Retrograde degeneration of myelinated axons and re-organization in the optic nerves of adult frogs (Xenopus laevis) following nerve injury or tectal ablation

Summary The optic nerve proximal to the lesion (toward the retina) was examined by light and electron microscopy in adultXenopus laevis after various types of injury to optic nerve fibres. Intraorbital resection, transection or crush of the optic nerve or ablation of the contralateral optic tectum all resulted in marked alterations in the myelinated axon population and in the overall appearance of the nerve proximal to the site of injury. Examination of the nerves from 3 days to 6 months postoperatively indicated that a progressive, retrograde degeneration of myelin and loss of large-diameter axons occurred throughout the retinal nerve stump regardless of the type of injury or distance of the injury from the retina. The retinal stump of nerves receiving resection or transection showed a nearly complete loss of myelin and large-diameter axons while the degree of degeneration was subtotal in nerves receiving crush injury or after lesions farther from the retina (i.e. tectal ablation). In addition, the entire retinal nerve stump after all types of injury was characterized by the appearance of an actively growing axon population situated circumferentially under the glia limitans. The latter fibres are believed to represent regrowing axons which are being added onto the nerve, external to the original axon population and are suspected to modify actively the glial terrain and glia limitans.     

Laminin in traumatized peripheral nerve: Basement membrane changes during degeneration and regeneration

Summary The changes in Schwann cell basement membrane associated with degeneration and regeneration during 50 weeks after transection of rat sciatic nerve were studied immunohistochemically with antibodies to laminin. In half of the animals, regeneration was prevented by suturing the nerve stumps aside, whereas in the rest spontaneous regeneration was allowed. Axonal regeneration was monitored with anti-neurofilament protein antibodies.In control nerves, basement membranes surrounding Schwann cells were visualized as circular, laminin-positive structures within the endoneurium. By 8 weeks after transection, Schwann cells had formed columns which were laminin-positive throughout their cross-sectional area and indistinguishable from basement membrane zones in both non-regenerating and regenerating nerves. As axons repopulated the distal stump, the normal shape of Schwann cell basement membrane tubes was slowly restored in freely regenerating nerves. In non-regenerating nerves, however, a striking atrophy of Schwann cell columns was observed. Regenerating axons were only seen inside laminin-positive tubular structures in all phases after 8 weeks in regenerating nerves. On the other hand, restoration of normal shape in laminin-positive basement membrane zones was coincident with appearance of axons in the distal stump, but it did not take place in chronically degenerating nerves.The results show that chronic degeneration leads to an atrophy of Schwann cell columns and results in a decrease in laminin immunoreactivity associated with them.     

Observations on the process of degeneration of the afferent connections to the sensori-motor cortex of the monkey.

The ultrastructural features of degeneration of the afferent connections to the sensorimotor cortex of the primate have been studied with the electron microscope. Thalamo-cortical terminals show cytoplasmic darkening and swelling of vesicles, and neurofilaments and glycogen are often prominent in the larger terminals, although true neurofilamentous hypertrophy is rare. The terminals of the commissural and association projections similarly show darkening and some swelling of vesicles but are usually smaller and neurofilaments and glycogen are rarely seen. Ethanolic-phosphotungstic acid specifically stains degenerating axons and axon terminals. Degeneration occurs more rapidly in the terminals of the association projections than in those of the longer commissural projection although terminals at different stages of degeneration are present simultaneously in both instances. Micropinocytosis of degenerating terminals has been found and both glial and neuronal profiles are involved in this. Exposed post-synaptic thickenings have been seen and these induce subsurface cisternae in the neuronal structures to which they become apposed, including axon initial segments and unmyelinated axons.Damage to a part of the central nervous system thus causes consequential changes in other regions, and these changes are relevant to the interpretation of experimental studies and to the possibility of reorganization of connections following injury.     

Electron microscopic study of the interaction of axons and glia at the site of anastomosis between the optic nerve and cellular or acellular sciatic nerve grafts

Summary The interactions between retinal ganglion cell (RGC) axons and glia at the site of optic nerve section and at the junctional zone between optic nerve and cellular or acellular peripheral nerve (PN) grafts have been studied electron microscopically. After transection, RGC axons, accompanied by processes of astrocyte cytoplasm, grew out from the proximal optic nerve stump into the scar tissue that developed between proximal and distal stumps. However, axons failed to cross the scar, and none entered the distal stump. By 3 days post lesion (DPL), bundles of RGC axons, accompanied by astrocytes and oligodendrocytes, grew out from the proximal optic nerve stump into the junctional zone between optic nerve and either type of PN graft. The bundles of RGC axons and growth cones that grew towards acellular PN grafts degenerated within 10–20 DPL; by 30 DPL a small number of axons persisted within the end of the proximal optic nerve stump. No axons were seen within the acellular PN grafts. These results suggest that reactive axonal sprouting, axon outgrowth and glial migration from the proximal optic nerve stump are events that occur during an acute response to injury, and that they are independent of the presence of Schwann cells. However, it would appear that few axons entered either scar or junctional zone unless accompanied by glia. There was little evidence that axon outgrowth was laminin-dependent.The bundles that grew towards cellular PN grafts encountered cells that we have identified as Schwann cells within the junctional zone: the axons in these bundles survived and entered the cellular grafts. Schwann cells migrated into the junctional zone from the cellular PN graft. It is probable that Schwann cells facilitated RGC axon entry into the graft directly by both cell contact and the secretion of neuronotrophic factors, and indirectly by modifying the CNS glia in the junctional zone.     

Neurofilamentous hypertrophy of intramedullary axonal arbors in intact spinal motoneurons undergoing peripheral sprouting

An incomplete motor nerve injury or a partial loss of motoneurons leads to a partial denervation of skeletal muscle. As part of a compensatory response, the remaining intact motoneurons undergo peripheral sprouting and increase their motor unit size. Our knowledge about the responses in the more proximal parts of these sprouting motoneurons is sparse, however. We investigated the effects of an incomplete transection of the medial gastrocnemius (MG) nerve in the adult cat on the morphology of the intramedullary axon and axon collateral systems of the remaining intact MG motoneurons. At twelve weeks following the partial transection of the MG nerve, intracellular recording and labeling techniques were used to deposit horseradish peroxidase into single intact MG motoneurons for detailed morphological studies. The light microscopic appearance and caliber of the intramedullary stem motor axons of the intact MG motoneurons were indistinguishable from controls. The number and size of the intramedullary motoraxon collateral systems were also unchanged. However, frequent and marked hypertrophy of the distal portions of the motoraxon collaterals was encountered. Electron microscopic studies of the hypertrophied collaterals demonstrated abnormal accumulations of disorganized neurofilaments arranged in bundles or whorls. The morphological changes were indistinguishable from the neurofilamentous hypertrophy that has previously been reported in Wallerian degeneration, in experimental and human motor neuron disease and in some regenerating axonal processes of spinal motoneurons. We conclude that, neurofilamentous hypertrophy of the intramedullary arbors of motor axons may also be part of a reactive and non-degenerative response in intact motoneurons undergoing compensatory peripheral sprouting.     

Ultrastructural and morphometric analysis of long-term peripheral nerve regeneration through silicone tubes

Summary Light and electron microscopy were used to investigate long-term regeneration in peripheral nerves regenerating across a 10 mm gap through silicone tubes. Schwann cells and axons co-migrated behind an advancing front of fibroblasts, bridging the 10 mm gap between 28 and 35 days following nerve transection. Myelination of regenerated fibres started between 14 and 21 days after transection and occurred in a manner similar to that reported during development. Although these early events were successful in producing morphologically normal-appearing regenerated fibres, complete maturation of many of these fibres was never achieved. Axonal distortion by neurofilaments, axonal degeneration and secondary demyelination were seen at 56 days following nerve transection. These changes progressed in severity with time as more axons advanced through the distal stump towards their peripheral target. Since regeneration occurs in the absence of endoneurial tubes, and because constrictive forces act on the nerve during regeneration, we suggest that these extrinsic factors limit the successful advancement of axons through the distal stump to their target organ.     

Fine structure of the axon collaterals of Huber in frog spinal ganglia

Summary The fine structure of intracapsular axon collaterals in the spinal ganglia ofRana nigromaculata nigromaculata has been studied. The collaterals originated from the initial tract of the axon, before its bifurcation. They were about 0.8 m in diameter and contained numerous longitudinally aligned neurofilaments, a few mitochondria and vesicles. The end bulbs of axon collaterals contained spirally arranged bundles of neurofilaments; mitochondria, agranular and granulated vesicles, and dense bodies occurred underneath the plasma membrane. The axon collaterals generally lay in the periaxonal labyrinth formed by processes of the polar cells around the initial tract of the axon and the axonal pole of the spinal ganglion cell. The significance of the intracapsular axon collaterals is discussed.     

Comparison of matrix metalloproteinase expression during Wallerian degeneration in the central and peripheral nervous systems

The matrix metalloproteinases (MMPs) are a large family of zinc-dependent enzymes which are able to degrade the protein components of the extracellular matrix. They can be placed into subgroups based on structural similarities and substrate specificity. Aberrant expression of these destructive enzymes has been implicated in the pathogenesis of immune-mediated neuroinflammatory disorders. In this study we investigate the involvement of MMPs, from each subgroup, in Wallerian degeneration in both the central and peripheral nervous systems. Wallerian degeneration describes the process initiated by transection of a nerve fibre and entails the degradation and removal of the axon and myelin from the distal stump. A similar degenerative process occurs as the final shared pathway contributing to most common neuropathies. MMP expression and localisation in the peripheral nervous system are compared with events in the CNS during Wallerian degeneration. Within 3 days after axotomy in the peripheral nervous system, MMP-9, MMP-7 and MMP-12 are elevated. These MMPs are produced by Schwann cells, endothelial cells and macrophages. The temporospatial expression of activated MMP-9 correlates with breakdown of the blood-nerve barrier. In the CNS, 1 week after optic nerve crush, four MMPs are induced and primarily localised to astrocytes, not microglia or oligodendrocytes. In the degenerating optic nerve, examined at later time points (4, 8, 12 and 18 weeks), MMP expression was down-regulated. The absence of MMPs in oligodendrocytes and mononuclear phagocytes during Wallerian degeneration may contribute to the slower removal of myelin debris observed in the CNS. The low level of the inactive pro-form of MMP-9 in the degenerating optic nerve may explain why the blood-brain barrier remains intact, while the blood-nerve barrier is rapidly broken down. We conclude that the difference in the level of expression, activation state and cellular distribution of MMPs may contribute to the different sequence of events observed during Wallerian degeneration in the peripheral compared to the CNS.     

An immunocytochemical study of calpain II in the hippocampus of rats injected with kainate

The distributions of the kainate/dl-alpha-amino-3-hydroxy-5-methylisoxazolepropionic acid (KA/AMPA) receptors GluR1 and calcium-activated neutral protease II (calpain II) in the hippocampus of normal and kainate-lesioned rats were studied by immunocytochemistry. There was a reduction in GluR1 immunoreactivity and a slight increase in calpain II immunoreactivity on the dendrites of pyramidal neurons in CA fields affected by the kainate at 18 h postinjection. Calpain II immunoreactivity was associated with amyloid fibrils at electron microscopy. These fibrils were most often intracellular, in membrane-bound profiles, some of which were contacted by axon terminals and were identified as degenerating dendrites. There was extensive destruction of mitochondrial membranes in degenerating profiles, and accumulations of amyloid fibrils were often localised in mitochondria in a calpain-positive profile. This was unlike other, calpain-negative degenerating profiles, that contained tubulovesicular profiles or multilamellar bodies, where mitochondrial membranes were preserved. Many more calpain-positive profiles were observed at electron microscopy 6 days after kainate injection. The enzyme was present in macrophages and astrocytes in lesioned areas.     

Relationship between cerebral nystagmogenic area and spinal cord

The effect of section of the spinal cord upon the cerebral nystagmogenic area was investigated in rabbits. Cervical transection was found to result in an increase of excitability of such area, which is demonstrated by lowering of the threshold for cerebral eye nystagmus, whereas dorsal spinal transection does not affect the cerebral nystagmogenic area. Moreover,it was found that stimulation of the proximal stump of the spinal cord, at level of the dorsal quadrants, depresses the excitability of the cerebral nystagmogenic area, while ventral quadrants stimulation has a facilitating effect on the CNA. Modification of the excitability of the cerebral nystagmogenic area was also studied recording NPPs (Non-Primary Potentials) and unitary responses to photic stimulation prior to and after cervical transection as well as following dorsal and ventral quadrants stimulation. The Authors suggest that since the nystagmogenic area is located in the perivisual fields it receives visual and sensitive afferent impulses from the upper part of the body and it, in turn, projects onto the oculomotor nuclei, so that it could be considered as an integrator center for extrinsic musculature of the eye.     

The projection of the lateral geniculate nucleus to area 17 of the rat cerebral cortex. I. General description.

Lesions were made in the lateral geniculate nucleus of the rat and the consequent degeneration in area 17 of the cerebral cortex was studied by light and electron microscopy. These lesions produced prominent degeneration of axon terminals in layer IV extending into layer III and a much lesser amount in layers I and VI. The darkened degenerating axon terminals forming asymmetric synaptic junctions and were frequently surrounded by hypertrophied astrocytic processes. These terminals appeared to be disposed randomly, forming no discernible patterns. In layer IV 83% of the synapsing, degenerating terminals formed junctions with dendritic spines, 15% with dendritic shafts, and 2% with neuronal perikarya. The dendritic shafts and neuronal perikarya appeared to belong to spine-free stellate cells. The dendrites giving rise to the spines receiving degenerating axon terminals could not be identified, for most of the spines appeared as isolated profiles that could not be traced back to their dendritic shafts. One example of a degenerating axon terminal synapsing with an axon initial segment was encountered. Small, degenerating myelinated axons were prevalent in layers VI, V and IV, but were only infrequent in the supragranular layers. These results are compared with those obtained in other studies of thalamocortical projections.     

Enhanced expression of the extracellular matrix molecule J1/tenascin in the regenerating adult mouse sciatic nerve

Summary We have investigated the expression of J1/tenascin in the sciatic nerve of the adult mouse under normal and regenerating conditions by immunocy tological and immunochemical methods. In the normal nerve, J1/tenascin expression was confined to the extracellular matrix at the node of Ranvier and in the perineurium. At 2 days after nerve transection, J1/tenascin was detectable in the fibroblast-containing caps of the distal and proximal nerve stumps, in the distal nerve stump along its entire length and in the distal end of the proximal nerve stump. In the nerve stumps immunoreactivity was predominantly associated with extracellular matrix consisting of collagen fibrils and Schwann cell basal laminae. Approximately 7 days after transection, the caps of the nerve stumps had usually grown together forming a bridge. This bridge consisted of a J1/tenascin-negative perineurium-like structure and an inner part of predominantly fibroblasts, endothelial cells and macrophages. All cell types in this inner part were embedded in a J1/tenascin-positive matrix of collagen fibrils indicating the prospective direction of growth of neural elements. A few days later, J1/tenascin in the bridge was confined to the extracellular matrix around small Schwann cell-containing nerve fascicles. In nerves chronically denervated for 19 days, J1/tenascin was poorly detectable in the cap of the distal stump, although Schwann cells had infiltrated this cap. Approximately 19 days after the lesion, J1/tenascin expression returned to control levels in the proximal nerve stump. In the distal nerve stump, J1/tenascin immunoreactivity reached a peak at approximately 14 days after nerve transection and vanished only at approximately 35 days, thus correlating with the time of active regrowth of axons into the distal nerve stump. This reduction was prevented by chronic denervation, suggesting that reinnervation of target structures may be related to the down-regulation of J1/tenascin. These combined observations suggest that J1/tenascin is differentially regulated in the individual parts of the regenerating nerve, possibly triggered by different cellular and molecular signals.     

Reticulon-like-1, the Drosophila orthologue of the hereditary spastic paraplegia gene reticulon 2, is required for organization of endoplasmic reticulum and of distal motor axons

Several causative genes for hereditary spastic paraplegia encode proteins with intramembrane hairpin loops that contribute to the curvature of the endoplasmic reticulum (ER), but the relevance of this function to axonal degeneration is not understood. One of these genes is reticulon2. In contrast to mammals, Drosophila has only one widely expressed reticulon orthologue, Rtnl1, and we therefore used Drosophila to test its importance for ER organization and axonal function. Rtnl1 distribution overlapped with that of the ER, but in contrast to the rough ER, was enriched in axons. The loss of Rtnl1 led to the expansion of the rough or sheet ER in larval epidermis and elevated levels of ER stress. It also caused abnormalities specifically within distal portions of longer motor axons and in their presynaptic terminals, including disruption of the smooth ER (SER), the microtubule cytoskeleton and mitochondria. In contrast, proximal axon portions appeared unaffected. Our results provide direct evidence for reticulon function in the organization of the SER in distal longer axons, and support a model in which spastic paraplegia can be caused by impairment of axonal the SER. Our data provide a route to further understanding of both the role of the SER in axons and the pathological consequences of the impairment of this compartment.     

Morphological investigations on axonal swellings and spheroids in various human diseases

Summary Axonal swellings and spheroids in various human diseases were studied by light and electron microscopy. 4 cases of infantile neuroaxonal dystrophy, 2 of degenerative diseases, 2 brain tumors and 3 of cerebrovascular disease were examined.Ultrastructurally most spheroids in infantile neuroaxonal dystrophy consisted of interconnected tubules, stacked membranotubular profiles, alternating layered membranes and accumulations of neurofilaments. Combinations of these four constituents were seen only in infantile neuroaxonal dystrophy. Torpedos (fusiform swelling of the axon of a Purkinje cell) consisted exclusively of neurofilaments. Spheroids in case 6 (mental retardation) and 7 (atypical teratoma) consisted of interwoven skeins of neurofilaments and grouped mitochondria. Spheroids in case 8 (demyelination) and 9 (cerebrovascular disease) consisted of packed complex bodies and mitochondria. Spheroids in cases 10 and 11 (cerebrovascular disease) consisted of degenerating organelles only. The morphological differences between cases 9, 10 and 11 probably depends on the severity and timing of the cerebral injury.Most spheroids show similar histological and histochemical properties, but ultrastructural study may give some clue to the origin of the bodies.     

Unusual neurofibrillary accumulations induced by beta,beta'-iminodipropionitrile (IDPN)

Beta,beta'-iminodipropionitrile (IDPN) is a synthetic compound known to induce massive focal accumulations of neurofilaments almost exclusively in the proximal segments of motor and sensory axons. The present paper deals with a cat intoxicated with IDPN for five weeks. Swellings filled with skeins of maloriented neurofilaments were observed in the initial (non-myelinated) segment of intraparenchymal spinal axons in addition to the swellings consistently located in the first internodes. Filamentous masses in the initial segment were in some cases intermingled with focal accumulations of mitochondria and smooth vesicles to give the proximal axon the classical appearance of 'spheroid'. In some motoneurons, clumps of neurofilaments occupied the axon hillock and extended well into the perikaryon and some dendritic stems, making the borderlines between cell body and processes undistinguishable. Cytoplasmic organelles were displaced and rearranged in the presence of somal neurofibrillary changes. In a few neurons, regardless of the presence of swellings in the axon initial segment, signs of central chromatolysis were noticed, in the form of nuclear caps of Nissl substance and eccentricity of the nucleus. These findings are discussed with reference to other neurotoxic models of neurofibrillary pathology and the immunochemical differences recently detected between neurofilaments in perikarya, dendrites and axons.     

On the degeneration of rat neuromuscular junctions after nerve section

1. A study was made of functional and structural changes during degeneration of end-plates in the rat diaphragm after phrenic nerve section at two levels.2. For 8-10 hr after cutting the nerve in the neck, all end-plates retain the ability to transmit impulses. During the following 8-10 hr, an increasing number of end-plates lose this ability so that after a total of about 20 hr, no end-plates can transmit.3. Transmission failure occurs abruptly at most end-plates. This failure is usually accompanied by cessation of spontaneous miniature end-plate potentials (min.e.p.p.s), though in a few cases min.e.p.p.s persist after junctional transmission has failed. Several degenerating junctions were observed where the frequency of min.e.p.p.s was very low, suggesting an intermediate stage in min.e.p.p. failure.4. The time of junctional failure depends on the length of the degenerating nerve stump. For each additional centimetre of nerve, failure is delayed about 45 min.5. Changes in ultrastructure of nerve endings closely parallel those of function. For about 8-12 hr after cutting the nerve, nearly all end-plates appear normal. During the period when transmission is failing, some end-plates are clearly undergoing structural break-down. By the time functional failure is complete, all end-plates appear grossly abnormal.6. During degeneration, the contents of the axoplasm undergo disruption and the nerve terminal breaks up into small fragments. In contrast, the Schwann cell appears to become very active and its processes extend into the synaptic cleft to surround fragments of the nerve terminal. Ultimately, the Schwann cell completely replaces the axon at the end-plate.7. Increasing the length of the peripheral nerve stump delays the onset of structural break-down. Disruption of end-plates near the site of nerve entry into the muscle occurs before those farther away.8. It is suggested that end-plate degeneration is triggered by a signal which passes from the site of injury to the nerve terminal. The duration of the period after transection when end-plates appear to be normal would then reflect the time required for this signal to travel the length of the isolated nerve stump.     

Gracile nucleus of streptozotocin-induced diabetic rats

Summary This study reports ultrastructural changes in the gracile nucleus of male Wistar rats after streptozotocin-induced diabetes. During the acute phase (3–7 days) degenerating electron-dense dendrites and axon terminals were dispersed in the neuropil. Degenerating dendrites were characterized by an electron-dense cytoplasm, swollen mitochondria, dilated endoplasmic reticulum and scattered ribosomes. Degenerating axon terminals were characterized by an electron-dense cytoplasm and clustering of small spherical agranular vesicles. Degenerating axon terminals may form part of a synaptic glomerulus with a central electron-dense dendrite, or they may form the central element of a synaptic glomerulus. These degenerating profiles were absent in the gracile nucleus of the 3 and 7 days insulin-treated post-streptozotocin rats. Macrophages were present in the neuropil and were in the process of engulfing neuronal elements. During the medium phase (1–6 months), most of the degenerating dendrites and axon terminals had been engulfed or removed by macrophages. During the late phase (9–12 months) a second wave of degeneration occurred in the gracile nucleus, similar to the acute phase. During the medium and late phases, dystrophic axonal profiles were also significantly increased in the rats after streptozotocin treatment.It is concluded that the ultrastructural changes observed in the gracile nucleus in the present study were the result of streptozotocin-induced diabetes rather than a toxic effect of streptozotocin, even in the acute phase.     

The relation of axonal transport of mitochondria with microtubules and other axoplasmic organelles.

Axonal transport of mitochondria was studied in frog sciatic nerves incubated in agents selected for their known or alleged effect on microtubules or axonal flow. Quantitative data on mitochondria, microtubules, neurofilaments, endoplasmic reticulum, and cross-sectional area of the axon indicate that axonal transport of mitochondria is dependent on microtubules. When more than half of the microtubules are destroyed, the axonal transport of mitochondria is diminished in proportion to the destruction of microtubules. Axonal transport of mitochondria is not related to neurofilaments and endoplasmic reticulum. Changes in the cross-sectional area of axons, even upon reduction to half the normal size, do not noticeably affect mitochondrial transport. Cyanide which blocks oxidative metabolism also blocks axonal transport of mitochondria, but analysis of fine structure indicates that cyanide is destructive to microtubules as well.     

A glutamate-sensitive neuronal system originating from the area postrema terminates in and transports acetylcholinesterase to the nucleus of the solitary tract

Summary The possible cellular mechanism of action of systemically administered monosodium-l-glutamate and the projections of glutamate-sensitive area postrema neurons have been studied in rats. Parenteral administration of monosodium-l-glutamate induced a selective degeneration of a particular population of AChE-containing area postrema neurons. Electron microscopic cytochemistry and X-ray microanalysis revealed the presence of calcium-containing electron-dense deposits in the mitochondria of degenerating area postrema neurons indicating the possible pathogenetic role of an enhanced intracellular calcium level in the mechanism of monosodium-l-glutamate-induced nerve cell degeneration. Degeneration of area postrema neurons was followed by the appearance of degenerating axon terminals in a well-defined region of the nucleus of the solitary tract, the area subpostrema. Degenerating area postrema neurons and axon terminals were rapidly engulfed by phagocytes predominantly of microglial character. AChE activity, localized to the basal lamina of the capillaries of the area subpostrema under normal conditions, could no longer be detected in rats treated with monosodium-l-glutamate 3–4 weeks previously.These findings provide evidence for the existence of a particular population of glutamate-sensitive, AChE-containing area postrema neurons which project and transport AChE to the nucleus of the solitary tract. This specific neuronal pathway connecting the area postrema with the nucleus of the solitary tract may play an important role in some of the functions attributed to the area postrema. The results also strengthen the hypothesis that brain capillary AChE activity may be of neuronal origin.     

The Morphologic Effects of Dieldrin and Methyl Mercuric Chloride on Pars Recta Segments of Rat Kidney Proximal Tubules

This investigation was undertaken to evaluate the morphologic effects in rat kidney resulting from chronic exposure to low doses of the pesticide dieldrin, methyl mercuric chloride (CH₃HgCl) and the combination of dieldrin plus CH₃HgCl. Histologic and ultrastructural changes were confined to the proximal tubules. Alterations in these tubules were consistent and reproducible for each regimen and did not become more severe with duration of exposure. The straight segment of the proximal tubule (pars recta) was more severely affected by dieldrin and CH₃HgCl than the convoluted portion. Female rats were more markedly affected than males. Pars recta tubule cells of male and female rats exposed to dieldrin showed an increase of smooth endoplasmic reticulum (SER). Male rats displayed a greater increase in SER than females. Pars recta tubule cells of animals given CH₃HgCl also exhibited increased amounts of SER, degenerating mitochondria and cell death. Pars recta tubules of females were dilated and contained within the lumens many spherical, hematoxylin-positive staining, cytoplasmic masses, which were visible by light microscopy. These masses were characterized ultrastructurally by the presence of an SER aggregate in an area of material similar to cell matrix. In addition, cells of the pars recta of female animals contained electron-dense membranous cytosomes not present in control animals. Pars recta cells of males showed an increase in SER, but the dense membranous cytosomes observed in the pars recta cells of female rats were not seen. Rats exposed to dieldrin plus CH₃CgCl showed less morphologic alteration of the pars recta tubules than animals given methyl mercuric chloride; however, increased amounts of SER and more degeneration in tubule cells were observed in these animals when compared to control animals. The findings are discussed in relation to the conversion of CH₃HgCl to inorganic mercury in vivo and the known toxicity of inorganic mercury to the pars recta. Decreased tubular alteration in males and dieldrin-treated animals may be explained by sexual differences in renal enzyme levels or activities and the induction of microsomal enzyme systems by dieldrin.