Showing papers in "Journal of Neurocytology in 1974"
TL;DR: In freeze-fractured frog sartorius muscles, the long terminal branches of motor axons possess a series of narrow transverse ridges on their surface, bordered by rows of relatively large particles within the presynaptic membrane, which represent anen face view of the electron-dense cytoplasmic bands around which synaptic vesicles cluster.
Abstract: In freeze-fractured frog sartorius muscles, the long terminal branches of motor axons possess a series of narrow transverse ridges on their surface, bordered by rows of relatively large particles within the presynaptic membrane. By their exclusive location opposite muscle folds, it is apparent that these ridges represent anen face view of the electron-dense cytoplasmic bands around which synaptic vesicles cluster. In resting terminals there is no sign that vesicles underlie these ridges, except for an occasional bulge where a vesicle presses against the plasmalemma; but in terminals stimulated briefly in fixative, the ridges are surrounded by a number of small dimples where synaptic vesicles attach to the plasmalemma. Such ‘vesicle sites’ do not appear when Mg++ is used to prevent the transmitter release that results from stimulation, so they presumably represent sites of transmitter discharge. However, more vesicle sites appear in terminals stimulated in more slowly acting fixatives, so they appear to accumulate for some time during fixation and do not indicate the instantaneous level of transmitter release. Vesicle sites occur in a variety of sizes and shapes that may represent different stages of vesicle discharge. Dimples which appear during stimulation under Schwann processes, where endocytosis of coated vesicles has been found to occur, are sometimes larger than vesicle sites but otherwise look much the same; so it is not possible to readily distinguish between the freeze fracture images of synaptic vesicle discharge and coated vesicle formation at this synapse. The muscle membrane beneath nerve terminals is paved with clusters of relatively large particles which mostly appear on the cytoplasmic half of the membrane after fracture. These clusters of particles occur in regions of the postsynaptic membrane that are coated by plaques of electron-dense cytoplasmic fuzz. Immediately around the clusters are a number of tightly-packed, orthogonal aggregates of slightly smaller particles.
487 citations
TL;DR: The total number of neuroglial cells increases with age, however, the numbers of the various types do not increase by similar proportions and in animals 18 months and older, examples of the multipolar, the elongated, and the engorged spherical forms of microglia may be encountered.
Abstract: Neuroglial cells were examined in the rat auditory cerebral cortex to determine the effects of aging on astrocytes, oligodendrocytes and microglia. The animals ranged in age from 3 months to 29.5 months. Over this age range of 26.5 months none of the neuroglial cells exhibit striking abnormalities in the morphology or the number of their organelles other than each of these cell types accumulates a distinct membrane-bound inclusion material. Astrocytes accumulate considerably more of this material than oligodendrocytes, and demonstrate earlier and more frequent signs of it. Microglia show the most striking alterations in regard to the inclusion material and in animals of increasing age microglial cells engorged with the heterogeneous material become increasingly common. Concurrently there is a slow transformation of the microglial population from an array of multipolar forms to larger, elongated or spherical forms which contain variable amounts of the dense membrane-bound material. These alterations in the microglial cell population do not occur at a chronologically uniform rate, and in animals 18 months and older, examples of the multipolar, the elongated, and the engorged spherical forms of microglia may be encountered. The total number of neuroglial cells increases with age. However, the numbers of the various types do not increase by similar proportions. While there is little change in the populations of astrocytes and oligodendrocytes in rats between the ages of 3 and 27 months, in the same animals the number of microglia increases by about 65%.
293 citations
TL;DR: The sequestration and phagocytosis of unusual and abnormal axoplasmic organelles by Schwann and oligodendroglial cells is described, maximally developed in situations where there is centripetal axonal degeneration such as occurs in dying-back toxic disease and in the proximal stump of an amputated nerve.
Abstract: A mechanism is postulated and described for the sequestration and phagocytosis of unusual and abnormal axoplasmic organelles by Schwann and oligodendroglial cells. Axonal organelles involved in this process are clear and dense-core vesicles, membrane-bounded dense membranous bodies reminiscent of secondary lysosomes, enlarged mitochondria, glycogen-like granules and glycogen-filled mitochondrial remnants. The process of sequestration of these organelles begins with the formation of a ridge of ensheathing cell adaxonal cytoplasm adjacent to an internally coated region of axolemma. The ridge of adaxonal cytoplasm enlarges to form a thin sheet which indents the axon surface adjacent to the abnormal axonal organelles. The invaginating adaxonal cytoplasmic sheet surrounds the abnormal axonal organelles and segregates them from the remainder of the axon. The cytoplasmic sheet infolds on itself and sequesters groups of axoplasmic organelles to form an interdigitated profile when viewed in cross-section. Electron lucent areas correspond to sequestered axoplasm and electron dense areas to ensheathing cell cytoplasm. The membranes separating axoplasm and ensheathing cell cytoplasm in the interdigitated networks break down allowing the abnormal axoplasmic organelles to be phagocytosed by the ensheathing cell cytoplasm. The process occurs to a limited degree in the normal nervous system at paranodes but is much more developed in pathologic situations where there is early axonal disease. The process is maximally developed in situations where there is centripetal axonal degeneration such as occurs in dying-back toxic disease and in the proximal stump of an amputated nerve.
183 citations
TL;DR: The formation of ‘axoglial synapses’ may reflect errors in development while their disappearance may result when more specific mechanisms of synaptic differentiation become operational, and the formation of synaptic junctions occurs in a hierarchy of steps.
Abstract: Neurites have been observed in various types of fine structural associations with developing radial glial processes in the lateral marginal zones of embryonic mouse spinal cords. The least complex of the neurite-glial relationships involves a simple apposition of dendrites and dendritic growth cones with radial glial processes. We interpret this relationship to mean that dendrites, growing from the somata of the lateral motor nucleus, may preferentially move along the interface between radial glial processes and the adjacent axons of the marginal zone. A somewhat more specialized neurite-glial association is the occurrence ofpuncta adhaerentia between neurites and radial glial processes. Finally, the most elaborate morphological relationship observed is that of synapse-like contacts between axons and radial glial processes. These ‘axoglial synapses’ begin to appear at E11 and E12, are most numerous in E13 and E14 spinal cords, but they can not be found after embryonic day 15. Based on these observations, we make the following speculation: the formation of synaptic junctions occurs in a hierarchy of steps with a process of trial and error primarily involved in the initial stages of synapto-genesis, while more specific requirements are involved in the later steps of synaptic differentiation. Thus, the formation of ‘axoglial synapses’ may reflect errors in development while their disappearance may result when more specific mechanisms of synaptic differentiation become operational.
168 citations
TL;DR: The synaptic glomeruli of the rat gelatinosa are thus a synaptic relay station for primary afferents, whose terminals possess an extralysosomal acid phosphatase possibly concerned with the metabolism of a local synaptic transmitter.
Abstract: The rat substantia gelatinosa Rolandi (lamina II) was found to contain synaptic glomeruli at the centre of which was a dark terminal (C-terminal) full of round synaptic vesicles. The C-terminal was presynaptic to several dendritic profiles and to terminals containing sparse pleomorphic synaptic vesicles, and was occasionally postsynaptic at symmetrical contacts to other terminals containing numerous pleomorphic vesicles of which about one-third were elongated. Acid phosphatase activity was observed on, and between, the membranes of the synaptic vesicles of the central bouton, as well as in the hyaloplasm of the peripheral elements of the glomeruli. Dorsal root section caused electrondense degeneration of the central boutons. Acid phosphatase activity disappeared from synaptic vesicle membranes in C-boutons, and gradually filled their dense cytoplasmic matrix. The enzyme disappeared completely from the peripheral profiles. The synaptic glomeruli of the rat gelatinosa are thus a synaptic relay station for primary afferents, whose terminals possess an extralysosomal acid phosphatase possibly concerned with the metabolism of a local synaptic transmitter. Enzyme accumulation in degenerating knobs seems to be an autolytic process distinct from the normal function of this acid phosphatase.
165 citations
TL;DR: A general account is given of synaptic organization in the rat LGd, with special emphasis on the intrinsic neurons, and the vesicle-containing P-boutons to which they give rise, based on ultrastructural studies of normal adult rats and of experimental animals surviving from less than one day to more than four weeks following unilateral eye removal or ablation of the visual and peri-visual cortex.
Abstract: A general account is given of synaptic organization in the rat LGd, with special emphasis on the intrinsic neurons, and the vesicle-containing P-boutons to which they give rise. The account is based on ultrastructural studies of normal adult rats, and of experimental animals surviving from less than one day to more than four weeks following unilateral eye removal or ablation of the visual and peri-visual cortex. P-boutons originate as narrow stalks or as blunt outpushings from presynaptic dendritic shafts. Commonly a single appendage comprises a series of P-boutons interconnected by narrow intervaricose portions. There is a very close correspondence in size and appearance between such appendages characterized by electron microscopy and the complex dendritic appendages seen in Golgi impregnations. P-boutons represent the principal synaptic territory of the intrinsic neuron. They arepresynaptic to relay cell dendrites (or dendritic appendages), and to other P-boutons (or, less commonly, to presynaptic dendrites). They arepostsynaptic to other P-boutons (or, less commonly, to presynaptic dendrites), to axon terminals containing ‘flattened’ vesicles (F-boutons), and to large intraglomerular axon terminals containing spherical synaptic vesicles (R-boutons). Degeneration of R-boutons after enucleation indicates that most are the terminals of retinal afferents. The P-boutons are intermediate elements in various serial synapses, the most characteristic of which is the intraglomerular triplet (or triad) synapse. At triplet synapses, a P-bouton and a relay cell dendrite onto which the P-bouton synapses, both receive at least one synaptic contact from an R-bouton. The possible role of triplet synapses in rapid feed-forward inhibition is considered. P-boutons and presynaptic dendrites also establish reciprocal synaptic relationship with one another, although closely spaced reciprocal synapses (c. 0.25 μm apart) are rare. The F-bouton population originates at least in part from small myelinated axons. The F-boutons contain closely packed, cylindrical synaptic vesicles, establish Gray type 2 contacts and are never postsynaptic. They are greatly outnumbered by P-boutons within the glomeruli, but synapse extensively in the extraglomerular neuropil onto both intrinsic and relay cell somas and dendrites, and onto the initial axons of relay cells. The possibility that some F-boutons originate from brain stem afferents is considered.
157 citations
TL;DR: The morphologically distinct V nerve innervation of the SG layer and the internal circuitry of theSG glomeruli appear to be essential for the perception of thermal and painful stimuli.
Abstract: In the spinal trigeminal nucleus of the adult cat, the neuropil of the substantia gelatinosa (SG) layer is made up of axonal endings of V nerve axons, dendrites and axons of SG neurons and dendrites of marginal and magnocellular neurons. A typical SG glomerulus consists of a centrally located V nerve ending which contains large synaptic vesicles and forms asymmetrical axodendritic synapses on two kinds of dendritic spines and on dendritic shafts. The dendritic processes of the SG glomerulus are linked by two kinds of dendrodendritic synapses: type 2 dendritic spines, containing large synaptic vesicles, form slightly asymmetrical (intermediate) synapses on type I dendritic spines. Dendritic shafts, containing clusters of small synaptic vesicles, form symmetrical dendrodendritic synapses amongst themselves and on type I dendritic spines. Small axonal endings, containing small synaptic vesicles form intermediate axodendritic synapses on type I dendritic spines and on dendritic shafts. These small axons also form symmetrical axoaxonic synapses on the V nerve ending. The V nerve endings in the SG layer are dark and differ morphologically from the pale V nerve endings in other parts of the trigeminal nuclei.
143 citations
TL;DR: New and decisive criteria based on the differential morphology of synaptic vesicles and junctional membranes were sought to distinguish between the amacrine and bipolar processes in single sections.
Abstract: The synaptic morphology and organization of the inner plexiform layer of the salamander retina was examined in serial sections with the electron microscope. Processes were traced, whenever possible, from their cells of origin, and synapses were classified as ribbon or regular (non-ribbon, conventional). Amacrine processes always make regular synapses, while bipolar processes make both ribbon and regular synapses. For this reason, new and decisive criteria based on the differential morphology of synaptic vesicles and junctional membranes were sought to distinguish between the amacrine and bipolar processes in single sections. Amacrine processes contain a relatively uniform population of small round vesicles and they make regular symmetrical synapses. Bipolar processes contain vesicles that are generally larger and more pleomorphic than those of the amacrine processes, and they make ribbon synapses (monads, dyads and triads) and regular synapses of the asymmetrical type. Amacrine processes synapse upon other amacrine processes, bipolar axons, ganglion cell dendrites, and the perikarya of these three types of cells. Amacrine cells also give rise to somatodendritic synapses. Bipolar processes synapse upon amacrine processes, ganglion cell dendrites and other bipolar processes, but they have not been seen to synapse upon the somata of any cell. Both amacrine and bipolar processes engage in serial synapses, and these two groups often make reciprocal synapses with each other. Gap junctions have been found between two bipolar processes, between two amacrine processes, and less frequently, between a bipolar and an amacrine process.
142 citations
TL;DR: As regeneration progressed, astrocytic processes extended radially from the optic nerve's central cellular core and formed longitudinal compartments for regenerating axons that traversed the site of the lesion and extended into the distal stump within 13–15 days.
Abstract: Optic nerves of stage 54–56Xenopus laevis tadpoles were either transected or crushed, and subsequent Wallerian degeneration, regeneration, and remyelination were examined. After 4 days, normal myelinated fibres were no longer present in the distal stump, and only a few unmyelinated fibres remained. After 10–13 days, the distal nerve consisted mainly of a core of reactive astrocytes with enlarged processes and scattered oligodendrocytes which persisted throughout the degenerative period. Regenerating axons traversed the site of the lesion and extended into the distal stump within 13–15 days.
131 citations
TL;DR: Examination of the hypothesis that ACh may be released from this cytoplasmic pool during synaptic activation indicated an efflux of approximately 1.5–3.0 × 10−12 M Ach cm−2 synapsing membrane/impulse.
Abstract: Cat sympathetic ganglia were prepared for electron microscopy by perfusion fixation with glutaraldehyde in the presence of Mg++. At resting boutons de passage the populations of synaptic vesicles were 6400 per bouton. The vesicle distributions displayed many of the features of spheres in close-packing. Calculations based on a vesicle close-packing hypothesis gave a figure of 8000 vesicles per bouton. In ganglia stimulated for 20 min at 20/s the vesicle populations were reduced to 25%, and to 28.5% in ganglia in which acetylcholine (ACh) synthesis was inhibited by hemicholinium (HC-3). The reduction was to 34% when stimulation was for 1 min. In ganglia stimulated for 20 min at 1/s and 4/s the vesicle populations were reduced to 44% and 46% respectively. Even following 1 min stimulation at 4/s over half the boutons showed significant loss of vesicles. ACh stores in ganglia are known not to be depleted by any of these procedures except stimulation in the presence of HC-3. The fraction of ganglionic ACh stores known to be released by stimulation for 1 min at 20/s or 4/s and by 20 min stimulation at 1/s is substantially less than the fraction of vesicles lost. The observations therefore were not readily accounted for by the vesicle theory of transmitter storage and release. They were consistent with the idea the ACh is stored in vesicles at rest, but that during maintained activity over half the bouton ACh is free in the cytoplasm. The concentration of cytoplasmic ACh was calculated to be approximately 50–150 mm 1−1. Examination of the hypothesis that ACh may be released from this cytoplasmic pool during synaptic activation indicated an efflux of approximately 1.5–3.0 × 10−12 M Ach cm−2 synapsing membrane/impulse.
123 citations
TL;DR: It is suggested that this differential distribution of thorns can be correlated with the axonal inputs in the various cortical layers, and that the Meynert cell is designed to receive maximal information from layers I and II, and from layers V and VI, which are sources mainly of intracortical inputs.
Abstract: The solitary cells of Meynert are distinguished by their specific location in layer V of the striate cortex, very large size, argyrophilia, and the profusion of neurofilaments in their dendrites and perikarya. They occur with greater frequency in the macular region of the cortex, spaced a minimal distance of 110 μm apart, at a maximum density of about 8000/cm2. In the perifoveal cortex, Meynert cells are spaced about 400 μm apart and packed at a density of approximately 625/cm2. Each Meynert cell has an apical dendrite and many large basal dendrites. The perikaryon and primary segments of all dendrites are spine-free; however, more distally a total of 36000 spines are present, differentially disposed upon the dendritic surfaces. The basal dendrites bear over 77% of the spines on the Meynert cell, although they account for only 66% of the total length of the dendritic arborization. The first part of the apical dendrite is the most densely decorated with appendages, accounting for almost 10% of the spines on the whole dendritic tree. The apical dendrite becomes progressively less spiny as it passes through the superficial part of layers IV and III; less than 2.5% of the total number of spines of the Meynert cell project from this part of the apical dendrite. When the dendrite reaches layer II it bursts into an umbel of rapidly tapering branches. These are highly spinose, accounting for 8–13% of the cell's total, dispersed over only 23% of the linear dendritic length. It is suggested that this differential distribution of thorns can be correlated with the axonal inputs in the various cortical layers, and that the Meynert cell is designed to receive maximal information from layers I and II, and from layers V and VI, which are sources mainly of intracortical inputs. Thus the Meynert cell may be principally concerned with integrative information. In the perifoveal cortex, the basal dendrites of adjacent Meynert cells overlap considerably, and the apical terminal bouquet dendrites do not. In the macular cortex, because of the increased frequency of these neurons, both basal and apical terminal dendritic fields overlap. A model is developed to illustrate these hypotheses.
TL;DR: The axonal changes are similar to those that occur during the degeneration of myelinated axons and all axons degenerate completely within 7 days, and this time course correlates closely with previous electrophysiological observations.
Abstract: Observations have been made on the ultrastructural changes that occur in the cervical and abdominal portions of the rabbit vagus nerve following transection The axonal changes are similar to those that occur during the degeneration of myelinated axons and all axons degenerate completely within 7 days This time course correlates closely with previous electrophysiological observations The degeneration of the axons takes place external to the Schwann cells, but these cells tend to react by surrounding the degenerating axons with multiple flattened processes This debris is removed both by Schwann cells and macrophages Most of the degenerate material has disappeared by 15 days after transection Compact columns of Schwann cells analogous to the bands of Bungner are not formed
TL;DR: The calculation is described (performed by a computer), and the methods available for counting synapses are discussed, and the results obtained are discussed.
Abstract: The several methods that have been used to count synapses by electron microscopy are put in error by the inevitable failure to recognize and count all of the smaller profiles of synapses present in a section. The smaller profiles are produced by near-tangential sectioning of synapses of larger diameter, whereas equatorial sections produce large profiles that are always recognized. The expected number of smaller profiles can be calculated from the observed numbers of larger profiles by the method of Coupland (1968). The estimated density is then dependent on the more reliable observations only, but the iterative calculation required is very tedious. This paper describes the calculation (performed by a computer), and discusses the methods available for counting synapses, and the results obtained.
TL;DR: Synaptogenesis in the cerebellar cortex and in the spinal cord of the chick embryo is compared and the possible origin of the early cerebellary synaptic connections is discussed.
Abstract: Synaptogenesis was studied in the cerebellar cortex of chicken embryos from 7 1/2 days through 8, 8 1/2, 9, 10, 12, 14, and 18 days of incubation and one day post hatching (day 22). The earliest axo-dendritic synapses were found in the prospective molecular layer at 7 1/2 days; they were characterized by a slight, symmetrical membrane thickening and very few synaptic vesicles (2–3 per section). By day 9 the number of synapses had increased, there were more synaptic vesicles per terminal (ca. 10 per section) and the membrane thickenings were slightly asymmetrical. At day 10 morphologically mature synapses appeared, e.g. more synaptic vesicles and further membrane differentiation was observed. Synapses on dendritic spines of Purkinje cells appeared at day 18 and were abundant in the hatchling.
TL;DR: In the hamster adrenal medulla, nerve endings terminating on adrenaline- cells (A-cells) are different from those terminating on noradrenaline-cells (NA-cells), which means that each A-cell seems to receive only one nerve ending.
Abstract: In the hamster adrenal medulla, nerve endings terminating on adrenaline-cells (A-cells) are different from those terminating on noradrenaline-cells (NA-cells) Nerve endings on NA-cells are packed in clusters; they are spherical with a diameter of 1–2 μm Each NA-cell receives several nerve endings Nerve endings on A-cells are elongated, ramified and much larger; their longer axis often reaches 10 μm Each A-cell seems to receive only one nerve ending Synaptic areas and synaptic complexes on NA-cells cover a surface twice as large as those on A-cells
TL;DR: Structural changes in neurons in the cervical anterior horn of ageing mice have been studied and a probable sequence of morphologic events accompanying the transformation of lysosome-like granules into mature lipofuscin pigment granules is suggested.
Abstract: Structural changes in neurons in the cervical anterior horn of ageing mice have been studied. The main age-related neurocytological change observed during this study is progressive accumulation of lipofuscin. The pigment was present in some nerve cells as early as six weeks after birth. Various types of membrane-bound granules encountered in the anterior horn cells were characterized on the basis of their shape, size and fine structure into primary lysosome-like (L1) granules (dense bodies), autophagic vacuole-like (L2) granules and mature (L3) pigment granules of complex substructure and irregular configuration.L1,L3 andL3 types of granules appear to represent respectively early, intermediate and mature stages in a developmental continuum of lipofuscin pigment granules. Transitional stages suggest that matureL3 pigment granules evolve by gradual alteration of lysosome-likeL1 andL2 granules. A probable sequence of morphologic events accompanying the transformation of lysosome-like granules into mature lipofuscin pigment granules is suggested.
TL;DR: The surfaces of cells of the ependyma and choroid plexus in the lateral ventricle of the mature rat brain have been examined with a scanning electron microscope on tissue dried by the critical point method.
Abstract: The surfaces of cells of the ependyma and choroid plexus in the lateral ventricle of the mature rat brain have been examined with a scanning electron microscope on tissue dried by the critical point method. The ventricular surfaces of the choroid plexus cells are domed and have a tangle of thin processes resembling microvilli. The cilia possessed by these cells could not be identified. Overlying the choroid plexus are a number of free cells. These cells have many fine processes extending from them. They are probably macrophages and have been equated with the Kolmer or epiplexus cells which are known to rest on the surface of the choroidal epithelium.
TL;DR: An analysis of the median percentage covering of dendrites by contiguous elements of the neuropil reveals that as the dendrite diameter decreases, astrocytes increase from 43 to 75%, axons decrease from 21% to zero, boutons decreases from 28% to 0, and d endrites decrease from 10% tozero.
Abstract: There is an extensive bundle of dendrites with a rostro-caudal axis in the ventro-lateral lamina of the sixth lumbar segment in each side of the rat spinal cord. Such a bundle has a diameter of about 250 μm and contains over 1400 parallel dendrites., each with a diameter of less than 8 μm, interspersed between neuronal somata. The volume fraction of dendrites in the bundle neuropil is about 55%, the remainder being equally distributed between astrocytes, synaptic boutons, and axons, most of which are unmyelinated. An analysis of the median percentage covering of dendrites by contiguous elements of the neuropil reveals that as the dendrite diameter decreases from 4 to 0.2 μm (mean=2 μm), astrocytes increase from 43 to 75%, axons decrease from 21% to zero, boutons decrease from 28% to zero, and dendrites decrease from 10% to zero. There is a mean of 18 synaptic boutons per 100 μn2 of the overall dendritic surface, but larger boutons tend to be more frequent on larger dendritic profiles. Apposed dendrites and their somata may have either puncta adhaerentia or confronting subsurface cisternae. Synaptic types in the rat are similar to those reported for the cat. The morphological findings are discussed with respect to previously proposed interaction between neural elements.
TL;DR: It is found that microtubular continuity is not universally valid and an upper limit to the tubule density is found to be in the neighbourhood of 400 microtubules per μm2.
Abstract: The ubiquitous cell organelle, the microtubule, has been assigned a putative role in the phenomenon of axoplasmic transport. In this regard the following question can be posed and subjected to experimental test. Do microtubules extend throughout the length of an axon as single, continuous strands, or are they present in the form of long, but discontinuous lengths? The size of the tubules mandates the use of the electron microscope in an attempt to answer this question. Tubule number and density have been measured in selected single axons of the crayfish abdominal nerve cord in two types of geometry: (1) ‘before’ and ‘after’ an axon splits into two branches; (2) at two macroscopically separated points along unbranched axons which may however undergo large changes in their diameters between the two fiducial points. The results for condition (1) are that a conservation law of tubule number holds across the branch point. For condition (2) the conservation law holds for most of the cases examined, but there are clear violations in the case of several axons. The conclusion is therefore that microtubular continuity is not universally valid. Other results are that microtubule density is uniform across the axonal area, that microtubule density obeys left-right symmetry for corresponding pairs of axons, and that axons with similar functions (e.g. motor axons, interneurons) form groups within which the microtubule densities are similar. A limited number of axons from other species of organisms were measured and the microtubule number and density recorded and included in an overall survey. From this collection, an upper limit to the tubule density is found to be in the neighbourhood of 400 microtubules per μm2.
TL;DR: A concordance of methods for designating laminae in the primate striate cortex is developed and it is suggested that Brodmann's original scheme be followed as the most generally useful and applicable.
Abstract: This paper develops a concordance of methods for designating laminae in the primate striate cortex. The classic notation schemes of Ramony Cajal, Brodmann, and von Bonin and four recent modifications are described in words and photomicrographs. The recent trend toward adopting a lamination scheme that effects a division of cells into recognizable layers and at the same time reflects their connections is considered. Since a single, consistent plan for lamination of the visual cortex would facilitate communication among investigators, it is suggested that Brodmann's original scheme be followed as the most generally useful and applicable.
TL;DR: The spinal electromotor nucleus was studied by electron microscopy in normal eels and in eels after high spinal section to explain the physiological observation that the synchronous neural volley leading to organ discharge is subject to a greater delay in transmission through the electrom motor nucleus in rostral regions of the cord.
Abstract: The spinal electromotor nucleus was studied by electron microscopy in normal eels and in eels after high spinal section. Electromotor neurons are surrounded by small myelinated fibres that degenerate after spinal section. These fibres form electrotonic synapses (characterized by gap junctions) on somata and dendrites of the electromotor neurons. Some terminals synapse with two cell processes and form a pathway for electrotonic coupling of the two postsynaptic elements. Direct dendrodendritic electrotonic synapses also occur. Within the cord electromotor axons have short internodes (ĨOμm). In rostral regions electromotor axons are smaller in diameter, have shorter internodes and are more thinly myelinated than in caudal regions. In rostral regions preterminal axons are smaller in diameter than in caudal regions. Axons of the bulbospinal (electromotor) tract are slightly larger in caudal regions. All these differences are in the right direction to explain the physiological observation that the synchronous neural volley leading to organ discharge is subject to a greater delay in transmission through the electromotor nucleus in rostral regions of the cord. This delay plays an essential role in synchronizing the discharge in widely separated regions of the organ. The very slow conduction in preterminal fibres that is indicated from physiological experiments could arise if these fibres were to have very short internode lengths similar to those observed in the electromotor axons.
TL;DR: The retinula cell axons entering the synaptic region of the optic lamina in the crayfish form large expanded bag-like terminals which are organized with other neural elements into structural units called ‘cartridges’.
Abstract: The retinula cell axons entering the synaptic region of the optic lamina in the crayfish form large expanded bag-like terminals which are organized with other neural elements into structural units called ‘cartridges’ The cytoplasm of the terminals contains synaptic and coated vesicles, ER cisternae, clusters of tubular elements, and mitochondria Several mitochondria are often found associated with a single large rod-shaped inclusion present within each terminal The rod-like formation could be demonstrated in both light and EM material, it is composed of 85–95 A filaments and averages I μm in width and 65 μm in length The terminal synaptic contacts are characterized by a bar-shaped presynaptic density and three postsynaptic elements Some synaptic vesicles appear aligned along the bar density which measures approximately 800 A in width and 075 μm in length Each terminal synapse has three postsynaptic elements which have an electron-dense fringe along their membrane bordering the synaptic cleft From the planes of section through this contact a composite reconstruction is presented Also present along the central border of the terminals are numerous small invaginated processes, some of which extend almost to the middle of the terminal No membrane specializations were found along these processes and they have been tentatively identified as neuronal
TL;DR: The mossy endings of the cerebellum of the rat have been studied by electron microscopy, using preparative procedures involving fixations either at acid or neutral pH, irrespective of whether initial fixation was with osmium or aldehyde.
Abstract: The mossy endings of the cerebellum of the rat have been studied by electron microscopy, using preparative procedures involving fixations either at acid or neutral pH. Counts showed that endings initially fixed with osmium tetroxide showed significantly more numerous coated vesicles and reticulosomes than those initially fixed with aldehyde. Acid pH did not significantly alter the percentages of coated vesicles or reticulosomes when compared with neutral pH, irrespective of whether initial fixation was with osmium or aldehyde. Differences in the appearance of coated vesicles, reticulosomes and cytonet using the different methods are described.
TL;DR: Dendro-dendritic synapses have been found in the suprachiasmatic nucleus of the rat and it is established that the presynaptic dendrites receive additional synaptic contacts from at least two types of axons.
Abstract: Dendro-dendritic synapses have been found in the suprachiasmatic nucleus of the rat. These synapses are similar to ‘symmetrical’ or Gray-type II synapses. The existence of reciprocal dendro-dendritic synapses was also established. The presynaptic dendrites receive additional synaptic contacts from at least two types of axons.
TL;DR: It is suggested that a function of the interneurons may be to synthesize and store monoamines and to discharge them on appropriate stimuli, especially from preganglionic afferents (which synapse upon themin vivo), thus making immediately available to the principal neurons a local catecholamine pool.
Abstract: This inquiry is directed toward the component of small granular neurons (SG cells, SIF cells, or interneurons) which characterizes sympathetic ganglia. Superior cervical and stellate ganglia from newborn rats, and paravertebral chain ganglia from newborn rats and embryonic chicks, have been maintained in organotypic tissue culture for periods of 3 to 12 weeks, and studied with the light microscope both living and fixed. These observations have been correlated with their fine structure and their responses to chemical agents affecting adrenergic neurons, as determined by histofluorescence findings. The interneurons develop normally in the isolated ganglia without preganglionic innervation, establishing junctional relationships with each other and with principal neurons. They store within their perikarya conspicuous dense-cored vesicles (∼14OO A −x), and exhibit intense catecholamine fluorescence. These small neurons are resistant to agents that extinguish fluorescence in principal neurons by interfering with biosynthesis or depleting catecholamine stores. It is suggested that a function of the interneurons may be to synthesize and store monoamines and to discharge them on appropriate stimuli, especially from preganglionic afferents (which synapse upon themin vivo), thus making immediately available to the principal neurons a local catecholamine pool.
TL;DR: Differences in shape, size, intracytoplasmic organelles and nuclear characteristics were found in detail and the proportions of the different cell types were very similar to those in the motor and somatic sensory areas.
Abstract: Three types of cell groups were observed in area 17 of the visual cortex of the rhesus monkey with electron microscopy: pyramidal, large non-pyramidal and small non-pyramidal neurons. Differences in shape, size, intracytoplasmic organelles and nuclear characteristics were found in detail. The few synapses on the somata of pyramidal cells are associated with symmetrical membrane contacts and the somata of large and small non-pyramidal neurons have both symmetrical and asymmetrical synapses, varying in number. A quantitative study showed that approximately 60% of the cells were pyramidal, 7% large non-pyramidal and 33% small non-pyramidal, and that the proportions of the different cell types were very similar to those in the motor and somatic sensory areas.
TL;DR: Two methods of marking neuronal processes have been explored: degenerating axons that are undergoing the dark degenerative change can be recognized readily within the thick sections, however, they stand out less clearly than in thin sections.
Abstract: Synaptic relationships between neuronal processes have been studied in thick sections (up to 1.0 mum) with high voltage electron microscopy. Some of the problems that arise in the study of such thick sections have been considered. (1) Where two parts of a single process are in continuity through a narrow stalk, as a dendritic stem and one of its spines, the continuity can generally not be seen if the stalk is included within the section or is crossed in the section by other processes. (2) Membranes that are viewed face-on cannot be seen in the thick sections and, therefore, processes belonging to two separate nerve cells may appear as though they are in cytoplasmic continuity. (3) Synaptic junctions that are viewed face-on cannot be seen in the thick sections, and junctions lying close to the same angle are also invisible; the relative number of invisible junctions depends upon a number of factors, including the section thickness and the size and contrast properties of the junctional specializations. Two methods of marking neuronal processes have been explored: (1) degenerating axons that are undergoing the dark degenerative change can be recognized readily within the thick sections. However, they stand out less clearly than in thin sections. (2) Neuronal processes that have been impregnated by an extra-rapid modification of the Golgi method can be easily traced in thick sections. It is possible to study some of the synaptic contacts made by these impregnated processes, especially if the processes have been only lightly impregnated or if the silver dichromate deposits are secondarily removed.
TL;DR: Comparison of the above findings with developing spinal cord tissue from kittens and previously reported works, has shown that phenomena similar to the above also occurin situ, and their higher frequency in vitro might be a feature of the tissue culture environment.
Abstract: Ultrastructural examination has revealed the existence of certain aberrant appearances common to both normal and experimental organotypic cultures of spinal cord and cerebellar tissue. Astrocytes frequently possessed cytoplasmic membranous whorls which originated as concentric arrays of smooth ER enclosing a portion of cytoplasm. The membranes forming these inclusions gradually became compacted and the central island of cytoplasm degenerated. The process of inclusion formation was analogous to autophagy. Giant mitochondria were found within astrocytes located deep in the explants and within neurons. Their presence in astrocytes was attributed to a decrease in oxygen tension and lower concentrations of nutrients in comparison to the surface strata of the tissue. Their size ranged between 4 and 8 μm. Another aberrant phenomenon occurringin vitro was the myelination of neuronal somata. This involved both granule cells and Purkinje cells in cerebellar explants and was rare in spinal cord tissue. Ependymal cells formed bizarre microcysts around chambers into which microvilli and cilia protruded. Comparison of the above findings with developing spinal cord tissue from kittens and previously reported works, has shown that phenomena similar to the above also occurin situ. Their higher frequencyin vitro might be a feature of the tissue culture environment.
TL;DR: The general characteristics of the club boutons of type 5 appear to be similar to those of the M-knob found on the spinal motoneuron in the same species (Conradi, 1969; McLaughlin, 1972b): these may therefore represent primary muscle spindle afferent terminals.
Abstract: Nucleus dorsalis of the adult cat atL3 has been examined electron microscopically, and six types of terminal bouton recognized. Types 1 and 2 consist of elongated giant boutons (5–25 μm) containing numerous mitochondria and neurofilaments, and circular and elliptical vesicles respectively, which establish repeated synaptic contacts along the surface of the Clarke cell soma and primary dendrites.
TL;DR: The polarity of the cilia as determined by the arrangement of their axial filaments was in the caudal direction along the central canal in all the vertebrates examined in this study, which indicates that the cerebrospinal fluid is moved caUDalwards as a result of the activity ofThe cilia.
Abstract: The numbers and the polarity of the ependymal cilia in the central canals of goldfish, frogs, turtles, birds, rats, and rabbits had been studied with the electron microscope. The number of cilia per cell was characteristic for each species. A single cilium, usually situated in the centre of the hexagonal apex of the cell, was present in goldfish, frogs, turtles and birds. Two cilia were regularly found in rats, whereas in rabbits single- and multiple-ciliated ependymal cells occurred.