Comparison of the Tripartite Organization of Synaptic Terminals in Intraocular Septal Transplants and in the Septal Area of the Brain

Abstract

The septal area of the brain is functionally connected to other limbic structures and is involved in cognitive processes. Intraocular neural transplants provide an experimental model for studying the endogenous morphofunctional properties of the septum. The aim of the present work was to compare the structure of synaptic contacts in septal intraocular neural transplants and in the septal area of the brain in situ. Neurons are regarded as thee-component complexes including not only pre- and postsynaptic components, but also the astrocyte processes encircling them. Ultrastructural studies showed that despite the absence of normal afferent and efferent connections in intraocular neural transplants, their synapses reproduced the tripartite organization. Morphometric analysis of their parameters showed a decrease in the statistical mean area and perimeter of the presynaptic compartment as compared with normal. At the same time, the extent of encirclement by perisynaptic astrocyte processes in transplants was, conversely, significantly greater than in the septum in situ. The mean extents of active zones in synaptic profiles in transplanted neurons was greater than that in controls. The morphometric data obtained here indicate coordinated regulation of the sizes of the three compartments of synapses depending on the functional state of the synaptic apparatus overall.