Three-dimensional analysis of the intracellular architecture by scanning electron microscopy

The two-dimensional observation of ultrathin sections from resin-embedded specimens provides an insufficient understanding of the three-dimensional (3D) morphological information of membranous organelles. The osmium maceration method, developed by Professor Tanaka's group >40 years ago, is the only technique that allows direct observation of the 3D ultrastructure of membrane systems using scanning electron microscopy (SEM), without the need for any reconstruction process. With this method, the soluble cytoplasmic proteins are removed from the freeze-cracked surface of cells while preserving the integrity of membranous organelles, achieved by immersing tissues in a diluted osmium solution for several days. By employing the maceration method, researchers using SEM have revealed the 3D ultrastructure of organelles such as the Golgi apparatus, mitochondria and endoplasmic reticulum in various cell types. Recently, we have developed new SEM techniques based on the maceration method to explore further possibilities of this method. These include: (i) a rapid osmium maceration method that reduces the reaction duration of the procedure, (ii) a combination method that combines agarose embedding with osmium maceration to elucidate the 3D ultrastructure of organelles in free and cultured cells and (iii) a correlative immunofluorescence and SEM technique that combines cryosectioning with the osmium maceration method, enabling the correlation of the immunocytochemical localization of molecules with the 3D ultrastructure of organelles. In this paper, we review the novel osmium maceration methods described earlier and discuss their potential and future directions in the field of biology and biomedical research.

The Regulated Secretion and Models of Intracellular Transport: The Goblet Cell as an Example

Transport models are extremely important to map thousands of proteins and their interactions inside a cell. The transport pathways of luminal and at least initially soluble secretory proteins synthesized in the endoplasmic reticulum can be divided into two groups: the so-called constitutive secretory pathway and regulated secretion (RS) pathway, in which the RS proteins pass through the Golgi complex and are accumulated into storage/secretion granules (SGs). Their contents are released when stimuli trigger the fusion of SGs with the plasma membrane (PM). In specialized exocrine, endocrine, and nerve cells, the RS proteins pass through the baso-lateral plasmalemma. In polarized cells, the RS proteins secrete through the apical PM. This exocytosis of the RS proteins increases in response to external stimuli. Here, we analyze RS in goblet cells to try to understand the transport model that can be used for the explanation of the literature data related to the intracellular transport of their mucins.

Applications of Scanning Electron Microscopy Using Secondary and Backscattered Electron Signals in Neural Structure

Scanning electron microscopy (SEM) has contributed to elucidating the ultrastructure of bio-specimens in three dimensions. SEM imagery detects several kinds of signals, of which secondary electrons (SEs) and backscattered electrons (BSEs) are the main electrons used in biological and biomedical research. SE and BSE signals provide a three-dimensional (3D) surface topography and information on the composition of specimens, respectively. Among the various sample preparation techniques for SE-mode SEM, the osmium maceration method is the only approach for examining the subcellular structure that does not require any reconstruction processes. The 3D ultrastructure of organelles, such as the Golgi apparatus, mitochondria, and endoplasmic reticulum has been uncovered using high-resolution SEM of osmium-macerated tissues. Recent instrumental advances in scanning electron microscopes have broadened the applications of SEM for examining bio-specimens and enabled imaging of resin-embedded tissue blocks and sections using BSE-mode SEM under low-accelerating voltages; such techniques are fundamental to the 3D-SEM methods that are now known as focused ion-beam SEM, serial block-face SEM, and array tomography (i.e., serial section SEM). This technical breakthrough has allowed us to establish an innovative BSE imaging technique called section-face imaging to acquire ultrathin information from resin-embedded tissue sections. In contrast, serial section SEM is a modern 3D imaging technique for creating 3D surface rendering models of cells and organelles from tomographic BSE images of consecutive ultrathin sections embedded in resin. In this article, we introduce our related SEM techniques that use SE and BSE signals, such as the osmium maceration method, semithin section SEM (section-face imaging of resin-embedded semithin sections), section-face imaging for correlative light and SEM, and serial section SEM, to summarize their applications to neural structure and discuss the future possibilities and directions for these methods.

Three-dimensional reconstruction of the Golgi apparatus in osteoclasts by a combination of NADPase cytochemistry and serial section scanning electron microscopy

The three-dimensional morphology of the Golgi apparatus in osteoclasts was investigated by computer-aided reconstruction. Rat femora were treated for nicotinamide adenine dinucleotide phosphatase (NADPase) cytochemistry, and light microscopy was used to select several osteoclasts in serial semi-thin sections to investigate the Golgi apparatus by backscattered electron-mode scanning electron microscopy. Lace-like structures with strong backscattered electron signals were observed around the nuclei. These structures, observed within the Golgi apparatus, were attributed to the reaction products (i.e., lead precipitates) of NADPase cytochemistry. Features on the images corresponding to the Golgi apparatus, nuclei, and ruffled border were manually traced and three-dimensionally reconstructed using ImageJ/Fiji (an open-source image processing package). In the reconstructed model, the Golgi apparatus formed an almost-continuous structure with a basket-like configuration, which surrounded all the nuclei and also partitioned them. This peculiar three-dimensional morphology of the Golgi apparatus was discovered for the first time in this study. On the basis of the location of the cis- and trans-sides of the Golgi apparatus and the reported results of previous studies, we postulated that the nuclear membrane synthesized specific proteins in the osteoclasts and, accordingly, the Golgi apparatus accumulated around the nuclei as a receptacle.

Three-dimensional morphology of the Golgi apparatus in osteoclasts: NADPase and arylsulfatase cytochemistry, and scanning electron microscopy using osmium maceration

This study was designed to observe osteoclasts in the rat femora by light and electron microscopic cytochemistry for nicotinamide adenine dinucleotide phosphatase (NADPase) and arylsulfatase, and scanning electron microscopy using osmium maceration to assess the three-dimensional morphology of the Golgi apparatus in osteoclasts. The Golgi apparatus showed strong NADPase activity and surrounded each nucleus with the cis-side facing the nucleus. The Golgi apparatus could be often traced for a length of 20 μm or longer. Observations of serial semi-thin sections confirmed that a single line of reaction products (=lead precipitates) intervened somewhere between any two neighboring nuclei. The nuclear membrane showed strong arylsulfatase activity as well as rough endoplasmic reticulum and lysosomes. Scanning electron microscopy showed that the Golgi apparatus covered the nucleus in a porous sheet-like configuration. Under magnification, the cis-most saccule showed a sieve-like configuration with fine fenestrations. The saccules decreased fenestration numbers toward the trans-side and displayed a more plate-like appearance. The above findings indicate the following. (1) The Golgi saccules of osteoclasts have a three-dimensional structure comparable with that generally seen in other cell types. (2) The Golgi apparatus forms a porous multi-spherical structure around nuclei. Within the structure, in most cases a Golgi stack partitions the room into several compartments in each of which a nucleus fits. (3) The nuclear membrane synthesizes some kinds of proteins more stably and sufficiently than the rough endoplasmic reticulum. Consequently, the Golgi apparatus accumulates around nuclei with the cis-side facing the nucleus.

Q&A: Array tomography

Array tomography encompasses light and electron microscopy modalities that offer unparalleled opportunities to explore three-dimensional cellular architectures in extremely fine structural and molecular detail. Fluorescence array tomography achieves much higher resolution and molecular multiplexing than most other fluorescence microscopy methods, while electron array tomography can capture three-dimensional ultrastructure much more easily and rapidly than traditional serial-section electron microscopy methods. A correlative fluorescence/electron microscopy mode of array tomography furthermore offers a unique capacity to merge the molecular discrimination strengths of multichannel fluorescence microscopy with the ultrastructural imaging strengths of electron microscopy. This essay samples the first decade of array tomography, highlighting applications in neuroscience.

Backscattered electron imaging of resin-embedded sections

Scanning electron microscopes have longer focal depths than transmission electron microscopes and enable visualization of the three-dimensional (3D) surface structures of specimens. While scanning electron microscopy (SEM) in biological research was generally used for the analysis of bulk specimens until around the year 2000, more recent instrumental advances have broadened the application of SEM; for example, backscattered electron (BSE) signals under low accelerating voltages allow block-face and section-face images of tissues embedded in resin to be acquired. This technical breakthrough has led to the development of novel 3D imaging techniques including focused ion beam SEM, serial-block face SEM and serial section SEM. Using these new techniques, the 3D shapes of cells and cell organelles have been revealed clearly through reconstruction of serial tomographic images. In this review, we address two modern SEM techniques: section-face imaging of resin-embedded tissue samples based on BSE observations, and serial section SEM for reconstruction of the 3D structures of cells and organelles from BSE-mode SEM images of consecutive ultrathin sections on solid substrates.

Methods for array tomography with correlative light and electron microscopy

The three-dimensional ultra-structure is the comprehensive structure that cannot be observed from a two-dimensional electron micrograph. Array tomography is one method for three-dimensional electron microscopy. In this method, to obtain consecutive cross sections of tissue, connected consecutive sections of a resin block are mounted on a flat substrate, and these are observed with scanning electron microscopy. Although array tomography requires some bothersome manual procedures to prepare specimens, a recent study has introduced some techniques to ease specimen preparation. In addition, array tomography has some advantages compared with other three-dimensional electron microscopy techniques. For example, sections on the substrate are stored semi-eternally, so they can be observed at different magnifications. Furthermore, various staining methods, including post-embedding immunocytochemistry, can be adopted. In the present review, the preparation of specimens for array tomography, including ribbon collection and the staining method, and the adaptability for correlative light and electron microscopy are discussed.