Stages of growth of hamster retinofugal axons: implications for developing axonal pathways with multiple targets

Neurons in many regions of the CNS (e.g., cortical areas, thalamic nuclei) are heterogeneous with regard to their afferent and efferent connections. Using the hamster retinofugal system as a model, we investigated the mechanisms by which such connectional heterogeneity arises during ontogeny. Retinal ganglion cell axons were labeled with 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (Dil) in paraformaldehyde-fixed tissue. The fluorescent label was photoconverted to a diaminobenzidine reaction product. The morphology of the axons, including their trajectories, branching patterns, and growth cones, was studied at the level of the dorsal lateral geniculate nucleus (LGd) from embryonic day 14 to adulthood. In adult hamsters, axons of retinal ganglion cells (RGCs) are spatially segregated at the level of the lateral geniculate nucleus into a superficial optic tract, situated just beneath the pia, and an internal optic tract consisting of fascicles running parallel to the pia within the geniculate. All retinofugal axons project to the midbrain, but only superficial optic tract axons emit collaterals to the LGd. During development, axons in both divisions of the optic tract emit collaterals to the LGd, but by postnatal day 15, collaterals of internal optic tract axons are virtually entirely eliminated, whereas those of superficial optic tract axons have elaborated terminal arbors. Thus, the heterogeneity among different classes of RGCs with respect to their efferent connections emerges by the selective stabilization, by each class, of a unique subset of connections from an initially widespread set shared by all classes. Thalamic collaterals of RGC axons emerge along established axon trunks, not by bifurcation of the growing tip. This occurs after the axons have grown past the thalamus and, presumably, entered their targets in the midbrain. Growth cones at the tips of elongating axon trunks are larger in size and have a more "complex" morphology compared to the growth cones on collaterals. Axons of RGCs develop in 3 morphologically distinct growth states. First, they elongate to their most distant targets in the midbrain. Then, they simultaneously emit unbranched or poorly branched collaterals to multiple targets. Finally, they elaborate terminal arbors in their definitive targets and eliminate their other collaterals. This developmental strategy may be paradigmatic for the formation of long CNS pathways with multiple targets. Furthermore, these data document, at the single-axon level, the steps in the elaboration and withdrawal of transient neuronal projections.