THE GANGLIONIC EMINENCE MAY BE AN INTERMEDIATE TARGET FOR CORTICOFUGAL AND THALAMOCORTICAL AXONS

In the nervous system of many species, growing axons associate transie ntly with cellular groupings along their path. Whether this mechanism applies to the development of corticothalamic and thalamocortical proj ections is unknown. Using carbocyanine dyes, we studied the early grow th of both corticofugal and thalamocortical fibers in hamster embryos. At embryonic day 11.5 (E11.5), corticofugal fibers invade the lateral ganglionic eminence (LGE), and thalamocortical fibers invade the medi al ganglionic eminence (MGE). At this age, both sets of fibers are not yet in contact with each other. At the same time, neurons in each sub division of the GE grow toward the cortex and thalamus. During the nex t 24 hr, corticofugal and thalamocortical fibers remain within the con fines of the GE, where they course at different radial levels and bear large and complex growth cones. In the LGE, corticofugal fibers are o ften found in close association with cells that are likely to be neuro nal. Starting on E13.5, both early projections from the GE decrease, a nd corticothalamic and thalamocortical fibers invade their definitive target regions. To lest whether the GE specifically orients the growth and trajectories of cortical fibers even in the absence of the recipr ocal thalamic projection, we cocultured explants of cortex and GE from either hamster or mouse embryos. These experiments showed that the GE , but not other tested brain regions, is able specifically to orient t he growth of cortical axons. We therefore suggest that the GE may be a n intermediate target in the pathfinding of axons between the cortex a nd the thalamus.