INDIVIDUAL AXON MORPHOLOGY AND THALAMOCORTICAL TOPOGRAPHY IN DEVELOPING RAT SOMATOSENSORY CORTEX

The morphology of individual thalamocortical axons in developing rat p rimary somatosensory cortex was studied using lipophilic tracers. Ante rograde labeling with lipophilic dyes demonstrated a topographical org anization of thalamocortical projections exiting the thalamus as early as embryonic day (E)16; retrograde labeling studies demonstrated topo graphy of these projections as they reached the cortex as early as E18 . At E17, axons course tangentially within the intermediate zone and t urn or branch near the deepest layer of cortex (layer VIb), suggesting the presence of guidance cues in this region. Axons appear to grow an d branch progressively within layers VIb and VIa during the following days; axons in the intermediate zone may give rise to radially directe d branches. Individual axons appear to grow steadily and progressively into the cortex, with the leading front of axons at the transition zo ne between the cortical plate (CP) and the differentiating cortical la yers. At birth (PO), thalamocortical axons extend radially through lay ers VIa and V and emit branches within these layers; some axons reach the CP. By P1, layer IV has begun to differentiate and axons begin to form a few simple branches in the vicinity of the layer IV cells. Over the ensuing week, axons generate more branches within layer TV, but t he tangential extent of individual axon arbors does not exceed the wid th of a barrel. By P7, individual axons overlap within barrel clusters , and individual axons span the width of a cluster. These observations indicate that thalamic afferents develop by progressive growth of arb ors that remain spatially restricted, rather than by overbranching and retracting arbors. (C) 1996 Wiley-Liss, Inc.