Three‐dimensional analysis of the structure and composition of CA3 branched dendritic spines and their synaptic relationships with mossy fiber boutons in the rat hippocampus

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Abstract
This paper is the third in a series to quantify differences in the composition of subcellular orgunelles and three-dimensional structure of dendritic spines that could contribute to their specific biological properties. Proximal apical dendritic spines of the CAS pyramidal cells receiving synaptic input from mossy fiber (MF) boutons in the adult rat hippocampus were evaluated in three sets of serial electron micrographs. These CAS spines are unusual in that they have from 1 to 16 branches emerging from a single dendritic origin. The branched spines usually contain subcellular organelles that are rarely found in adult spines of other brain regions including ribosomes, multivesicular bodies (MVB), mitochondria, and microtubules. MVBs occur most often in the spine heads that also contain smooth endoplasmic reticulum, and ribosomes occur most often in spines that have spinules, which are small nonsynaptic protuberances emerging from the spine head. Most of the branched spines are surrounded by a single MF bouton, which establishes synapses with multiple spine heads. The postsynaptic densities (PSDs) occupy about 10–15% of the spine head membrane, a value that is consistent with spines from other brain regions, with spines of different geometries, and with immature spines. Individual MF boutons usually synapse with several different branched spines, all of which originate from the same parent dendrite. Larger branched spines and MF boutons are more likely to synapse with multiple MF boutons and spines, respectively, than smaller spines and boutons. Complete three-dimensional reconstructions of representative spines with 1, 6, or 12 heads were measured to obtain the volumes, total surface areas, and PSD surface areas. Overall, these dimensions were larger for the complete branched spines than for unbranched or branched spines in other brain regions. However, individual branches were of comparable size to the large mushroom spines in hippocampal area CA1 and in the visual cortex, though the CA3 branches were more irregular in shape. The diameters of each spine branch were measured along the cytoplasmic path from the PSD to the origin with the dendrite, and the lengths of branch segments over which the diameters remained approximately uniform were computed for subsequent use in biophysical models. No constrictions in the segments of the branched spines were thin enough to reduce charge transfer along their lengths. The complex geometry of these spines and their subcellular constituents could modulate postsynaptic responses, however, during synaptic activation or plasticity such as long term potentiation, in ways that can now be evaluated on the basis of the complete dimensions and descriptions reported here.