An electron microscopic study of normal bovine spinal ganglia and nerves

Ultrastructure of dorsal root ganglia

Dorsal root ganglia (DRG) contains thousands of sensory neurons that transmit information about our external and internal environment to the central nervous system. This includes signals related to proprioception, temperature, and nociception. Our understanding of DRG has increased tremendously over the last 50 years and has established the DRG as an active participant in peripheral processes. This includes interactions between neurons and non-neuronal cells such as satellite glia cells and macrophages that contribute to an increasingly complex cellular environment that modulates neuronal function. Early ultrastructural investigations of the DRG have described subtypes of sensory neurons based on differences in the arrangement of organelles such as the Golgi apparatus and the endoplasmic reticulum. The neuron-satellite cell complex and the composition of the axon hillock in DRG have also been investigated, but, apart from basic descriptions of Schwann cells, ultrastructural investigations of other cell types in DRG are limited. Furthermore, detailed descriptions of key components of DRG, such as blood vessels and the capsule that sits at the intersection of the meninges and the connective tissue covering the peripheral nervous system, are lacking to date. With rising interest in DRG as potential therapeutic targets for aberrant signalling associated with chronic pain conditions, gaining further insights into DRG ultrastructure will be fundamental to understanding cell-cell interactions that modulate DRG function. In this review, we aim to provide a synopsis of the current state of knowledge on the ultrastructure of the DRG and its components, as well as to identify areas of interest for future studies.

Perikaryal surface specializations of neurons in sensory ganglia

Slender projections, similar to microvilli, are the main specialization of the perikaryal surface of sensory ganglion neurons. The extent of these projections correlates closely with the volume of the corresponding nerve cell body. It is likely that the role of perikaryal projections of sensory ganglion neurons, which lack dendrites, is to maintain the surface-to-volume ratio of the nerve cell body above some critical level for adequate metabolic exchange. Satellite cells probably have the ability to promote, or provide a permissive environment for, the outgrowth of these projections. It is not yet known whether the effect of satellite cells is mediated by molecules associated with their plasma membrane or by diffusible factors. Furthermore, receptor molecules for numerous chemical agonists are located on the nerve cell body surface, but it is not known whether certain molecules are located exclusively on perikaryal projections or are also present on the smooth surface between these projections. Further study of the nerve cell body surface and of the influence that satellite cells exert on it will improve our understanding of the interactions between sensory ganglion neurons and satellite neuroglial cells.

Nervus terminalis ganglion of the bonnethead shark (Sphyrna tiburo): evidence for cholinergic and catecholaminergic influence on two cell types distinguished by peptide immunocytochemistry

The nervus terminalis is a ganglionated vertebrate cranial nerve of unknown function that connects the brain and the peripheral nasal structures. To investigate its function, we have studied nervus terminalis ganglion morphology and physiology in the bonnethead shark (Sphyrna tiburo), where the nerve is particularly prominent. Immunocytochemistry for gonadotropin-releasing hormone (GnRH) and Leu-Pro-Leu-Arg-Phe-NH2 (LPLRFamide) revealed two distinct populations of cells. Both were acetylcholinesterase positive, but LPLR-Famide-immunoreactive cells consistently stained more darkly for acetylcholinesterase activity. Tyrosine hydroxylase immunocytochemistry revealed fibers and terminal-like puncta in the ganglion, primarily in areas containing GnRH-immunoreactive cells. Consistent with the anatomy, in vitro electrophysiological recordings provided evidence for cholinergic and catecholaminergic actions. In extracellular recordings, acetylcholine had a variable effect on baseline ganglion cell activity, whereas norepinephrine consistently reduced activity. Electrical stimulation of the nerve trunks suppressed ganglion activity, as did impulses from the brain in vivo. During electrical suppression, acetylcholine consistently increased activity, and norepinephrine decreased activity. Muscarinic and, to a lesser extent, alpha-adrenergic antagonists both increased activity during the electrical suppression, suggesting involvement of both systems. Intracellular recordings revealed two types of ganglion cells that were distinguishable pharmacologically and physiologically. Some cells were hyperpolarized by cholinergic agonists and unaffected by norepinephrine; these cells did not depolarize with peripheral nerve trunk stimulation. Another group of cells did depolarize with peripheral trunk stimulation; a representative of this group was depolarized by carbachol and hyperpolarized by norepinephrine. These and other data suggest that the bonnethead nervus terminalis ganglion contains at least two cell populations that respond differently to acetylcholine and norepinephrine. The bonnethead nervus terminalis ganglion appears to differ fundamentally from sensory and autonomic ganglia but does share some features with the neural circuits of forebrain GnRH systems.

Clusters of nerve cell bodies enclosed within a common connective tissue envelope in the spinal ganglia of the lizard and rat

A careful search for groups of nerve cell bodies enclosed within a common connective envelope was made in the spinal ganglia of the lizard and rat using a serial-section technique. Nerve cell bodies sharing a common connective envelope were found to be more common in the lizard (9.4%) than in the rat (5.6%). These nerve cell bodies were arranged in pairs, or, less frequently, in groups of three. At times, they appeared to be in immediate contact, with no intervening satellite cells; at others, they remained separated from one another by a satellite cell sheet. The clusters of nerve cell bodies enclosed within a common connective envelope probably result from the arrest of developmental processes in the spinal ganglion. It is possible that, as a result of the cell arrangement here described, certain neurons electrically influence other sensory neurons at the level of the ganglion.

The perikaryal projections of rabbit spinal ganglion neurons. A comparison of thin section reconstructions and scanning microscopy views

Shape, length and width of the perikaryal projections of spinal ganglion neurons from adult rabbits fixed in situ by perfusion have been evaluated by means of serial section electron microscopy. The results thus obtained have been compared with those obtained by enzymatic removal of ganglionic connective tissue and satellite cells followed by direct observation of the true neuronal surface under the scanning electron microscope. The comparison has shown that the perikaryal projections exhibit a similar shape and similar size with both techniques.

Scanning electron-microscope observations of the perikaryal projections of rabbit spinal ganglion neurons after enzymatic removal of connective tissue and satellite cells

The true surface of rabbit spinal ganglion neurons has been made directly accessible to scanning electron-microscope observation after removal of both the connective tissue and satellite cells that normally cover it. The neuronal surface is characterized by a profusion of slender projections whose shapes have been determined and whose length and width have been quantified. Controls carried out with transmission electron microscopy demonstrate that the procedure employed in this study satisfactorily preserves neuronal structure.

False-positive artifacts of tracer strategies distort autonomic connectivity maps

The widespread use of new axonal transport tracing techniques in the ANS has resulted in substantially revised and amended descriptions of ANS organization. The present review suggests, however, that at least some of the results on which proposed revisions of ANS anatomy have been based have incorporated artifacts and therefore should be cautiously interpreted. The peripheral nervous system and viscera are composed in part of connective and endothelial tissues that are porous or 'leaky' to solutes with appropriate chemical characteristics, including the major tracer compounds. As a result, several extra-axonal routes for redistribution of label from the application site into other tissues are present. These include (1) diffusion through tissue membranes to enter directly adjacent tissues and (2) leakage into extracellular fluids within the body cavity, vasculature, lymphatics, exocrine ducts, or organ lumens to migrate to more distant tissues. As a consequence of the extreme sensitivity of the methods used, such redistribution of even minute amounts of label can produce false positives. Review of autonomic neuroanatomy suggests additional mechanisms, including tracer uptake by fibers of passage, can produce artifactual staining. Based on these surveys of tissue composition, tracer characteristics and sources of artifact, experimental controls and criteria for identifying and avoiding labeling artifacts are described. Since no single procedure is foolproof for ANS experimentation, the routine application of multiple controls, particularly ones which restrict or prevent tracer diffusion, are needed.

A quantitative electron microscope study of the perikaryal projections of sensory ganglion neurons. I. Cat and rabbit

With a quantitative method and serial sections a study was carried out under the electron microscope of the perikaryal projections of the neurons in the thoracic spinal ganglia of cat and rabbit. These projections usually appear as finger-shaped evaginations which run roughly parallel to the surface of the nerve cell body. Their length ranges between 0.3 and 3.25 microns, and they show a nearly circular cross section with a rather uniform transverse diameter having an average value of about 0.2 microns. Both in cat and rabbit a very high correlation was found between the surface area of perikaryal projections and both the volume and smoothed surface area of the corresponding nerve cell body. Perikaryal projections increase the surface area of the nerve cell body by 43% in cat and 39.5% in rabbit. These findings support the idea that perikaryal projections in sensory ganglion neurons are normal formations, which maintain the surface-to-volume ratio above the critical level for metabolic exchanges.

A study of virus-like particles present in bovine nerve sheath tumours

The morphology of virus-like particles present in bovine nerve sheath tumours is described, and their origin and significance discussed. Virus-like particles were investigated using transmission electron microscopy and tissue culture. The particles were found in a number of tumours and were present either as aggregates in pseudonuclear inclusions or as budding forms associated with the plasma membrane. The latter were found only in cultured tissue. Although other cellular structures were considered it was thought that the particles represented virus, with those in pseudonuclear inclusions being an intermediate form. Both bovine syncytial virus and oncovirinae were considered as possibilities in the aetiology of the condition.

A study of the perineurial diffusion barrier of a peripheral ganglion

The perineurial diffusion barrier to horseradish peroxidase (HRP) and ferritin was investigated in superior cervical ganglia of rats and mice. The ganglion was surrounded by a delicate epineurium and 2-5 perineurial lamellae joined by zonulae occludentes and desmosomes. Following local application of tracers the animals were killed after 5, 30, and 60 min and the distribution of HRP and ferritin was studied by light and electron microscopy. The inner layers of the ganglionic perineurium prevented diffusion of both HRP and ferritin perineurial lamellae investing the ganglion. HRP had often extended to the innermost lamella 60 min after application. HRP and ferritin were present in vesicles of ganglionic perineurial cells. There was no passage of tracers via intercellular junctions.

The ultrastructure of bovine peripheral nerve sheath tumours

Bovine peripheral nerve sheath tumours from 30 cattle were similar ultrastructurally to human schwannomas and neurofibromas. Bovine neurofibromatous tissue had large amounts of extracellular material, primarily collagen and electron lucent granular material. The principal cells had basal laminae and a disorganized proliferation of the plasmalemma. Axons were consistently seen and were surrounded by the plasmalemma of principal cells. The principal cells seemed to be Schwann cells or variants of them. Bovine schwannomas had areas similar to Antoni type A tissue with sparse extracellular material, few, if any, axons, and an apparent organized layering of cytoplasmic processes clad in a basal lamina. Cell nuclei often formed palisades. The principal cells in bovine schwannomas might be derived either from Schwann cells or perinuerial cells. Bovine schwannomas appeared together with bovine neurofibromatous tissue in affected nerves.

The altrastructure of Auerbach's plexus in the guinea-pig. I. Neuronal elements

The ultrastructural features of nerve cell bodies and axon profiles within Auerbach's plexus in the stomach, ileum, caecum and colon of the guinea-pig have been examined. Nerve cell bodies have been tentatively classified into nine different types according to their size, distribution of organelles, location and relationship to satellite cells. Except for cell size, no attempt has been made to correlate ultrastructual with light microscopical observations. On the basis of vesicular size, shape and content, eight morphologically distinct types of axon profile have been identified as well as two profile types which are thought to reflect different physiological conditions. The axons contained various populations of small, mostly granular vesicles; small, round agranular vesicles; small, flattened vesicles; large flattened or elongated vesicles; and three types of large vesicle with granular contents distinguished by size. Some correlation between types of axon profile and two types of nerve cell body was recognized. However, more than one type of axon profile usually formed synapses with one type of cell body, and a precise correlation was not determined.