Quantitative relationships between nerve and satellite cells in spinal ganglia: an electron microscopical study. II. Reptiles

The histomorphological and stereological assessment of rat dorsal root ganglion tissues after various types of sciatic nerve injury

Peripheral nerve injuries lead to significant changes in the dorsal root ganglia, where the cell bodies of the damaged axons are located. The sensory neurons and the surrounding satellite cells rearrange the composition of the intracellular organelles to enhance their plasticity for adaptation to changing conditions and response to injury. Meanwhile, satellite cells acquire phagocytic properties and work with macrophages to eliminate degenerated neurons. These structural and functional changes are not identical in all injury types. Understanding the cellular response, which varies according to the type of injury involved, is essential in determining the optimal method of treatment. In this research, we investigated the numerical and morphological changes in primary sensory neurons and satellite cells in the dorsal root ganglion 30 days following chronic compression, crush, and transection injuries using stereology, high-resolution light microscopy, immunohistochemistry, and behavioral analysis techniques. Electron microscopic methods were employed to evaluate fine structural alterations in cells. Stereological evaluations revealed no statistically significant difference in terms of mean sensory neuron numbers (p > 0.05), although a significant decrease was observed in sensory neuron volumes in the transection and crush injury groups (p < 0.05). Active caspase-3 immunopositivity increased in the injury groups compared to the sham group (p < 0.05). While crush injury led to desensitization, chronic compression injury caused thermal hyperalgesia. Macrophage infiltrations were observed in all injury types. Electron microscopic results revealed that the chromatolysis response was triggered in the sensory neuron bodies from the transection injury group. An increase in organelle density was observed in the perikaryon of sensory neurons after crush-type injury. This indicates the presence of a more active regeneration process in crush-type injury than in other types. The effect of chronic compression injury is more devastating than that of crush-type injury, and the edema caused by compression significantly inhibits the regeneration process.

Primary Postnatal Dorsal Root Ganglion Culture from Conventionally Slaughtered Calves

Neurological disorders in ruminants have an important impact on veterinary health, but very few host-specific in vitro models have been established to study diseases affecting the nervous system. Here we describe a primary neuronal dorsal root ganglia (DRG) culture derived from calves after being conventionally slaughtered for food consumption. The study focuses on the in vitro characterization of bovine DRG cell populations by immunofluorescence analysis. The effects of various growth factors on neuron viability, neurite outgrowth and arborisation were evaluated by morphological analysis. Bovine DRG neurons are able to survive for more than 4 weeks in culture. GF supplementation is not required for neuronal survival and neurite outgrowth. However, exogenously added growth factors promote neurite outgrowth. DRG cultures from regularly slaughtered calves represent a promising and sustainable host specific model for the investigation of pain and neurological diseases in bovines.

The structure of the perineuronal sheath of satellite glial cells (SGCs) in sensory ganglia

In sensory ganglia each nerve cell body is usually enveloped by a satellite glial cell (SGC) sheath, sharply separated from sheaths encircling adjacent neurons by connective tissue. However, following axon injury SGCs may form bridges connecting previously separate perineuronal sheaths. Each sheath consists of one or several layers of cells that overlap in a more or less complex fashion; sometimes SGCs form a perineuronal myelin sheath. SGCs are flattened mononucleate cells containing the usual cell organelles. Several ion channels, receptors and adhesion molecules have been identified in these cells. SGCs of the same sheath are usually linked by adherent and gap junctions, and are functionally coupled. Following axon injury, both the number of gap junctions and the coupling of SGCs increase markedly. The apposed plasma membranes of adjacent cells are separated by 15–20 nm gaps, which form a potential pathway, usually long and tortuous, between connective tissue and neuronal surface. The boundary between neuron and SGC sheath is usually complicated, mainly by many projections arising from the neuron. The outer surface of the SGC sheath is covered by a basal lamina. The number of SGCs enveloping a nerve cell body is proportional to the cell body volume; the volume of the SGC sheath is proportional to the volume and surface area of the nerve cell body. In old animals, both the number of SGCs and the mean volume of the SGC sheaths are significantly lower than in young adults. Furthermore, extensive portions of the neuronal surface are not covered by SGCs, exposing neurons of aged animals to damage by harmful substances.

The perineuronal glial tissue of spinal ganglia. Quantitative changes in the rabbit from youth to extremely advanced age

The volumes of the nerve cell bodies and those of the enveloping satellite cell sheaths from spinal ganglia were determined by morphometric methods applied to electron micrographs in young, adult, old and very old rabbits. The mean volume of the nerve cell bodies increased progressively with age; this is probably related to the increase with age of the body size of the rabbits studied. The mean volume of the satellite cell sheaths did not differ significantly in young, adult and old animals, but was significantly smaller in very old animals. It is extremely unlikely that this marked reduction in the volume of the satellite cell sheath is the result of a pathological process. The mean value of the volume ratio between the satellite cell sheaths and the related nerve cell bodies did not differ significantly in young and adult animals, but was significantly smaller in old and very old animals. This ratio was particularly low in very old animals. Our analysis showed that in each age group the volume of the satellite cell sheath is linearly related to the volume of the related nerve cell body. This result suggests that in rabbit spinal ganglia the quantitative relations between glial and nervous tissue are tightly controlled throughout life. It is suggested that ganglionic neurons release signals to influence and control the volume of their associated glial tissue. Since satellite cells have important support roles for the neurons they surround, it is likely that the marked reduction in the volume of perineuronal sheaths in the extremely advanced age is accompanied by a reduction of those roles, with negative consequences for neuronal activity.

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.

Morphological and morphometrical changes in dorsal root ganglion neurons innervating the regenerated lizard tail

The variations occurring in neurons from dorsal root ganglia that provide innervation to the regenerated tail of the lizard (vicarious ganglia) are analysed. Vicarious ganglion neurons, when compared to control ganglion neurons (i.e. ganglia from the same animal that were not involved in the reinnervation process), show a size increase of the soma (cell hypertrophy) which applies to all cell types and subtypes. No statistically significant differences in the relative percentage of neurofilament-poor (type D) and neurofilament-rich (type L) neurons were found between vicarious dorsal root ganglia compared to controls in all animals. On the contrary, within L neuron sub-types, a statistically significant increase in sub-type L2 (very rich in neurofilaments), and the appearance of sub-type L3 neuron which is not detectable in controls, were demonstrated in vicarious dorsal root ganglia. In spite of these variations in size and percentage distribution, no structural and ultrastructural differences of the various cell types and sub-types are detectable, except for the appearance of the sub-type L3 neurons. However, this neuron sub-type might not be considered specific of hypertrophy since the same morphological features have been observed, in normal conditions, in lizard dorsal root ganglia from cervical and lumbar spinal levels that provide innervation to limb plexuses.

A paracrine effect for neuron-derived BDNF in development of dorsal root ganglia: stimulation of Schwann cell myelin protein expression by glial cells

Addition of neurons to cultures of non-neuronal cells derived from quail embryonic dorsal root ganglia causes a 2.5-fold increase in the proportion of cells that express the glial marker Schwann cell myelin protein (SMP) when compared to cultures devoid of neurons. This effect is mediated by BDNF because incubation with a trkB immunoadhesin that sequesters BDNF, but not with trkA or trkC immunoadhesins, abolishes this stimulation. This neuronal activity can be mimicked by treatment with soluble BDNF that stimulates specifically the conversion of SMP-negative glial cells into cells that express this phenotype. That BDNF is the endogenous neuron-derived factor affecting glial development is further supported by the observation that BDNF is extensively expressed in developing sensory neurons of the avian ganglia both in vivo and in vitro, but not by the satellite cells. These results show for the first time a paracrine role for neuronal BDNF on differentiation of peripheral glial cells. This effect of BDNF is likely to be mediated by the p75 neurotrophin receptor because: (1) p75 immunoreactive protein is expressed by a subset of satellite cells; (2) neutralization of p75 abolishes the BDNF-induced stimulation; (3) a treatment of non-neuronal cell cultures with equimolar concentrations of either soluble NGF or NT-3 also affects the proportion of cells that become SMP-positive. Whereas NGF stimulates the acquisition of this glial antigen to a similar extent as BDNF, NT-3 inhibits its expression, suggesting that distinct neurotrophins signal differentially through p75. These findings also suggest that the definitive phenotype of peripheral glia is determined by a balance between positive and inhibitory signals arising in adjacent neurons.

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.

An electron microscope study of quantitative relationships between axon and Schwann cell sheath in myelinated fibres of peripheral nerves

The quantitative relationships between the cross-sectional area of the Schwann cell sheath (myelin included) and that of its related axon were studied by electron microscopy in the nerve fibres of the spinal roots of lizard (Lacerta muralis). In both ventral and dorsal roots the cross-sectional area of the Schwann cell sheath (myelin included) was found to be directly proportional to that of its related axon (correlation coefficients between 0.88 and 0.92). The ratio between the cross-sectional area of the Schwann cell sheath (myelin included) and that of its related axon tends to diminish as the cross-sectional area of the latter increases. Thus, under normal conditions, in myelinated fibres of the spinal roots of the lizard a quantitative balance exists between the nerve tissue and its associated glial tissue. This result agrees with those previously obtained in the spinal ganglia of the lizard, gecko, cat and rabbit. Some of the mechanisms probably involved in the control of the quantitative balance between nerve tissue and its associated glial tissue in peripheral nerves are presented and discussed.

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.

Proliferative and degenerative events in the early development of chick dorsal root ganglia. I. Normal development

Development of the chick dorsal root ganglia was examined in 4.5- to 9.5-day embryos. Tritiated thymidine (3H-TdR) and autoradiography was used to analyze proliferative activity and the Feulgen procedure to analyze degenerative activity in ganglia 12-17. Proliferative activity was found to be elevated through 4.5 days of incubation when as many as 14% of the ganglionic cells become labelled following a one-hour exposure to 3H-TdR. By 6.5 to 7.5 days proliferative activity decreases to 2-4% in the lateroventral (LV) regions and to approximately 1% in the mediodorsal (MD) regions of the ganglia. However, there appears to be increased proliferative activity by the end of the experimental period at 9.5 days. Birthdate studies demonstrate that large-scale neuronal production occurs between 4.5 and 6.5 days in the LV regions and between 4.5 and 7.5 days in the MD regions. After those times ganglionic proliferative activity must be largely nonneuronal in nature. This nonneuronal proliferation is greater in LV than in MD regions and in brachial than in nonbrachial ganglia. Degenerative activiy was found to be absent from the ganglia until after 4.5 days of incubation. It then increases rapidly, and by 5.5 days 5% of the LV cells in nonbrachial ganglia are degenerating. Degenerative activity then declines but is still present at 9.5 days. In contrast to results of an earlier study (Hamburger and Levi-Montalcini, '49), degenerative activity was also found in the LV region of brachial ganglia and the MD regions of brachial and nonbrachial ganglia. The pattern of LV degenerative activity in brachial ganglia is similar to that in nonbrachial ganglia, but the level of activity is lower. In the MD regions degenerative activity increases throughout the experimental period, and by 9.5 days as many as 4% of the MD cells are degenerating.

Non-synaptic chemical neurotransmission

Images with apparently gemmulofugal polarity in the EPL of the olfactory bulb are the result of sectioning, along misleading planes, gemmulopetal synapses containing postsynaptic vesicles. Unless one accepts a bidirectional conduction for chemical synapses, the internal granule cells lack actual gemmulofugal synapses and the neurotransmitter contained on their vesicles must act at non-synaptic membranes.