Developmental mechanisms patterning thalamocortical projections: intrinsic, extrinsic and in between.

Roger Sperry proposed 40 years ago that topographic neural connections are established through complementary expression of chemoaffinity labels in projecting neurons and their final targets. This led to the identification of ephrins as key molecular cues controlling the topography of retinotectal projections. Recent studies have revealed a surprising twist to this model, shedding light on the developmental mechanisms patterning the projections between the thalamus and the cortex: ephrins, unexpectedly expressed in an intermediate target, control the establishment of topography of axonal projections between these two structures. The same cues are re-used later to control the mapping of thalamocortical projections within a given cortical area, which strikingly illustrates how a limited set of genes can contribute to generate several levels of complexity of a neuronal network.