continue
d in the nerve and in single glomeruli of adults. Second, the influenc
e of laminin on neurite outgrowth was examined in vitro using olfactor
y receptor cells harvested from E14 rat embryos. We developed an in vi
tro 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-coate
d regions of coverslips, the primary neurites of olfactory receptor ce
lls were 52% longer than on the poly-L-lysine control substrates. In a
ddition, the direction of the neurite outgrowth was influenced by lami
nin. Fifty-six percent of all receptor cells located in a defined area
surrounding a laminin zone extended neurites onto laminin. In contras
t, only 7% of all receptor cells located in the corresponding laminin
zone extended a neurite onto poly-L-lysine. In summary, these data sug
gest that laminin provides a favorable substrate far the extension of
the primary neurite from olfactory receptor cells and the direction of
their extension. Therefore, laminin may be a factor underlying contin
uous olfactory receptor cell axon outgrowth and its pathfinding in the
olfactory system. (C) 1997 John Wiley & Sons, Inc.