PREFERENTIAL TERMINATION OF CORTICORUBRAL AXONS ON SPINE-LIKE DENDRITIC PROTRUSIONS IN DEVELOPING CAT

The formation of synaptic contacts is a crucial event during neural de velopment and is thought to be achieved by complex interactions betwee n incoming axons and the neurons in the target. We have focused on spi ne-like dendritic protrusions (SLDPs), which are transient pleomorphic protrusive structures seen in developing brains. Although the functio nal significance of SLDPs remains unknown, accumulating in vitro evide nce suggests that the SLDP plays an important role in synaptogenetic i nteractions with axons. As a test of this idea, the present study was performed to examine whether the SLDPs are the preferential sites of s ynapse formation in vivo. The ultrastructure of biocytin-labeled corti corubral (CR) terminals was examined in serial thin sections during th e period of synaptogenesis in newborn cats. We found that a major prop ortion (86%) of the CR synapses was formed on SLDPs. The presynaptic t erminals were often invaginated by fine processes extending from the t ips of SLDPs. Synaptic structures presumably of cortical origin were a lso found on SLDPs of HRP-labeled rubrospinal cells, suggesting that S LDPs postsynaptic to labeled CR terminals originate at least in part f rom rubrospinal cells. Taken together, these results indicate that SLD Ps may represent preferred sites of synapse formation and support the notion that SLDPs play a role in synaptogenic interactions during brai n development.