Laminin alpha 2 chain (M chain) is found within the pathway of avian and murine retinal projections

Laminin-1 is found at the end-feet of neuroepithelial cells along the outer margin of the optic pathway during early stages of development. Prior to the establishment of most retinal projections in vivo, laminin-1 expression becomes restricted to basement membranes associated with the eye and optic pathway. We report that, in contrast to the alpha 1, beta 1, and gamma 1 chains of laminin-1, laminin alpha 2 chain (formerly laminin M chain) is expressed within the pathway of avian and murine retinal ganglion cell (RGC) growth cones as they extend into the optic nerve, across the optic chiasm and into the brain. Expression of laminin alpha 2 chain is reduced soon after formation of the visual projections but nevertheless maintained at non-basal lamina sites within the adult optic nerve. Laminin alpha 2 chain, in contrast to laminin-1 chains, is also highly expressed in the developing avian tectobulbar pathway. Chick optic nerve derived type-1 astrocytes in culture express laminin alpha 2 chains as extracellular fibrils on their surface. Laminin alpha 2 chain was also detected on the surface of cultured embryonic retinal neurons and developing RGCs. These results suggest that astrocytes and/or RGCS may synthesize laminin alpha 2 chain along the developing optic pathway, and imply that laminin alpha2--in a complex with non-beta1 and non-gamma1 laminin chains-may serve as an adhesive substrate and possibly as a guidance cue for elongating RGC growth cones in vivo.