Collateral branch formation related to cellular structures in the axon tract during corticopontine target recognition

The corticopontine projection develops exclusively by collateral branches that form along the length of corticospinal axons days after they have passed their hindbrain target, the basilar pons. In vitro evidence suggests that the basilar pons releases a diffusible activity that initiates and directs the growth of collateral branches. This study investigates whether contact-dependent mechanisms may also influence the formation of collateral branches. By using immunocytochemistry, electron microscopy, and neuronal tracing techniques, we examined the region of the axon tract, the cerebral peduncle, overlying the basilar pons for cellular structures that correlate spatially and temporally with collateral branch formation. We found that radial glia are excluded from the tract. Oligodendrocyte precursors are found only at low density. Although mature astrocytes are absent, immature astrocytes are present throughout the tract. However, our evidence does not suggest a direct role for glial cell types in collateral branch formation. In contrast, dendrites of basilar pontine neurons are transiently present in the tract during the time of collateral branch formation. Although collateral 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 reveals sites of increased collateral branch formation near neuronal cell bodies or dendrites. However, cell processes, whether dendritic or otherwise, are rarely found in direct contact with collateral branch points. A common and unexpected feature is the bundles of corticopontine collateral branches, oriented transversely to their parent corticospinal axons and directed across the tract to the basilar pons. Dendrites were often apposed to or embedded within the transverse bundles. These findings suggest that dendrites are not essential for collateral branch formation but that they may enhance this process and define discrete preferred locations for collateral branch initiation and elongation within the cerebral peduncle.