![]() The space between synapses is occupied by astrocytic processes ( Kettenmann et al., 1996 Chao et al., 2002 Witcher et al., 2007 Verkhratsky and Nedergaard, 2018). Spines have a neck and a head with an electron dense PSD. Dendritic spines are highly dynamic structures that can be classified into several groups based on their morphology (e.g., mushroom spines, thin spines etc.) ( Hering and Sheng, 2001). ![]() The dendritic spine is the postsynaptic terminal of a glutamatergic synapse. The presynaptic terminal is an axonal varicosity that is closely apposed to the postsynaptic terminal. Synaptic contacts are highly specialized neuronal compartments. Taken together, these results suggest that astrocytic leaflets preferentially contact synapses as opposed to the dendritic shaft, an arrangement that might favor neurotransmitter spillover and extrasynaptic receptor activation along dendritic shafts. ![]() In contrast, the VF of astrocytic branchlets was similarly low around these three neuronal compartments. The VF of astrocytic leaflets was highest around dendritic spines, lower around axonal boutons and lowest around dendritic shafts. The 2D projections of astrocytic branchlets had smaller spatial complexity and entropy than leaflets, consistent with the higher structural complexity and less organized distribution of leaflets. We used complexity and entropy measures to characterize astrocytic branchlets and leaflets. Diffusion simulations showed that such synaptic microenvironment favors glutamate spillover and extrasynaptic receptor activation at smaller spines. This suggests that the astrocytic coverage of small and larger spines is similar in hippocampal neuropil. The distance to the maximum of the astrocyte volume fraction (VF) correlated with the size of the spine when calculated from the center of mass of the postsynaptic density (PSD) or from the edge of the PSD, but not from the spine surface. Here we use three-dimensional (3D) reconstructions from serial section electron microscopy (EM) of rat CA1 hippocampal neuropil to determine the astrocytic coverage of dendritic spines, shafts and axonal boutons. Leaflets are small and flat processes that protrude from branchlets and fill the space between synapses. Astrocytic branchlets are rod-like processes containing mitochondria and endoplasmic reticulum, capable of generating inositol-3-phosphate (IP 3)-dependent Ca 2+ signals. 6All-Russian Research Institute of Medicinal and Aromatic Plants, Moscow, Russiaĭistal astrocytic processes have a complex morphology, reminiscent of branchlets and leaflets.5Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Moscow, Russia. ![]() Lobachevsky State University of Nizhny Novgorod, Nizhny Novgorod, Russia ![]() 4Institute of Information Technologies, Mathematics and Mechanics, N.3Department of Biology, University at Albany, The State University of New York (SUNY), Albany, NY, United States.Lomonosov Moscow State University, Moscow, Russia 2Department of Biophysics, Faculty of Biology, M.Nikolay Gavrilov 1 Inna Golyagina 1 Alexey Brazhe 2 Annalisa Scimemi 3 Vadim Turlapov 4 Alexey Semyanov 1,5,6 * ![]()
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