GROWTH OF CALLOSAL TERMINAL ARBORS IN PRIMARY VISUAL AREAS OF THE CAT

In kittens ranging in age between postnatal day (P) 5 and P150, callos al axons originating near the 17/18 border were anterogradely labelled with biocytin and reconstructed from serial sections. At the end of t he first postnatal week most of the axons begin to invade the cortex n ear the 17/18 border with multiple branches; some axons already span t he grey matter up to layer I. Branches tend to grow into the grey matt er in loose bundles less than or equal to 100 mu m in diameter, separa ted by empty spaces of comparable width. In the following weeks additi onal branches are produced in the grey matter; this appears to blur th e initial bundled distribution, although by the end of the first postn atal month the branches are distributed in discrete patches similar to the adult terminal columns. Although a few boutons (presumably synapt ic boutons) are found in the white matter/subplate region at earlier a ges, they appear in the grey matter from P12 onwards. Their number per axon increases with age, reaching adult values about the end of the f irst month. Subsequently the number of boutons continues to increase a nd remains above adult values at P50, P65 and P80; it then decreases, reaching adult levels by P150. During the first month boutons tend to be more numerous in the infragranular layers, but then the trend rever ses in favour of the supragranular layers. In most cases, the distribu tion of boutons spares layer IV partially or completely. From the onse t boutons are distributed in radial 'columns' whose diameter increases with age. They maintain selective laminar and columnar distributions through the period of rapid and exuberant increase. These distribution s do not appear to be sharpened further by the reduction in the number of boutons to adult levels. On the whole, callosal terminal arbors di fferentiate through stages of exuberant, albeit progressively constrai ned, growth involving both progressive and regressive events. Comparis ons with previous work suggest that visual activity might finely shape the arbor, from the onset of synaptogenesis onwards.