Molecular Mechanisms Separating Two Axonal Pathways during Embryonie Development of the Avian Optic Tectum

During embryonic development of the avian optic tec-tum, retinal and tectobulbar axons form an orthogonal array of nerve processes. Growing axons of both tracts are transiently very closely apposed to each other. De-spite this spatial proximity, axons from the two pathways do not intermix, but instead restrict their growth to de-fined 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 playa role in segregating both axonal populations: Retinal and tec-tobulbar axons differ in their ability to use the extracellu-lar matrix protein laminin as a substrate for axonal elon-gation; the environment in the optic tectum is generally permissive for retinal axons, but is specifically nonper-missive for tectobulbar axons, resulting in a strong fas-ciculation of the laUer; and growth cones of temporal retinal axons are reversibly inhibited in their motility by direct contact with the tectobulbar axon's membrane