ROLE OF DIRECTED GROWTH AND TARGET SELECTION IN THE FORMATION OF CORTICAL PATHWAYS - PRENATAL DEVELOPMENT OF THE PROJECTION OF AREA V2 TO AREA V4 IN THE MONKEY

In experiments combining retrograde tracers and histochemistry, we hav e looked at the prenatal development of the cortical pathway linking a reas V2 and V4. Transient expression of acetylcholinesterase in fetal area V2 reveals the separate compartments that project to V4 (temporal directed pathway) and V5 (parietal directed pathway). During early st ages of pathway formation, V2 neurons projecting to area V4 are cluste red in the appropriate compartments. During the phase of rapid axonal growth, there is a selective increase of connections originating from the appropriate compartments leading to a strongly clustered organizat ion at the peak of connectivity. During this phase, injections involvi ng the white matter also showed clustering, but this was somewhat redu ced in comparison to that of gray matter injections. The growth phase is followed by an elimination phase during which there is a tendency f or a preferential loss of intercluster connections, which may sharpen the early formed pattern. These results demonstrate the primary role o f axonal guidance and target recognition mechanisms followed by a limi ted extent of selective elimination during the formation of functional cortical pathways in the primate isocortex. Compared to previous find ings, these results suggest that the developmental restriction of call osal connections is not a universal model of cortical development. In the present report, the directed growth and early specification of fee d-forward connections contrast with the prolonged remodelling of monke y feedback projections, suggesting two distinct developmental strategi es of pathway formation in the monkey. (C) 1996 Wileg-Liss, Inc.