Ultrastructural morphology of synapses formed by corticostriatal terminals in the avian striatum

We studied the ultrastructural morphology of corticostriatal projections from two different avian 'neocortical' regions, namely, the hyperstriatum accessorium (HA) and the pallium externum (PE). Biotinylated dextran amine (BDA) was used to label the corticostriatal projection from either HA or PE to the striatum. The corticostriatal axons from both the PE and HA possessed numerous beaded varicosities with the striatum. These varicosities were filled with numerous round vesicles characterizing them as terminals. These terminals formed asymmetric synapses with spine heads and with dendrites of striatal neurons. The axospinous synapses outnumbered the axodendritic synapses by more than two to one. The diameters of labeled axons were typically 250-500 nm. The labeled terminals were typically 400-750 nm in diameter. No obvious differences between the ultrastructural morphology of the HA and the PE corticostriatal projections were observed. These data show that corticostriatal terminals and their synaptic contacts in birds are similar to those described in mammals.