Relationships between synaptic junctions, puncta adhaerentia and the spine apparatus at neocortical axo-spinous synapses. A serial section study.

A total of 80 cotical axo-spinous synaptic junctions were reconstructed from serial sections and about 100,000 were analyzed in single sections. Special attention was paid to the occurrence of puncta adhaerentia associated with perforated, annulate or horseshoe-shaped (= complex) synaptic junctions and to the presence and proximity of the spine apparatus. Further evidence is presented that the spine apparatus has no relationship to simple (round or oval) synaptic specializations, but is present in association with at least 91% of complex junctions. The spine apparatus points towards the punctum adhaerens which in at least 71% of cases seems to be an integral part of the complex synapse. Direct continuity was found between the dense material of the spine apparatus and the punctum adhaerens. It is suggested, in accordance with other recent studies, that expansion of the synaptic active zone occurs by the addition and transformation of puncta adhaerentia. The spine apparatus may participate in this dynamic process as a possible donor of specific postsynaptic proteins.