A study in microtomy for electron microscopy

Insights and therapeutic advances in pancreatic cancer: the role of electron microscopy in decoding the tumor microenvironment

Pancreatic cancer is one of the most lethal cancers, with a 5-year overall survival rate of less than 10%. Despite the development of novel therapies in recent decades, current chemotherapeutic strategies offer limited clinical benefits due to the high heterogeneity and desmoplastic tumor microenvironment (TME) of pancreatic cancer as well as inefficient drug penetration. Antibody- and nucleic acid-based targeting therapies have emerged as strong contenders in pancreatic cancer drug discovery. Numerous studies have shown that these strategies can significantly enhance drug accumulation in tumors while reducing systemic toxicity. Additionally, electron microscopy (EM) has been a critical tool for high-resolution analysis of the TME, providing insights into the ultrastructural changes associated with pancreatic cancer progression and treatment responses. This review traces the current and technological advances in EM, particularly the development of ultramicrotomy and improvements in sample preparation that have facilitated the detailed visualization of cellular and extracellular components of the TME. This review highlights the contribution of EM in assessing the efficacy of therapeutic agents, from revealing apoptotic changes to characterizing the effects of novel compounds like ionophore antibiotic gramicidin A on cellular ultrastructures. Moreover, the review delves into the potential of EM in studying the interactions between the tumor microbiome and cancer cell migration, as well as in aiding the development of targeted therapies like antibody-drug conjugates (ADCs) and aptamer-drug conjugates (ApDCs).

ER-phagy: mechanisms, regulation and diseases connected to the lysosomal clearance of the endoplasmic reticulum

ER-phagy (reticulo-phagy) defines the degradation of portions of the endoplasmic reticulum (ER) within lysosomes or vacuoles. It is part of the self-digestion (i.e., auto-phagic) programs recycling cytoplasmic material and organelles, which rapidly mobilize metabolites in cells confronted with nutrient shortage. Moreover, selective clearance of ER subdomains participates to the control of ER size and activity during ER stress, the re-establishment of ER homeostasis after ER stress resolution and the removal of ER parts, in which aberrant and potentially cytotoxic material has been segregated. ER-phagy relies on the individual and/or concerted activation of the ER-phagy receptors, ER peripheral or integral membrane proteins that share the presence of LC3/Atg8-binding motifs in their cytosolic domains. ER-phagy involves the physical separation of portions of the ER from the bulk ER network, and their delivery to the endolysosomal/vacuolar catabolic district. This last step is accomplished by a variety of mechanisms including macro-ER-phagy (in which ER fragments are sequestered by double-membrane autophagosomes that eventually fuse with lysosomes/vacuoles), micro-ER-phagy (in which ER fragments are directly engulfed by endosomes/lysosomes/vacuoles), or direct fusion of ER-derived vesicles with lysosomes/vacuoles. ER-phagy is dysfunctional in specific human diseases and its regulators are subverted by pathogens, highlighting its crucial role for cell and organism life.

Axonal Organelles as Molecular Platforms for Axon Growth and Regeneration after Injury

Investigating the molecular mechanisms governing developmental axon growth has been a useful approach for identifying new strategies for boosting axon regeneration after injury, with the goal of treating debilitating conditions such as spinal cord injury and vision loss. The picture emerging is that various axonal organelles are important centers for organizing the molecular mechanisms and machinery required for growth cone development and axon extension, and these have recently been targeted to stimulate robust regeneration in the injured adult central nervous system (CNS). This review summarizes recent literature highlighting a central role for organelles such as recycling endosomes, the endoplasmic reticulum, mitochondria, lysosomes, autophagosomes and the proteasome in developmental axon growth, and describes how these organelles can be targeted to promote axon regeneration after injury to the adult CNS. This review also examines the connections between these organelles in developing and regenerating axons, and finally discusses the molecular mechanisms within the axon that are required for successful axon growth.

The Locus Coeruleus- Norepinephrine System in Stress and Arousal: Unraveling Historical, Current, and Future Perspectives

Arousal may be understood on a spectrum, with excessive sleepiness, cognitive dysfunction, and inattention on one side, a wakeful state in the middle, and hypervigilance, panic, and psychosis on the other side. However, historically, the concepts of arousal and stress have been challenging to define as measurable experimental variables. Divergent efforts to study these subjects have given rise to several disciplines, including neurobiology, neuroendocrinology, and cognitive neuroscience. We discuss technological advancements that chronologically led to our current understanding of the arousal system, focusing on the multifaceted nucleus locus coeruleus. We share our contemporary perspective and the hypotheses of others in the context of our current technological capabilities and future developments that will be required to move forward in this area of research.

A perfect confluence of physiology and morphology: discovery of the transverse tubular system and inward spread of activation in skeletal muscle

By early 1954, there existed a plausible model of muscle contraction called the sliding filament model. In addition, the nature of muscle excitation was understood. Surprisingly, the link between the membrane excitation and contraction was entirely unknown. This dilemma has been called the time-distance paradox. The path to discovery of the missing link between excitation and contraction was a rocky one involving the simultaneous but independent development of physiological and morphological studies. From the viewpoint of physiology, significant events included the most thrilling moment of a scientific life, confirmation of a hypothesis that was wrong, a major surprise and shock, a result not expected from evolutionary relationships, and disappointment and confusion before clarity. From the viewpoint of morphology, there was the exciting beginning and rapid development of biological electron microscopy, heroic experiments, the importance of sample preparative procedures, and discovery of clues from the old light microscopic literature. However, it was the confluence of physiology and morphology that brought clarity and a major advance in understanding, leading to the discovery of the transverse tubular system and inward spread of activation in skeletal muscle.

A brief history of how microscopic studies led to the elucidation of the 3D architecture and macromolecular organization of higher plant thylakoids

Microscopic studies of chloroplasts can be traced back to the year 1678 when Antonie van Leeuwenhoek reported to the Royal Society in London that he saw green globules in grass leaf cells with his single-lens microscope. Since then, microscopic studies have continued to contribute critical insights into the complex architecture of chloroplast membranes and how their structure relates to function. This review is organized into three chronological sections: During the classic light microscope period (1678-1940), the development of improved microscopes led to the identification of green grana, a colorless stroma, and a membrane envelope. More recent (1990-2020) chloroplast dynamic studies have benefited from laser confocal and 3D-structured illumination microscopy. The development of the transmission electron microscope (1940-2000) and thin sectioning techniques demonstrated that grana consist of stacks of closely appressed grana thylakoids interconnected by non-appressed stroma thylakoids. When the stroma thylakoids were shown to spiral around the grana stacks as multiple right-handed helices, it was confirmed that the membranes of a chloroplast are all interconnected. Freeze-fracture and freeze-etch methods verified the helical nature of the stroma thylakoids, while also providing precise information on how the electron transport chain and ATP synthase complexes are non-randomly distributed between grana and stroma membrane regions. The last section (2000-2020) focuses on the most recent discoveries made possible by atomic force microscopy of hydrated membranes, and electron tomography and cryo-electron tomography of cryofixed thylakoids. These investigations have provided novel insights into thylakoid architecture and plastoglobules (summarized in a new thylakoid model), while also producing molecular-scale views of grana and stroma thylakoids in which individual functional complexes can be identified.

The coordination of dietary valine and isoleucine on water holding capacity, pH value and protein solubility of fresh meat in finishing pigs

To investigate coordination of dietary levels of valine (Val) and isoleucine (Ile) on carcass traits and meat quality in pigs, 72 of 73.8 ± 1.6 kg crossbred barrows were randomly divided into a 2 (0.31% vs. 0.65% standard ileum digestible (SID) Val) × 2 (0.25% vs. 0.53% SID Ile) factorial arrangement. As a result, high dietary valine decreased myosin heavy-chain (MyHC)-I mRNA expression, pH_{24 h} value, bound water amount (T₂₁ peak area ratio), sarcoplasmic protein solubility and water holding capacity (WHC) of meat (P < .05). Meanwhile, high dietary isoleucine increased pH_{24 h} value, sarcomere length (P < .05) and tend to decrease drip loss (P = .07). The significant interactions between valine and isoleucine were observed on backfat thickness, water distribution forms and myofibrillar protein solubility (P < .05) of pork. High valine diets undermined WHC of meat probably through decreasing pH_{24 h} value and sarcoplasmic protein solubility considering their significant correlations with drip loss.

A primer on resolving the nanoscale structure of the plasma membrane with light and electron microscopy

Taraska reviews the imaging methods that are being used to understand the structure of the plasma membrane at the molecular level.

The plasma membrane separates a cell from its external environment. All materials and signals that enter or leave the cell must cross this hydrophobic barrier. Understanding the architecture and dynamics of the plasma membrane has been a central focus of general cellular physiology. Both light and electron microscopy have been fundamental in this endeavor and have been used to reveal the dense, complex, and dynamic nanoscale landscape of the plasma membrane. Here, I review classic and recent developments in the methods used to image and study the structure of the plasma membrane, particularly light, electron, and correlative microscopies. I will discuss their history and use for mapping the plasma membrane and focus on how these tools have provided a structural framework for understanding the membrane at the scale of molecules. Finally, I will describe how these studies provide a roadmap for determining the nanoscale architecture of other organelles and entire cells in order to bridge the gap between cellular form and function.

Molecular architecture of the presynaptic terminal

Neurotransmitter release at the presynaptic terminal is one of the fundamental processes in neuronal communication. It is a complex process comprising signaling pathways that exert a precise spatio-temporal coordination to prepare and bring synaptic vesicles to exocytosis. While many molecular components involved have been identified, their direct observation at different stages of the neurotransmitter release is lacking. Three-dimensional imaging by electron tomography provided remarkable views of the synaptic vesicles and the cytomatrix. Imaging fully hydrated, vitrified samples allowed a direct visualization, precise localization and a quantitative characterization of pleomorphic synaptic vesicle-bound complexes in situ, as well as the elucidation of their function in the neurotransmitter release.

Axonal endoplasmic reticulum is very narrow

The endoplasmic reticulum (ER) is an interconnected network of tubules and sheets. In most tissues of the body, ER tubules have a diameter of ∼60 nm. Using new methods for serial-section electron microscopy, a distinct class of very narrow, 20- to 30-nm-diameter tubules were found in neurons of both the central and peripheral nervous system. The narrow tubules appear to be the most abundant form of ER in axons, and are also found interspersed in the cell bodies and dendrites. At the site of branch points, there is a small sheet that has a similarly narrow lumen. The narrowness of the ER is likely to be important for the as yet poorly characterized functions of the axonal ER.

3-D EM exploration of the hepatic microarchitecture - lessons learned from large-volume in situ serial sectioning

To-date serial block-face scanning electron microscopy (SBF-SEM) dominates as the premier technique for generating three-dimensional (3-D) data of resin-embedded biological samples at an unprecedented depth volume. Given the infancy of the technique, limited literature is currently available regarding the applicability of SBF-SEM for the ultrastructural investigation of tissues. Herein, we provide a comprehensive and rigorous appraisal of five different SBF-SEM sample preparation protocols for the large-volume exploration of the hepatic microarchitecture at an unparalleled X, Y and Z resolution. In so doing, we qualitatively and quantitatively validate the use of a comprehensive SBF-SEM sample preparation protocol, based on the application of heavy metal fixatives, stains and mordanting agents. Employing the best-tested SBF-SEM approach, enabled us to assess large-volume morphometric data on murine parenchymal cells, sinusoids and bile canaliculi. Finally, we integrated the validated SBF-SEM protocol with a correlative light and electron microscopy (CLEM) approach. The combination of confocal scanning laser microscopy and SBF-SEM provided a novel way to picture subcellular detail. We appreciate that this multidimensional approach will aid the subsequent research of liver tissue under relevant experimental and disease conditions.

Three dimensional reconstruction by electron microscopy in the life sciences: An introduction for cell and tissue biologists

Early applications of transmission electron microscopy (TEM) in the life sciences have contributed tremendously to our current understanding at the subcellular level. Initially limited to two-dimensional representations of three-dimensional (3D) objects, this approach has revolutionized the fields of cellular and structural biology-being instrumental for determining the fine morpho-functional characterization of most cellular structures. Electron microscopy has progressively evolved towards the development of tools that allow for the 3D characterization of different structures. This was done with the aid of a wide variety of techniques, which have become increasingly diverse and highly sophisticated. We start this review by examining the principles of 3D reconstruction of cells and tissues using classical approaches in TEM, and follow with a discussion of the modern approaches utilizing TEM as well as on new scanning electron microscopy-based techniques. 3D reconstruction techniques from serial sections and (cryo) electron-tomography are examined, and the recent applications of focused ion beam-scanning microscopes and serial-block-face techniques for the 3D reconstruction of large volumes are discussed. Alternative low-cost techniques and more accessible approaches using basic transmission or field emission scanning electron microscopes are also examined.

Targeting the hallmarks of cancer with therapy-induced endoplasmic reticulum (ER) stress

The endoplasmic reticulum (ER) is at the center of a number of vital cellular processes such as cell growth, death, and differentiation, crosstalk with immune or stromal cells, and maintenance of proteostasis or homeostasis, and ER functions have implications for various pathologies including cancer. Recently, a number of major hallmarks of cancer have been delineated that are expected to facilitate the development of anticancer therapies. However, therapeutic induction of ER stress as a strategy to broadly target multiple hallmarks of cancer has been seldom discussed despite the fact that several primary or secondary ER stress-inducing therapies have been found to exhibit positive clinical activity in cancer patients. In the present review we provide a brief historical overview of the major discoveries and milestones in the field of ER stress biology with important implications for anticancer therapy. Furthermore, we comprehensively discuss possible strategies enabling the targeting of multiple hallmarks of cancer with therapy-induced ER stress.

Transcriptomic and physiological insights into the robustness of long filamentous cells of Methanosaeta harundinacea, prevalent in upflow anaerobic sludge blanket granules

Methanosaeta spp. are widely distributed in natural environments, and their filamentous cells contribute significantly to sludge granulation and the good performance of anaerobic reactors. A previous study indicated that Methanosaeta harundinacea 6Ac displays a quorum sensing-regulated morphological transition from short to long filaments, and more acetate is channeled into methane production in long filaments, whereas more is channeled into biomass synthesis in short filaments. Here, we performed transcriptomic and physiological analysis to gain insights into active methanogenesis in long filaments of M. harundinacea 6Ac. Both RNA sequencing (RNA-seq) and quantitative reverse transcription-PCR indicated that transcription of the genes involved in aceticlastic methanogenesis and energy metabolism was upregulated 1.2- to 10.3-fold in long filaments, while transcription of the genes for the methyl oxidative shunt was upregulated in short filaments. [2-(13)C]acetate trace experiments demonstrated that a relatively higher portion of the acetate methyl group was oxidized to CO2 in short filaments than in long filaments. The long filaments exhibited higher catalase activity and oxygen tolerance than the short ones, which is consistent with increased transcription of the oxidant-scavenging genes. Moreover, transcription of genes for cell surface structures was upregulated in the long filaments, and transmission electron microscopy revealed a thicker cell envelope in the filaments. RNA-seq determined a >2-fold upregulation of a variety of antistress genes in short filaments, like those encoding chaperones and DNA repair systems, which implies that the short filaments can be stressed. This study reveals the genetic basis for the prevalence of the long filamentous morphology of M. harundinacea cells in upflow anaerobic sludge blanket granules.

Is EM dead?

Since electron microscopy (EM) first appeared in the 1930s, it has held centre stage as the primary tool for the exploration of biological structure. Yet, with the recent developments of light microscopy techniques that overcome the limitations imposed by the diffraction boundary, the question arises as to whether the importance of EM in on the wane. This Commentary describes some of the pioneering studies that have shaped our understanding of cell structure. These include the development of cryo-EM techniques that have given researchers the ability to capture images of native structures and at the molecular level. It also describes how a number of recent developments significantly increase the ability of EM to visualise biological systems across a range of length scales, and in 3D, ensuring that EM will remain at the forefront of biology research for the foreseeable future.

Cryo-electron tomography: the challenge of doing structural biology in situ

Electron microscopy played a key role in establishing cell biology as a discipline, by producing fundamental insights into cellular organization and ultrastructure. Many seminal discoveries were made possible by the development of new sample preparation methods and imaging modalities. Recent technical advances include sample vitrification that faithfully preserves molecular structures, three-dimensional imaging by electron tomography, and improved image-processing methods. These new techniques have enabled the extraction of high fidelity structural information and are beginning to reveal the macromolecular organization of unperturbed cellular environments.

Stacked endoplasmic reticulum sheets are connected by helicoidal membrane motifs

The endoplasmic reticulum (ER) often forms stacked membrane sheets, an arrangement that is likely required to accommodate a maximum of membrane-bound polysomes for secretory protein synthesis. How sheets are stacked is unknown. Here, we used improved staining and automated ultrathin sectioning electron microscopy methods to analyze stacked ER sheets in neuronal cells and secretory salivary gland cells of mice. Our results show that stacked ER sheets form a continuous membrane system in which the sheets are connected by twisted membrane surfaces with helical edges of left- or right-handedness. The three-dimensional structure of tightly stacked ER sheets resembles a parking garage, in which the different levels are connected by helicoidal ramps. A theoretical model explains the experimental observations and indicates that the structure corresponds to a minimum of elastic energy of sheet edges and surfaces. The structure allows the dense packing of ER sheets in the restricted space of a cell.

Some retrospectives on early studies of plant microtubules

We pay tribute to the seminal paper 'A microtubule in plant cell fine structure' by Myron C. Ledbetter and Keith R. Porter (1963) by summarizing the very limited knowledge of plant cell ultrastructure that we had prior to that publication, and, by way of our three retrospective accounts, show how this paper stimulated and influenced subsequent research on plant microtubules. Micrographs of historical interest are presented that are either previously unpublished or from primary publications.

Methylobacterium bullatum sp. nov., a methylotrophic bacterium isolated from Funaria hygrometrica

A novel, pink-pigmented aerobic, facultatively methylotrophic bacterial strain (F3.2(T)) isolated from the phyllosphere of Funaria hygrometrica, was analyzed using a polyphasic approach. Cells were Gram-negative, motile rods, strictly aerobic and non-spore-forming and exhibited surface structures varying in quantity, distribution and morphology. The isolate grew at 10-33°C over a pH range of 5.5-8.0 and in the presence of less than 1.0% NaCl. Strain F3.2(T) shared less than 70% DNA-DNA binding to the next type strain of the genus Methylobacterium (M. adhaesivum DSM 17169(T)). In addition to the major cellular fatty acid C(18:1)ω7c (81.7%), present in all Methylobacterium species (and also members of the genus Alphaproteobacteria), a high value (11.7%) of the fatty acids (summed feature) C(16:1)ω7c and/or iso-C(15:0)2OH was determined. Phylogenetic analyses based on 16S rDNA and methanol dehydrogenase gene sequences, DNA-DNA hybridization values, chemotaxonomic and phenotypic characteristics indicate that the strain F3.2(T) represents a novel species within the genus Methylobacterium. We propose the name Methylobacterium bullatum sp. nov. for this species. The type strain is the strain F3.2(T) (DSM 21893(T)=LMG 24788(T)).

STRUCTURE AND DEVELOPMENT OF VIRUSES OBSERVED IN THE ELECTRON MICROSCOPE : IV. VIRUSES OF THE RI-APC GROUP

Representative viruses of the RI-APC group were observed with the electron microscope in thin sections of infected HeLa cells. The viral particles varied in density, were approximately 60 mµ in diameter and had a center to center spacing when close packed of about 65 mµ. Many of the less dense particles exhibited an internal body averaging 24 mµ in diameter. It was suggested that within the nucleus the virus differentiated from dense granular and reticular material and formed crystals. Disintegration of the crystals and disruption of the nuclear membrane with release of virus into the cytoplasm appeared to occur at any stage. No evidence to suggest development of the virus in the cytoplasm was obtained. It was possible to deduce the structure of the viral crystal from the electron micrographs. The viral particles are packed in a cubic body—centered lattice. Correlative histochemical observations in the light microscope which are now in progress revealed that the crystals and non-crystalline aggregates of virus were strongly Feulgen-positive.

From central to rudimentary to primary: the history of an underappreciated organelle whose time has come. The primary cilium

For the first time, the history of the central flagellum/primary cilium has been explored systematically and in depth. It is a long and informative story about the course of scientific discovery, memory loss and rediscovery. The progress of our story is saltatory, pushed onward by innovations in technology and retarded by socio-scientific issues of linguistic and temporal chauvinism. Over one hundred and fifty years passed between the discovery of this organelle and full appreciation of its important functions. The main character in our story is an organelle that was relegated to a very minor role in the cellular opera for a very long time, until its rather sudden promotion to a central role in orchestrating many of the sensory and signaling events of the cell. Although early investigators speculated on just such a role for the primary cilium as early as 1898, it was over one hundred years before proof for this hypothesis was forthcoming.

Electron tomography in life science

Electron tomography (ET) is a three-dimensional technique suitable to study pleomorphic biological structures with nanometer resolution. This makes the methodology remarkably versatile, allowing the exploration of a large range of biological specimens, both in an isolated state and in their cellular context. The application of ET has undergone an exponential growth over the last decade, enabled by seminal technological advances in methods and instrumentation, and is starting to make a significant impact on our understanding of the cellular world. While the attained results are already remarkable, ET remains a young technique with ample potential to be exploited. Current developments towards large-scale automation, higher resolution, macromolecular labeling and integration with other imaging techniques hold promise for a near future in which ET will extend its role as a pivotal tool in structural and cell biology.

In search of mitochondrial mechanisms: interfield excursions between cell biology and biochemistry

Developing models of biological mechanisms, such as those involved in respiration in cells, often requires collaborative effort drawing upon techniques developed and information generated in different disciplines. Biochemists in the early decades of the 20th century uncovered all but the most elusive chemical operations involved in cellular respiration, but were unable to align the reaction pathways with particular structures in the cell. During the period 1940-1965 cell biology was emerging as a new discipline and made distinctive contributions to understanding the role of the mitochondrion and its component parts in cellular respiration. In particular, by developing techniques for localizing enzymes or enzyme systems in specific cellular components, cell biologists provided crucial information about the organized structures in which the biochemical reactions occurred. Although the idea that biochemical operations are intimately related to and depend on cell structures was at odds with the then-dominant emphasis on systems of soluble enzymes in biochemistry, a reconceptualization of energetic processes in the 1960s and 1970s made it clear why cell structure was critical to the biochemical account. This paper examines how numerous excursions between biochemistry and cell biology contributed a new understanding of the mechanism of cellular respiration.

Biological ultrastructure research; the first 50 years

The second half of the 20th century has witnessed the birth and growth of biological ultrastructure research--a branch of cell biology in which electron microscopy plays an important role. After a humble start in around 1950, when only a limited arsenal of instrumentation was available, a wealth of auxiliary methodologies were developed and gradually put in use. Here we review these techniques: ultramicrotomy of "optimally" fixed and prepared samples, histochemical methods such as immuno-electron microscopy and electron microscope autoradiography, negative staining techniques, freeze-fracturing and other techniques. Closer to the millennium shift, various cryotechniques have gradually developed. Together with computer-based reconstruction methods they are likely to play increasingly more important roles in the future.

Some features of the submicroscopic morphology of synapses in frog and earthworm

Electron micrographs are presented of synaptic regions encountered in sections of frog sympathetic ganglia and earthworm nerve cord neuropile. Pre- and postsynaptic neuronal elements each appear to have a membrane 70 to 100 A thick, separated from each other over the synaptic area by an intermembranal space 100 to 150 A across. A granular or vesicular component, here designated the synaptic vesicles, is encountered on the presynaptic side of the synapse and consists of numerous oval or spherical bodies 200 to 500 A in diameter, with dense circumferences and lighter centers. Synaptic vesicles are encountered in close relationship to the synaptic membrane. In the earthworm neuropile elongated vesicles are found extending through perforations or gaps in the presynaptic membrane, with portions of vesicles appearing in the intermembranal space. Mitochondria are encountered in the vicinity of the synapse, and in the frog, a submicroscopic filamentary component can be seen in the presynaptic member extending up to the region where the vesicles are found, but terminating short of the synapse itself.

Electron microscopy of early cytoplasmic changes due to influenza virus

Recently improved methods for visualization of thin tissue sections by electron microscopy have been applied to the study of early changes in the bronchial epithelium of mice infected by inhalation of aerosols of influenza virus. In confirmation of previous findings by the authors, inclusion bodies have been demonstrated in ciliated and non-ciliated cells of infected bronchial epithelium. In addition to 3 strains of mouse-adapted Type A virus, 2 unadapted strains gave qualitatively the same results. The inclusion bodies were found to be composed largely of particles of a size estimated to correspond to the known size of influenza virus. The viral lesion of the cytoplasm was also associated with linear formations which were thought to be abnormal forms of endoplasmic reticulum. Well developed microvilli were found on the ciliated borders of ciliated cells, but no evidence was found of viral growth in this region.

The fine structure of neurons

1. Thin sections of representative neurons from intramural, sympathetic and dorsal root ganglia, medulla oblongata, and cerebellar cortex were studied with the aid of the electron microscope. 2. The Nissl substance of these neurons consists of masses of endoplasmic reticulum showing various degrees of orientation; upon and between the cisternae, tubules, and vesicles of the reticulum lie clusters of punctate granules, 10 to 30 mmicro in diameter. 3. A second system of membranes can be distinguished from the endoplasmic reticulum of the Nissl bodies by shallower and more tightly packed cisternae and by absence of granules. Intermediate forms between the two membranous systems have been found. 4. The cytoplasm between Nissl bodies contains numerous mitochondria, rounded lipid inclusions, and fine filaments.