Molecular mechanisms separating two axonal pathways during embryonic development of the avian optic tectum

During embryonic development of the avian optic tectum, retinal and
tectobulbar axons form an orthogonal array of nerve processes.
Growing axons of both tracts are transiently very closely apposed
to each other. Despite this spatial proximity, axons from the two
pathways do not intermix, but instead restrict their growth to
defined areas, thus forming two separate plexiform layers, the
stratum opticum and the stratum album centrale. In this study we
present experimental evidence indicating that the following three
mechanisms might play a role in segregating both axonal
populations: Retinal and tectobulbar axons differ in their ability
to use the extracellular matrix protein laminin as a substrate for
axonal elongation; the environment in the optic tectum is generally
permissive for retinal axons, but is specifically nonpermissive for
tectobulbar axons, resulting in a strong fasciculation of the
latter; and growth cones of temporal retinal axons are reversibly
inhibited in their motility by direct contact with the tectobulbar
axon's membrane.