Neurobiological applications of high voltage electron microscopy

Effective synaptome analysis of itch-mediating neurons in the spinal cord: A novel immunohistochemical methodology using high-voltage electron microscopy

Transmission electron microscopy (TEM) is used for three-dimensional (3-D) analysis of synaptic connections in neuroscience research. However, 3-D reconstruction of the synapses using serial ultrathin sections is a powerful but tedious approach requiring advanced technical skills. High-voltage electron microscopy (HVEM) allows examination of thicker sections of biological specimens due to the increased penetration of the more accelerated electrons, which is useful to analyze the 3-D structure of biological specimens. However, it is still difficult to visualize the neural networks and synaptic connections in 3-D using HVEM because of insufficient and non uniform heavy metal staining in the membranous structures in semi-thin sections. Here, we present the successful chemical 3-D neuroanatomy of the rat spinal dorsal horn at the ultrastructural level as a first step for effective synaptome analysis by applying a high-contrast en bloc staining method to immune-HVEM tomography. Our new approach made it possible to examine many itch-mediating synaptic connections and neural networks in the spinal cord simultaneously using HVEM tomography. This novel 3-D electron microscopy is very useful for the analysis of synaptic structure and the chemical neuroanatomy at the 3-D ultrastructural level.

High-voltage electron microscopy reveals direct synaptic inputs from a spinal gastrin-releasing peptide system to neurons of the spinal nucleus of bulbocavernosus

The spinal nucleus of bulbocavernosus (SNB) is a sexually dimorphic motor nucleus located in the anterior horn of the fifth and sixth lumbar segments of the spinal cord that plays a significant role in male sexual function. We recently found that a sexually dimorphic expression of gastrin-releasing peptide (GRP) in the lumbar spinal cord regulates male copulatory reflexes. Although it is reported that these systems are both profoundly regulated by circulating androgen levels in male rats, no direct evidence has been reported regarding GRP synaptic inputs onto SNB motoneurons. The aim of the current study was to determine the axodendritic synaptic inputs of spinal GRP neurons to SNB motoneurons. Immunoelectron microscopy, combined with a retrograde tracing technique using high-voltage electron microscopy (HVEM), provided a three-dimensional visualization of synaptic contacts from the GRP system in the lumbar spinal cord onto SNB motoneurons. HVEM analysis clearly demonstrated that GRP-immunoreactive axon terminals directly contact dendrites that extend into the dorsal gray commissure from the SNB. These HVEM findings provide an ultrastructural basis for understanding how the spinal GRP system regulates male sexual behavior.

Three-dimensional organization of neuronal and glial processes: high voltage electron microscopy

Neurons and glia cells in the mammalian central nervous system have many complicated processes. They are too fine for light microscopic study and too complicated and widely spread for thin section electron microscopy. High-voltage electron microscopic (HVEM) stereo observation of thick Golgi preparation provides detailed 3-D images of their processes. Three-dimensional fine structures of astocytic processes in the neuropile and on the surface of neuronal somata, and those of the ruffed cell axon initial segment and thorny excrescences of CA3 pyramidal cell dendrites, are elucidated with the aid of HVEM stereoscopy of thick Golgi preparations. In addition, some results obtained by 3-D morphometrical analysis of dendritic spines using HVEM stereo images are shown. The examples presented here clearly show the usefulness of high-voltage electron microscope stereo observation of thick specimens for detailed morphological and morphometric study of the central nervous system.

Understanding the neostriatal microcircuitry: high-voltage electron microscopy

High voltage electron microscopy (HVEM) and HVEM tomography of selectively stained cell processes in the neostriatum have offered an alternative to serial thin section reconstruction for accurate 3-D visualization and measurement of axons, dendrites, and dendritic spines. Tissue preparation is simple and rapid, allowing examination of large numbers of specimens required for quantitation of neuronal morphology. The resolution of the images exceeds that available from any light microscopic technique and is appropriate for measurement of the finest axons and dendritic spine necks. HVEM tomography allows the direct measurement of dendritic surface area, required for computational modeling of synaptic integration.

Three-dimensional morphometrical study of dendritic spines of the granule cell in the rat dentate gyrus with HVEM stereo images

Number, length, and diameters of dendritic spines of the granule cell in the dorsal leaf of the rat dentate gyrus were measured by using high-voltage electron microscope stereo images of 5-micron-thick Golgi preparations with the aid of a three-dimensional image analyzer system. Spine densities of 2.02 +/- 0.28, 2.28 +/- 0.33, and 3.36 +/- 0.35 per 1 micron at distal, middle, and proximal portions of the dendrite were obtained. These values were about 1.6-fold of the previous light microscopical report. Mean three-dimensional spine length were 1.244 +/- 0.506 micron, 1.262 +/- 0.563 micron, and 1.254 +/- 0.584 micron at distal, middle, and proximal portions, respectively, which were about 1.4 times longer than those measured in two dimensions. By using measured morphometrical parameters of spines such as lengths, diameters, and population densities, total spine surface areas of 2.401 micron 2, 2.806 micron 2, and 4.180 micron 2 per 1 micron of the dendrite at distal, middle, and proximal portions, respectively, were obtained. The total surface area of dendrite was about doubled by the addition of the spines at each dendritic portion. The advantageous features and the problems of the present method are discussed.

High-voltage electron microscopy of E-PTA-stained synaptic junctions in the rat frontal cortex

The E-PTA-stained synaptic junctions in the adult rat frontal cortex were examined with high-voltage electron microscopy (HVEM). Perforated whole synaptic junctions were clearly shown in the stereo image. The E-PTA staining procedure provides a useful marker for studies of the 3-dimensional structure of synaptic junctions by means of HVEM.

Three-dimensional structure of astrocytes in the rat dentate gyrus

Structural features of astrocytes in the rat dentate gyrus were studied by means of light and high-voltage electron microscopy of Golgi-impregnated materials, conventional electron microscopy, combined Golgi-electron microscopy, and immunohistochemistry for glial fibrillar acidic protein. Astrocytes in the dentate gyrus were of the protoplasmic type and were classified into six subtypes based on the location of their somata; i.e., astrocytes in the polymorph layer, in the subgranular zone, in the granular cell layer, at the border of the granular cell layer and the molecular layer, in the molecular layer, and subjacent to the pia surface. Stereoscopic observations of 5-micron-thick sections of Golgi-impregnated materials revealed three-dimensional structural details of astroglial processes not apparent in either the light microscope or in conventional thin-section electron microscopy. Most of them were basically thin sheets, varying in shape and size in accordance with their sites. In the granule cell layer, thin veillike sheets or lamellae, originating from three kinds of astrocytes (subtypes 2, 3, and 4), intervened between granule cell somata, whereas in the plexiform and molecular layers small leafletlike appendages originating from astrocytes (subtypes 1, 2, 4, and 5) intermingled with one another, making spongelike conglomerates. Thus astrocytes in the subgranular zone and those at the border of the granule cell layer and the molecular layer showed prominent regional differentiation of their processes among layers. In addition to these sheetlike processes, thin threadlike processes were also common.