A specialized type of neuron in the visual cortex of cat: a Golgi and electron microscope study of chandelier cells.

The axonal arborization of chandelier cells is characterized by its conspicuous, vertically oriented, bouton aggregates. The efferent synaptic relationships established by these terminal formations were investigated by electron microscopy of Golgi preparations after gold toning and deimpregnation. In all cases examined from layers II and III of cat areas 17 and 18, the terminal formations, here denominated specific terminal portions (stp), make symmetric synapses upon axon initial segments of pyramidal neurons. Some identified stp's were reconstructed from ultrathin serial sections with the aid of a microcomputer-based system, and the number of synaptic contacts established on axon initial segments was evaluated. No evidence was found that parts of the axonal tree other than stp's also engage in synaptic contacts. Specific terminal portions are rather variable in complexity. However, the synaptic contacts they engage in are constant and the complexity of stp's from the same axonal arborization varies. It is, therefore, clear that all stp's are terminal axonal formations of a unique, specialized type of neuron. Computer techniques and conventional Golgi observations were used to study further details of chandelier cell morphology. Axonal plexuses are preferentially, although not exclusively, local and distribute within spheric, ovoid, or disk-shaped spaces. In most chandelier cells, the main axonal trunk descends to the white matter, where we have been unable to follow it further.