COLLATERAL BRANCH FORMATION RELATED TO CELLULAR STRUCTURES IN THE AXON TRACT DURING CORTICOPONTINE TARGET RECOGNITION

The corticopontine projection develops exclusively by collateral branc hes that form along the length of corticospinal axons days after they have passed their hindbrain target, the basilar pens. In vitro evidenc e suggests that the basilar pons releases a diffusible activity that i nitiates and directs the growth of collateral branches. This study inv estigates whether contact-dependent mechanisms may also influence the formation of collateral branches. By using immunocytochemistry, electr on microscopy, and neuronal tracing techniques, we examined the region of the axon tract, the cerebral peduncle, overlying the basilar pens for cellular structures that correlate spatially and temporally with c ollateral branch formation. We found that radial glia are excluded fro m the tract. Oligodendrocyte precursors are found only at low density. Although mature astrocytes are absent, immature astrocytes are presen t throughout the tract. However, our evidence does not suggest a direc t role for glial cell types in collateral branch formation. In contras t, dendrites of basilar pontine neurons are transiently present in the tract during the time of collateral branch formation. Although collat eral branches are observed in regions of the tract devoid of dendrites , the orientation and location of most collateral branches correlates at the light microscopic level with dendrites. Electron microscopy rev eals sites of increased collateral branch formation near neuronal cell bodies or dendrites. However, cell processes, whether dendritic or ot herwise, are rarely found in direct contact with collateral branch poi nts. A common and unexpected feature is the bundles of corticopontine collateral branches, oriented transversely to their parent corticospin al axons and directed across the tract to the basilar pens. Dendrites were often apposed to or embedded within the transverse bundles. These findings suggest that dendrites are not essential for collateral bran ch formation but that they may enhance this process and define discret e preferred locations for collateral branch initiation and elongation within the cerebral peduncle. (C) 1998 Wiley-Liss, Inc.