Postnatal development of axosomatic synapses in the rat visual cortex: Morphogenesis and quantitative evaluation

Postnatal development of axosomatic synapses was studied in the rat visual cortex in order to obtain experimental data that may explain how the unequal distribution of asymmetric and symmetric synapses evolves on the soma of cortical neurons. Three types of synaptic junctions were identified: (1) asymmetric or type 1 synapses, with postsynaptic densities ≥ 20 nm, (2) symmetric type 2 synapses, and (3) symmetric synapses with an intermediate structure. The third synapse type had a structure similar to that of type 1 synapses, although the postsynaptic densities were thinner than 20 nm.