Pictures in cell biology: Three-dimensional reconstruction of synaptic ultrastructure

Abstract

A fundamental aspect of understanding the mechanisms of synaptic transmission in the central nervous system is knowing the dimensions and connectivity of synapses and the changes in structure that occur during development and plasticity. Improved computer-based quantitative analysis of brain anatomy at the ultrastructural level, especially the analysis of synaptic structure and connectivity, has the potential to clarify many of the issues in a way that no other technique can. Most excitatory synapses in the adult brain are located on the bulbous heads of dendritic spines, which exist in a variety of shapes and sizes. During development, before the formation of spines, cortical dendrites extend very long, thin processes, often without a bulbous head, called filopodia. As the cortex matures, these dendritic filopodia disappear and spines emerge. FIGURE 1, FIGURE 2 illustrate our recent efforts using three-dimensional reconstructions to investigate the relationships between neural components and their functions. See also our Web site http://synapses.tch.harvard.edu/ for further information.FIGURE 1Illustration of the use of serial-section electron microscopy and three-dimensional analysis to look at the role of dendritic filopodia in the formation of excitatory synaptic contacts and dendritic spines in hippocampal area CA1 in developing rats. The analysis revealed that numerous dendritic filopodia form synaptic contacts with axons and with filopodia extending from axons. Recent studies have suggested that these dendritic filopodia are involved in locating presynaptic targets and consolidating them into stable synapses. This figure shows a three-dimensional reconstruction of a dendritic filopodium created from the serial electron micrographs. The inset uses an electron micrograph to illustrate the corresponding synaptic connection between the filopodium and the adjacent axon. See Ref. 1xFiala, J.C. et al. J. Neurosci. 1998; 18: 8900–8911PubMedSee all ReferencesRef. 1 for more details. Bars: inset, 0.5 μm; main image, 1 μm.View Large Image | Download PowerPoint SlideFIGURE 2Smooth endoplasmic reticulum (SER) is important for regulating Ca2+, which is present at high levels in activated dendritic spines. We measured the dimensions and organization of the SER in rat hippocampal spines and dendrites using serial electron microscopy and three-dimensional analysis. This figure illustrates the three-dimensional reconstruction of the SER (purple) in a rat hippocampal CA1 dendritic segment. The plasma membrane of the dendrite is not visible in (a) or (b), although the membrane of the attached spine is present in (a), which shows that the SER in the dendrite is contiguous with the SER entering the thin neck of a large dendritic spine (grey). The SER in the head of the spine [seen in (b) where the spine membrane is invisible] is thought to provide synapse-specific regulation of Ca2+ and other molecules. This particular dendritic spine has a highly irregular synaptic area (red). See Ref. 2xSpacek, J. and Harris, K.M. J. Neurosci. 1997; 17: 190–203PubMedSee all ReferencesRef. 2 for more details. Bar, 0.5 μm.View Large Image | Download PowerPoint Slide