3-DIMENSIONAL ORGANIZATION OF SMOOTH ENDOPLASMIC-RETICULUM IN HIPPOCAMPAL CA1 DENDRITES AND DENDRITIC SPINES OF THE IMMATURE AND MATURE RAT

Recent studies have shown high levels of calcium in activated dendriti c spines, where the smooth endoplasmic reticulum (SER) is likely to be important for regulating calcium. Here, the dimensions and organizati on of the SER in hippocampal spines and dendrites were measured throug h serial electron microscopy and three-dimensional analysis. SER of so me form was found in 58% of the immature spines and in 48% of the adul t spines. Less than 50% of the small spines at either age contained SE R, suggesting that other mechanisms, such as cytoplasmic buffers, regu late ion fluxes within their small volumes. In contrast, >80% of the l arge mushroom spines of the adult had a spine apparatus, an organelle containing stacks of SER and dense-staining plates. Reconstructed SER occupied 0.001-0.022 mu m(3), which was only 2-3.5% of the total spine volume; however, the convoluted SER membranes had surface areas of 0. 12-2.19 mu m(2), which were 12 to 40% of the spine surface area. Coate d vesicles and multivesicular bodies occurred in some spines, suggesti ng local endocytotic activity. Smooth vesicles and tubules of SER were found in continuity with the spine plasma membrane and margins of the postsynaptic density (PSD), respectively, suggesting a role for the S ER in the addition and recycling of spine membranes and synapses. The amount of SER in the parent dendrites was proportional to the number o f spines and synapses originating along their lengths. These measureme nts support the hypothesis that the SER regulates the ionic and struct ural milieu of some, but not all, hippocampal dendritic spines.