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Axons in the rat molar pulp have been examined morphometrically to determine axonal size and the degree of axonal exposure and axoaxonal apposition in tissue fixed by perfusion using 2% glutaraldehyde in cacodylate buffers ranging in molarity form 0.025 M to 0.4 M. Between 31.2% and 45.0% of the axons were incompletely ensheathed. This proportion of axons exposed was linearly related to the buffer molarity (P less than 0.05) and was approximately double that found in more central axons. Between 32.3% and 45.0% of the axons were in contact with other axons. This proportion was not linearly related to buffer molarity but was least ten times higher than that observed in more centrally positioned nerve fibers in the inferior alveolar nerves. Increasing buffer molarity reduced the size of the axons, a relationship not found in the more central axons. It is suggested that axonal exposure and axoaxonal apposition are constant, significant features of pulpal nerve fibers that may be related to the onset and spread of nociceptive activity. The permeability properties of pulpal axons may differ from those of more centrally placed axons.
Anat Rec 1981 Nov
PMID:The effect of buffer molarity on axonal exposure and axoaxonal apposition in the rat molar pulp. 730 28

The mechanisms underlying directed axonal movement in the developing central nervous system are largely unknown. Histochemical methods for transmission and scanning electron microscopy were used to study the surface of the developing optic tectum in the chick embryo at the time of optic fiber ingrowth. A highly structured extracellular matrix consisting of fibrillar and granular components was seen in normal and in uninnervated specimens that had been fixed in solutions containing the cationic dyes Alcian blue, ruthenium red, or safranin O. The strong affinity of these stains for glycosaminoglycans suggests that the matrix contains such macromolecular aggregates. With routine fixation methods the matrix was not seen, but empty extracellular spaces were apparent. The tectal matrix was particularly prominent ahead of the growing front of optic fibers. Its location was thus appropriate for interacting with pioneering axons that cross the surface of the developing tectum along its anterior-posterior axis. Matrix fibrils were organized in a stacked alignment predominantly parallel to the tectal surface, but otherwise their orientation appeared random. The matrix possibly bears on the guidance of optic fibers. However, its geometry suggests that this may involve a mechanism more specific than mechanical contact guidance.
Anat Rec 1980 May
PMID:Structure and orientation of extracellular matrix in developing chick optic tectum. 742 8

The purpose of this investigation was to test the hypothesis that activation of the immune system in rats will lead to changes in the density of innervation in lymph nodes. In order to reduce the variability between animals, the rats were reared under sterile conditions and immunostimulation was effected by subcutaneous application of bovine albumin in a region draining to the axillary lymph nodes of both sides. Control animals received an equivalent application of sterile physiological saline. The animals were sacrificed 10 days and 27 days and 4 months after immunostimulation. The nerves in the axillary lymph nodes were quantified by light microscopy in silver impregnated sections and at the ultrastructural level on ultrathin sections. The survival times were chosen so that the first group was in the ascending phase of antibody production, the second group at the peak, and the third group in the declining phase. Both at the light and ultrastructural levels, there were statistically significant differences in the density of innervation of medulla between the groups, with a particularly pronounced increase in the group 4 months after immunostimulation. At the ultrastructural level, there was also an increase in the density of incompletely ensheathed axonal profiles in the parenchyma of the medulla, while the nerves associated with blood vessels were not increased. We conclude that immunostimulation leads to morphological changes in the innervation of the medulla of axillary lymph nodes, that are consistent with the concept of functional activation of the autonomic nervous system through the immune system.
Anat Rec 1994 Feb
PMID:Plasticity of innervation of the medulla of axillary lymph nodes in the rat after antigenic stimulation. 815 7

The maturation of the left vagal nerve was studied in the fetal lamb by transmission electron microscopy and by computer-assisted morphometry of sections of the entire nerve at seven gestational ages between 79 and 145 days (term is 147 days) and in the adult ewe. The number of unmyelinated axons per Schwann cell progressively decreased from 25 to 55 at 79 days to 1 to 5 at near-term. Unmyelinated axons of various sizes were enclosed within a single Schwann cell at all ages, but the mean axonal diameter increased in inverse relation to the number of unmyelinated axons. A few Schwann cells enclosed two myelinated axons, but in most instances myelination did not begin until a 1:1 ratio was achieved; some single axons with a Schwann cell remained unmyelinated in the adult. Myelinated fibers were rare at 79 days but myelination progressed rapidly thereafter until the adult ratio of myelinated: unmyelinated fibers was reached at about 100 days; myelinated axons were not uniformly distributed. The myelin sheaths and axons of small fibers progressively increased in diameter in late gestation, but new large fibers were not added. Early myelinating fibers and immature unmyelinated axons contained more microtubules than neurofilaments; neurofilaments predominated in mature axons with or without myelin. Cross-linkages between neurofilaments were already evident by 79 days. Maturation of the vagal nerve thus occurs first by an increase in number of myelinated fibers and then by an increase in the size of each fiber in this fixed population. The bimodal distribution in the size histogram of myelinated fibers is not achieved until 134 days gestation and correlates well with physiological maturation of respiratory patterns.
Anat Rec 1993 Dec
PMID:Vagal nerve maturation in the fetal lamb: an ultrastructural and morphometric study. 831 Dec 66

The fine structure of the periaqueductal gray (PAG) of the rabbit was examined using the transmission electron microscope. On the basis of synaptic polarity, vesicle size, and the nature of the pre- and post-synaptic elements, 10 essentially different synaptic types could be discerned (6 axo-dendritic, 2 axo-somatic, 1 axo-axonic, and 1 dendro-dendritic). Synaptic contacts on the soma of PAG neurons were small and covered, on average, only 1.6% of the soma surface. The most striking feature of the synaptic structure of the PAG was that more than 94.1% of all synapses were axo-dendritic. Of these, 83.5% were of the symmetrical type. Most of these contacts occurred on buttons of small to medium size, and contained either round vesicles of medium size or pleomorphic vesicles of medium size. Boutons containing only flattened vesicles were quite rare. Boutons contacting larger dendrites were generally small-to-medium in size, made asymmetric-type synaptic contacts, and contained pleomorphic vesicles of medium-to-large size. Medium-sized dendrites were contacted principally by small boutons exhibiting either symmetrical or asymmetrical junctions containing medium-sized pleomorphic vesicles, and in addition a few of these boutons contained both large, and small, round vesicles. Dendritic spines were generally provided with only one synaptic contact, stretching the entire width of the spinous process. Boutons and the spines on dendrites were approximately the same size. Synapses between two vesicle-containing structures (axo-axonic or dendro-dendritic synapses) were rare (1.4%). They were generally asymmetric and contained round vesicles of medium size. Complex synapses, where a glial sheet enclosed the synapse, were occasionally observed. Also seen were multiple synapses, with up to 11 contacts on a single dendritic profile. Large dense-core vesicle were seen in approximately 40% of all synapses, whereas small dense-core vesicles were only found in about 3%. Data is provided on how different synaptic features relate to ventral, lateral, dorsal, and medial PAG. Principally this is in relation to neuron size, glia cell content, axonal characterization, and vesicular type.
Anat Rec 1993 Jul
PMID:Quantitative ultrastructural analysis of the periaqueductal gray in the rabbit. 836 62

A crossbreed dog with a history of a chronic progressive tetraparesis had an insulinoma-related peripheral polyneuropathy. Frequent feeding and treatment with corticosteroids resulted in recovery from a non-ambulatory to an almost completely normal clinical state, despite the persistence of hypoglycaemia and hyperinsulinism. After euthanasia for uncontrollable seizures, the presence of a beta-cell carcinoma of the pancreas and extensive axonal degeneration of the peripheral nerves was confirmed by histopathological examination. These findings may indicate an immune-mediated pathogenesis for insulinoma-related peripheral polyneuropathy.
Vet Rec 1997 Jul 26
PMID:Treatment of a dog with an insulinoma-related peripheral polyneuropathy with corticosteroids. 926 10

S100 protein in the vertebrate peripheral nervous system consists of homo- or heterodimers of S100alpha and S100beta proteins, the first predominating in neurons and the second in glial cells. Recently, however, occurrence of S100beta protein in neurons has been reported. The expression of S100 protein by Schwann cells, as well as their derivatives in sensory corpuscles, depends on the sensory axon (i.e., the Schwann cell-axon contact). The present study analyzed the distribution of S100alpha and S100beta proteins in human cutaneous sensory corpuscles and the effects of peripheral or central sensory axon severance in the expression of these proteins. Simple or double immunohistochemistry was carried out using a panel of antibodies against S100alpha, S100beta or S100alpha+beta proteins, and the sections were examined by light or laser confocal scanning microscopy. Skin samples were obtained from normal subjects and patients with spinal cord injury, nerve entrapment, and nerve sections plus graft. The lamellar cells of Meissner corpuscles as well as the inner-core lamellae of the Pacinian corpuscles displayed strong immunoreactivity (IR) for all antigens examined, the most intense labeling being obtained for S100beta protein. The pattern of immunostaining was unchanged after spinal cord injury, whereas the number of stained corpuscles as well as the intensity of IR for each antigen decreased in cutaneous sensory corpuscles after nerve injury, both entrapment and section plus graft. No evidence was found of axonal labeling. The present results provide evidence that Schwann-related cells in human cutaneous sensory corpuscles contain both S100alpha and S100beta and that the expression of these proteins is dependent on the functional and structural integrity of sensory fibers.
Anat Rec 1998 07
PMID:S100alpha and S100beta proteins in human cutaneous sensory corpuscles: effects of nerve and spinal cord injury. 966 63

Human skin, including nerves and sensory corpuscles, displays immunoreactivity (IR) for low- (p75) and high-affinity (TrkA-like) receptors for nerve growth factor (NGF), the best characterized member of the family of neurotrophins. This study was designed to analyze the changes induced by spinal cord and peripheral nerve injuries in the expression of neurotrophin receptors in digital skin, with special reference to nerves and sensory corpuscles. Skin biopsy samples were obtained from 1) the hand and toes of normal subjects, 2) below the level of the lesion of patients with spinal cord injury affecting dorsal and lateral funiculi, 3) the cutaneous territory of entrapped peripheral nerves (median and ulnar nerves), and 4) the cutaneous territory of sectioned and grafted nerves (median nerve). The pieces were formalin-fixed and paraffin-embedded, cut in serial sections, and processed for immunohistochemistry using antibodies against human p75 and TrkA proteins. The percentage of sensory corpuscles displaying IR for p75 and TrkA-like, as well as the intensity of IR developed within them, was assessed using quantitative image analysis. Spinal cord severance causes a decrease in p75 IR in Meissner and Pacinian corpuscles, whereas TrkA-like IR did not vary. In other nonnervous tissues (i.e., epidermis, sweat glands), both p75 and TrkA-like IR was diminished or even absent. Similar but more severe changes were encountered in the skin from the territory of entrapped nerves. Finally, in subjects with sectioned-grafted nerves, p75 IR was found close to controls in nerves, reduced in Meissner corpuscles, and absent in the inner core of the Pacinian ones; TrkA-like IR was in the perineurium, a small percentage of Meissner corpuscles (about 7%), and the outer core and capsule of the Pacinan corpuscles. In the nonnervous tissues, p75 IR was practically absent, whereas TrkA-like IR did not change. No changes in the expression of neurotrophin receptors were observed in Merkel cells of the different groups. Present results show the following: 1) expression of nerve p75 IR in human cutaneous sensory corpuscles is sensitive to central deafferentation, to blockade or difficulty in axonal transport, and to disruption of axonal continuity independently of possible restoration of axonal integrity due to grafts; 2) expression of TrkA-like IR in nerves and sensory corpuscles is sensitive only to nerve transection; 3) the corpuscular Schwann-related cells are the only cells involved in the above modifications, the perineurial cells remaining unchanged; 4) the expression of p75 and TrkA-like IR by Merkel cells is independent of normal innervation; 5) an adequate innervation of the skin seems to be necessary for the expression of p75 but not TrkA-like in nonneuronal cells, especially in the epidermis. A role for NGF in the maintenance of epidermis integrity is discussed.
Anat Rec 1998 07
PMID:p75 and TrkA neurotrophin receptors in human skin after spinal cord and peripheral nerve injury, with special reference to sensory corpuscles. 966 65

Spinocerebellar axons have been studied extensively in placental mammals, but there have been no full reports on their origin, laterality, or spinal course in any marsupial. We have used the North American opossum (Didelphis virginiana) to obtain such information and to ask whether any spinocerebellar neurons innervate both the anterior and posterior lobes of the cerebellum through axonal collaterals. To identify spinal neurons that project to the cerebellum, we employed the retrograde transport of Fluoro-Gold (FG) from the anterior lobe, the main target of spinocerebellar axons. In some cases, cerebellar injections of FG were combined with hemisections of the rostral cervical or midthoracic spinal cord, so that laterality of spinocerebellar connections could be established. To determine whether single neurons project to both the anterior lobe and the posterior lobe, injections of Fast Blue (FB) into the anterior lobe were combined with injections of Diamidino yellow (DY) or rhodamine B dextran (RBD) into the posterior lobe, or vice versa. Following injections of FG into the anterior lobe, neurons were labeled throughout the length of the spinal cord, which differed in laminar distribution and laterality of their projections. Among other areas, neurons were labeled in the central cervical nucleus, the nucleus centrobasalis, Clarke's nucleus, the dorsal horn dorsal spinocerebellar tract area, the spinal border region, and Stilling's nucleus. When anterior lobe injections of FB were combined with injections of RBD or DY into the posterior lobe, or vice versa, some double-labeled neurons were present in all major spinocerebellar groups. Cerebellar injections of FG also retrogradely labeled spinocerebellar axons, allowing us to document their locations in the gray matter as well as within the periphery of the lateral and ventral funiculi at all spinal levels. A few spinocerebellar axons also were found in the dorsal funiculus (a dorsal column-spinocerebellar tract), which appeared to originate from neurons in the dorsal part of Clarke's nucleus from the ninth thoracic segment to the first lumbar segment. Our results indicate that spinocerebellar axons in the marsupial opossum are generally comparable in origin, course, and laterality to the same axons in the placental mammals studied to date.
Anat Rec 1998 08
PMID:Origin, course, and laterality of spinocerebellar axons in the North American opossum, Didelphis virginiana. 971 88

The nervous systems of invertebrates and vertebrates consist of neuronal networks of varying complexity, and the elucidation of the organization of these networks is essential if we are to understand neural function. Up until the mid-19th Century gross dissection was the primary tool available to scientists to study the nervous system. The development of neurohistological techniques, electrical stimulation, and observation of neural function in humans and animals following injury added rapidly to our understanding of the nervous system during the following century. Over the last 3 decades investigators seeking to unravel the complexities of neural circuits have made use of analytical methods based upon the biological properties of neurons, including orthograde and retrograde axonal transport of tracer substances, the expression of particular genes and gene products that can be assessed with immunocytochemical or in situ methods, and the imaging of the utilization of oxygen or glucose by active populations of neurons. Advances in neuroscience have led to an enormous expansion in our knowledge of normal neural functioning and how that function is altered by injury or disease. Modern studies of neuronal organization have been at the center of our increased understanding of how the brain works.
Anat Rec 1998 10
PMID:Untying the Gordian knot: contemporary studies of neuronal organization. 981 Nov 21


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