Maria Ledda

Glial cell plasticity in sensory ganglia induced by nerve damage

Numerous studies have been done on the effect of nerve injury on neurons of sensory ganglia but little is known about the contribution of satellite glial cells (SCs) in these ganglia to post-injury events. We investigated cell-to-cell coupling and ultrastructure of SCs in mouse dorsal root ganglia after nerve injury (axotomy). Under control conditions SCs were mutually coupled, but mainly to other SCs around a given neuron. After axotomy SCs became extensively coupled to SCs that enveloped other neurons, apparently by gap junctions. Serial section electron microscopy showed that after axotomy SC sheaths enveloping neighboring neurons formed connections with each other. Such connections were absent in control ganglia. The number of gap junctions between SCs increased 6.5-fold after axotomy. We propose that axotomy induces growth of perineuronal SC sheaths, leading to contacts between SCs enveloping adjacent neurons and to formation of new gap junctions between SCs. These changes may be an important mode of glial plasticity and can contribute to neuropathic pain.

Satellite cell reactions to axon injury of sensory ganglion neurons: Increase in number of gap junctions and formation of bridges connecting previously separate perineuronal sheaths

This study investigated satellite cell changes in mouse L4 and L5 spinal ganglia 14 days after unilateral transection of sciatic and saphenous nerves. The ganglia were studied under the electron microscope in single and serial sections, and by dye injection. Satellite cell responses to axon injury of the neurons with which they are associated included the formation of bridges connecting previously separate perineuronal sheaths and the formation of new gap junctions, resulting in more extensive cell coupling. Some possible consequences of these satellite cell reactions are briefly discussed.

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

SummaryA 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 other, 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.

Augmentation in gap junction-mediated cell coupling in dorsal root ganglia following sciatic nerve neuritis in the mouse

Recent findings highlight the participation of central glial cells in chronic pain, but less is known of a comparable role for satellite glial cells (SGCs), in dorsal root ganglia (DRG). Our previous work showed that sciatic nerve axotomy augmented SGC coupling by gap junctions. The aim of the present research was to find out whether similar changes occur in a mouse inflammation model. Sciatic nerve neuritis was induced by complete Freunds adjuvant (CFA), and isolated ganglia were examined 1 week later. Cell coupling was monitored by intracellular injection of the fluorescent dye Lucifer Yellow. Changes in gap junctions were assessed quantitatively by electron microscopy. Withdrawal threshold in the foot on the side of the inflamed nerve decreased from an average of 3.9 g in control to 0.94 g using Von Frey hairs (P<0.05). In CFA-treated animals dye coupling incidence between SGCs belonging to different glial envelopes increased from 6.9% in controls to 22.5% (P<0.05). Whereas in controls there was no coupling between neurons or between neurons and SGCs, after CFA application the incidence of neuron-neuron and neuron-SGC coupling was 8%. Electron microscopy showed formation of bridges between SGC sheaths surrounding different neurons, which were completely absent in controls. The mean number of gap junctions/100 microm(2) of surface of the section occupied by SGCs increased from 0.215 in controls to 0.709 (P<0.01) in CFA-treated mice. The size of individual gap junctions remained the same. This is the first evidence for ultrastructural changes in SGCs following inflammation. The results support the idea that SGCs are sensitive to a variety of peripheral nerve injuries. We propose that the observed changes may alter signal transmission in DRG and thus may contribute to chronic pain.

Age-related reduction of the satellite cell sheath around spinal ganglion neurons in the rabbit

SummaryThe volumes of the nerve cell bodies and those of the enveloping satellite cell sheaths from spinal ganglia of young adult and aged rabbits were determined by morphometric methods using the electron microscope. The mean volume of the nerve cell bodies was greater in the old rabbits than in young adults; this is probably related to the larger body size of the old animals. The mean volume of the satellite cell sheaths was, however, smaller in the aged rabbits than in the young adults. Consequently the volume ratio between the satellite cell sheaths and the related nerve cell bodies was significantly smaller in the aged animals. Since satellite cells play an important role in the support of the neuron, the reduction in volume of the perineuronal sheath could be associated with a decrease in the trophic activity of satellite cells towards the enveloped neuron with consequences for neuronal activity. Furthermore, in the satellite cell sheaths of old rabbits, the number and extension of gaps that leave the neuronal surface directly exposed to the basal lamina were significantly increased. Since spinal ganglia lack a blood-nervous tissue barrier, only the satellite cell sheath controls the traffic of material to the nerve cell body. Because the neuronal surface unprotected by the satellite cell envelopment is significantly more extensive in the spinal ganglia of old rabbits than in those of young adults, the nerve cells of the former are more exposed to potential damage by harmful substances. A dense undercoating was seen very frequently beneath the portions of the neuronal plasma membrane not covered by satellite cells.

Aging is associated with an increase in dye coupling and in gap junction number in satellite glial cells of murine dorsal root ganglia

Glial cells in both central and peripheral nervous systems are connected by gap junctions, which allow electrical and metabolic coupling between them. In spite of the great current interest in aging of the nervous system, the effect of aging on glial cell coupling received little attention. We examined coupling between satellite glial cells in murine dorsal root ganglia using the dye coupling technique and electron microscopy. We studied mice at ages of postnatal 90-730 days. Dye coupling incidence between satellite glial cells associated with a single neuron increased from 24.2% at postnatal day 90 to 50.5% at postnatal day 730. Dye coupling between satellite glial cells that are in contact with two or more neurons increased from 2.7% at postnatal day 90 to 18.6% at postnatal day 730 (P<0.05). Examination of the ganglia with the electron microscope showed that the number of gap junctions per 100 microm2 of surface area of satellite glial cells increased from 0.22 at postnatal day 90 to 1.56 at postnatal day 730 (P<0.01). The mean length of individual gap junctions did not change with age. These results provide strong evidence for an increase of functional coupling between satellite glial cells during life. This increase is apparently due to an increase in the total area of the system of gap junctions connecting these cells.

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

SummaryThe true surface of rabbit spinal ganglion neurons has been made directly accessible to scanning electronmicroscope 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.

A Quantitative Study of Microtubules in Motor and Sensory Axons

The number, density and distribution of microtubules were compared in the myelinated motor and sensory axons of the spinal roots of lizard (Lacerta muralis). In both motor and sensory axons the average number and density of microtubules were found to be related to the axonal size: the average number of microtubules rose, while the microtubular density decreased with an increase in the cross-sectional area of the axon. More precisely, a linear relationship was observed between the logarithm of the microtubular density and the cross-sectional area of the axon. No significant differences in the microtubular number and density were found between motor and sensory axons of corresponding size. Microtubules were unevenly distributed throughout the cross section of both motor and sensory axons. In particular, a nonaccidental association between microtubules and mitochondria was found in both axon types.

Quantitative changes in mitochondria of spinal ganglion neurons in aged rabbits

Within the context of our research on the age-related structural changes in spinal ganglia, we studied the mitochondria of the neuronal perikaryon in the spinal ganglia of 12-, 42-, and 79-month-old rabbits. Both the volume of the perikaryon and the total mitochondrial mass within the perikaryon increased significantly passing from young adult to old animals. Hence, there is no net loss of mitochondria in these neurons with age. Since, however, the volume of the perikaryon increased by more than 63% while the total mitochondrial mass within the perikaryon increased by only 18%, the mean percentage of perikaryal volume occupied by mitochondria decreased with age. This decrease is only in very minor part a consequence of lipofuscin accumulation, so that the ratio between the total mitochondrial mass and the functionally active volume of cytoplasm decreased with age. Possible causes of this decrease are discussed briefly. Moreover, while the mitochondrial structure did not change, mitochondrial size increased with age. Finally, in each of the three age groups both the mean percentage volume of mitochondria and the mean mitochondrial size were very similar in large light and in small dark neurons.

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

SummaryShape, 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.