Ultrastructural studies of the nervous system after mercury intoxication. II. Pathological changes in the nerve fibers

Elemental biomapping of human tissues suggests toxic metals such as mercury play a role in the pathogenesis of cancer

Toxic metals such as mercury, lead, and cadmium have multiple carcinogenic capacities, including the ability to damage DNA and incite inflammation. Environmental toxic metals have long been suspected to play a role in the pathogenesis of cancer, but convincing evidence from epidemiological studies that toxic metals are risk factors for common neoplasms has been difficult to gain. Another approach is to map the location of potentially toxic elements in normal human cells where common cancers originate, as well as in the cancers themselves. In this Perspective, studies are summarized that have used elemental biomapping to detect toxic metals such as mercury in human cells. Two elemental biomapping techniques, autometallography and laser ablation-inductively coupled-mass spectrometry imaging, have shown that multiple toxic metals exist in normal human cells that are particularly prone to developing cancer, and are also seen in neoplastic cells of breast and pancreatic tumors. Biomapping studies of animals exposed to toxic metals show that these animals take up toxic metals in the same cells as humans. The finding of toxic metals such as mercury in human cells prone to cancer could explain the increasing global incidence of many cancers since toxic metals continue to accumulate in the environment. The role of toxic metals in cancer remains to be confirmed experimentally, but to decrease cancer risk a precautionary approach would be to reduce emissions of mercury and other toxic metals into the environment from industrial and mining activities and from the burning of fossil fuels.

Mercury levels in hair are associated with reduced neurobehavioral performance and altered brain structures in young adults

The detrimental effects of high-level mercury exposure on the central nervous system as well as effects of low-level exposure during early development have been established. However, no previous studies have investigated the effects of mercury level on brain morphometry using advance imaging techniques in young adults. Here, utilizing hair analysis which has been advocated as a method for biological monitoring, data of regional gray matter volume (rGMV), regional white matter volume (rWMV), fractional anisotropy (FA) and mean diffusivity (MD), cognitive functions, and depression among 920 healthy young adults in Japan, we showed that greater hair mercury levels were weakly but significantly associated with diminished cognitive performance, particularly on tasks requiring rapid processing (speed measures), lower depressive tendency, lower rGMV in areas of the thalamus and hippocampus, lower rWMV in widespread areas, greater FA in bilaterally distributed white matter areas overlapping with areas of significant rWMV reductions and lower MD of the widely distributed gray and white matter areas particularly in the bilateral frontal lobe and the right basal ganglia. These results suggest that even normal mercury exposure levels in Japan are weakly associated with differences of brain structures and lower neurobehavioral performance and altered mood among young adults.

First record on mercury accumulation in mice brain living in active volcanic environments: a cytochemical approach

The health effects of mercury vapor exposure on the brain in volcanic areas have not been previously addressed in the literature. However, 10% of the worldwide population inhabits in the vicinity of an active volcano, which are natural sources of elemental mercury emission. To evaluate the presence of mercury compounds in the brain after chronic exposure to volcanogenic mercury vapor, a histochemical study, using autometallographic silver, was carried out to compare the brain of mice chronically exposed to an active volcanic environment (Furnas village, Azores, Portugal) with those not exposed (Rabo de Peixe village, Azores, Portugal). Results demonstrated several mercury deposits in blood vessels, white matter and some cells of the hippocampus in the brain of chronically exposed mice. Our results highlight that chronic exposure to an active volcanic environment results in brain mercury accumulation, raising an alert regarding potential human health risks. These findings support the hypothesis that mercury exposure can be a risk factor in causing neurodegenerative diseases in the inhabitants of volcanically active areas.

Effects of methylmercury on spinal cord afferents and efferents-A review

Methylmercury (MeHg) is an environmental neurotoxicant of public health concern. It readily accumulates in exposed humans, primarily in neuronal tissue. Exposure to MeHg, either acutely or chronically, causes severe neuronal dysfunction in the central nervous system and spinal neurons; dysfunction of susceptible neuronal populations results in neurodegeneration, at least in part through Ca2+-mediated pathways. Biochemical and morphologic changes in peripheral neurons precede those in central brain regions, despite the fact that MeHg readily crosses the blood-brain barrier. Consequently, it is suggested that unique characteristics of spinal cord afferents and efferents could heighten their susceptibility to MeHg toxicity. Transient receptor potential (TRP) ion channels are a class of Ca2+-permeable cation channels that are highly expressed in spinal afferents, among other sensory and visceral organs. These channels can be activated in numerous ways, including directly via chemical irritants or indirectly via Ca2+ release from intracellular storage organelles. Early studies demonstrated that MeHg interacts with heterologous TRP channels, though definitive mechanisms of MeHg toxicity on sensory neurons may involve more complex interaction with, and among, differentially-expressed TRP populations. In spinal efferents, glutamate receptors of the N-methyl-D-aspartate (NMDA), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), and possibly kainic acid (KA) classes are thought to play a major role in MeHg-induced neurotoxicity. Specifically, the Ca2+-permeable AMPA receptors, which are abundant in motor neurons, have been identified as being involved in MeHg-induced neurotoxicity. In this review, we will describe the mechanisms that could contribute to MeHg-induced spinal cord afferent and efferent neuronal degeneration, including the possible mediators, such as uniquely expressed Ca2+-permeable ion channels.

The toxicology of mercury and its compounds

A concentrated review on the toxicology of inorganic mercury together with an extensive review on the neurotoxicology of methylmercury is presented. The challenges of using inorganic mercury in dental amalgam are reviewed both regarding the occupational exposure and the possible health problems for the dental patients. The two remaining "mysteries" of methylmercury neurotoxicology are also being reviewed; the cellular selectivity and the delayed onset of symptoms. The relevant literature on these aspects has been discussed and some suggestions towards explaining these observations have been presented.

Visual field losses in workers exposed to mercury vapor

Visual field losses associated with mercury (Hg) exposure have only been assessed in patients exposed to methylmercury. Here we evaluate the automated visual field in 35 ex-workers (30 males; 44.20+/-5.92 years) occupationaly exposed to mercury vapor and 34 controls (21 males; 43.29+/-8.33 years). Visual fields were analyzed with the Humphrey Field Analyzer II (model 750i) using two tests: the standard automated perimetry (SAP, white-on-white) and the short wavelength automated perimetry (SWAP, blue-on-yellow) at 76 locations within a 27 degrees central visual field. Results were analyzed as the mean of the sensitivities measured at the fovea, and at five successive concentric rings, of increasing eccentricity, within the central field. Compared to controls, visual field sensitivities of the experimental group measured using SAP were lower for the fovea as well as for all five eccentricity rings (p<0.05). Sensitivities were significantly lower in the SWAP test (p<0.05) for four of the five extra-foveal eccentricity rings; they were not significant for the fovea (p=0.584) or for the 15 degrees eccentricity ring (p=0.965). These results suggest a widespread reduction of sensitivity in both visual field tests. Previous reports in the literature describe moderate to severe concentric constriction of the visual field in subjects with methylmercury intoxication measured manually with the Goldman perimeter. The present results amplify concerns regarding potential medical risks of exposure to environmental mercury sources by demonstrating significant and widespread reductions of visual sensitivity using the more reliable automated perimetry.

Methylmercury induces apoptosis in cultured rat dorsal root ganglion neurons

Methylmercury is known to have devastating effects on the mammalian nervous system. In order to characterize the dose dependence of methylmercury-induced neurotoxicity, we first studied neurite outgrowth from rat dorsal root ganglia explants. In this model, methylmercury inhibited neurite outgrowth with a TD(50) of approximately 0.5 microM. We then used this relationship to optimize dosing for subsequent transcriptional profiling analyses in two independent neuronal model systems: dissociated sensory neurons and PC12 cells. As seen in previous studies, the expression of a number of genes associated with oxidative stress was altered following a 6h challenge with 1 microM methylmercury. When PC12 cells were subjected to a longer exposure (24h), a relative increase was noted in the representation of genes associated with cell cycling and apoptosis. To confirm the presence of apoptosis in cultured neurons, we then applied TUNEL staining and bis-benzimide staining techniques to primary cultures of dissociated sensory neurons. After 24h, 1 microM methylmercury increased both DNA end-labeling (P<0.01) and nuclear fragmentation (P<0.02). The latter effect appeared to be dose-dependent.

Oxidative damage to nucleic acids in motor neurons containing mercury

Heavy metals have been implicated in the pathogenesis of sporadic motor neuron disease (MND). We were interested to see if inorganic mercury leads to oxidative damage in motor neurons since free radicals have been suspected to be involved in MND, so a method to examine oxidatively-damaged DNA in situ was used to examine individual motor neurons. Mice were exposed to 500 microg/m3 of mercury vapour for 2 h. Two, five, or ten days later sections from formalin-fixed, paraffin-embedded blocks of cervical spinal cord were incubated in avidin-FITC. Sections were examined under a fluorescence microscope and photographs of pairs of mercury-exposed and control spinal motor neurons were analysed semi-quantitatively for the amount of fluorescence using an image analysis program. Avidin fluorescence was seen in the perikaryon of both control and mercury-exposed motor neurons. In each control-mercury pair (four pairs per group) significantly more perikaryal fluorescence was seen in mercury-containing than in control motor neurons (Mann-Whitney testing). Mercury within the motor neuron perikaryon therefore leads to increased avidin binding, an indicator of oxidative damage to DNA. The findings support the hypothesis that an environmental toxin such as mercury can enter and damage motor neurons.

Mercury and multiple sclerosis

It has occasionally been claimed that multiple sclerosis (MS) may be due to a chronic mercury intoxication, e.g. from mercury liberated from dental fillings. Therefore, the present communication compares the mercury content assayed by neutron activation in 8 macroscopically normal areas (frontal lobe) of MS autopsy brains with those of 8 control samples. No significant differences could be traced between the two groups concerning total mercury. However, the lipid-soluble mercury (preferably methyl mercury) expressed per cell unit (DNA) was found significantly decreased in MS. These data may be explained either by a wash-out of lipid soluble mercury due to break-down of the blood-brain barrier in MS or to abnormalities in methylation processes probably related to the vitamin B12 metabolism in MS.

Animal models of amyotrophic lateral sclerosis and the spinal muscular atrophies

The causes of human amyotrophic lateral sclerosis (ALS) and the spinal muscular atrophies (SMA) are, almost without exception, unknown. This ignorance has stimulated the search for animal models to obtain insight into the etiology, pathogenesis and biochemical mechanisms underlying the human disorders. None of the 38 animal models, described in this review, provides an exact animal copy of a specific human motor neuron disease. Most of the models reproduce certain structural or physiological aspects of their human counterparts. The various experimental models can be classified according to the pathogenetic mechanism involved and according to the structural changes observed. Models based on experimentally induced disease, include heavy metals and trace elements (lead intoxication in guinea pigs, rabbits, rats, cats and primates; mercury intoxication in rats; aluminium intoxication in rabbits; swayback in goat kids; calcium and magnesium deficient rabbits and primates and calcium deficient cynomolgus monkeys), toxins (IDPN, vincristine, vinblastine, podophyllotoxin, colchicine, maytansine, maytanprine, L-BMAA, lectins, adriamycin), nutritional factors (ascorbic acid deficient guinea pigs), virus infection (spongiform polioencephalomyelitis, attenuated poliovirus, lactate dehydrogenase-elevating virus), and immunological factors (immunization with motor neurons). Hereditary models comprise hereditary canine spinal muscular atrophy, hereditary neurogenic amyotrophy in the pointer dog, Stockard paralysis, Swedish Lapland dog paralysis, "wobbler" mouse, "shaker" calf, and hereditary spinal muscular atrophy in zebra foals, crossbred rabbits,

Differential effects of methylmercury on the synthesis of protein species in dorsal root ganglia of the rat

Dorsal root ganglia from control and methylmercury (MeHg)-treated rats were incubated in vitro with 35S-methionine ant the proteins synthesized were analyzed by two-dimensional electrophoresis. The double labelling method, in which proteins of control dorsal root ganglia labelled in vitro with 3H-leucine were added to each of the two samples as an internal standard, was used to minimize unavoidable errors arising from the resolving procedure itself. The results obtained showed that the effect of MeHg on the synthesis of proteins in dorsal root ganglia was not uniform for individual protein species in the latent period of MeHg intoxication. Among 200 protein species investigated, 157 showed inhibition of synthesis close to that of the total proteins in the tissue (68% of the control). Among the remaining protein species, 20 showed real stimulation of synthesis, whereas 7 were moderately inhibited and 16 were inhibited more strongly than the total proteins in the tissue. These results suggest that the effect of MeHg on the synthetic rates for protein species in dorsal root ganglia differs with the species, and that unusual elevation or reduction of the synthesis of some protein species caused by MeHg may lead to impairment of normal nerve functions.

Spinal roots of rats poisoned with methylmercury: physiology and pathology

The evoked potentials in the ventral and dorsal roots were recorded independently by stimulating the sciatic nerve of both control and methylmercury-poisoned rats. Poisoned rats showed markedly decreased amplitudes but normal latencies of the potentials evoked in the dorsal roots. Potentials evoked in the ventral roots had normal latencies and amplitudes. Pathological correlates indicated acute axonal degeneration of the dorsal roots, with a significant decrease of the large and small myelinated fiber densities. The ventral roots were histologically unremarkable. Our pathological confirmation of the electrophysiologic changes in the methylmercury-poisoned rats enables us to substantially assess the pathophysiological aspects of acute lesions in the spinal roots.

Decrease in protein phosphorylation in central and peripheral nervous tissues of methylmercury-treated rat

The protein phosphorylation in extracts of nervous tissues of rats acutely exposed to methylmercury chloride (seven daily injections of 10 mg methylmercury chloride/kg body weight) was examined. In the brain, the phosphorylating activity was dependent on cAMP and Mg2+. The effect of methylmercury on the phosphorylation of brain proteins, including tubulin and MAP-2, was hardly discernible. In peripheral nervous tissues such as the dorsal and ventral roots, sciatic nerves and dorsal root ganglia, the phosphorylating activity was dependent on Ca2+, and the maximal activity was obtained when the tissues were extracted in the presence of 1% Triton X-100. SDS-Polyacrylamide gel electrophoresis revealed that the major phosphorylated proteins in the peripheral tissues were myelin proteins. The effects of methylmercury were not uniform regarding protein species and tissues. The most marked changes were observed in sciatic nerves, in which phosphorylation of the 33 kDa, 28 kDa, 19 kDa, 18 kDa and 15 kDa proteins was significantly decreased in the symptomatic phase of intoxication.

Interactions of mitochondrial inhibitors with methylmercury on spontaneous quantal release of acetylcholine

The interaction of methylmercury (MeHg) with various inhibitors of mitochondrial function (dinitrophenol, 50 microM; dicoumarol, 100 microM; valinomycin, 20 microM; and ruthenium red, 20 microM) on spontaneous quantal release of acetylcholine was tested at the neuromuscular junction of the rat. The objective was to determine whether these mitochondrial inhibitors blocked the MeHg-induced increase of spontaneous release of acetylcholine, an effect measured electrophysiologically as increased miniature endplate potential (MEPP) frequency. MEPPs were recorded from myofibers of the rat hemidiaphragm using conventional, intracellular microelectrode recording techniques. When given alone, all four inhibitors increased MEPP frequency from resting levels of 1-2/sec (Hz) to approximately 10-60 Hz after a latency which ranged from 5 to 30 min. MEPP frequency subsequently returned to control levels. Subsequent concomitant application of MeHg (100 microM) with dinitrophenol, dicoumarol, or valinomycin increased MEPP frequency sharply to peak values of 40-60 Hz after 15-20 min. MEPP frequency subsided to pre-MeHg levels 10 min later. The time course and peak MEPP frequency elicited by MeHg after pretreatment with these uncouplers were similar to results obtained in preparations treated with MeHg alone. Ruthenium red, a putative specific inhibitor of the Ca2+ uptake uniporter in mitochondria, increased MEPP frequency to 12 Hz after 8.5 min when given alone. MEPP frequency returned to control levels approximately 10 min later. Subsequent application of MeHg and ruthenium red for up to 80 min failed to increase MEPP frequency. The inability of MeHg to increase MEPP frequency in ruthenium red-treated preparations was not due to depletion of acetylcholine nor to block of postjunctional receptors by ruthenium red since subsequent treatment with La3+ (2 mM) increased MEPP frequency to 12.5 Hz within 10 min. Thus, ruthenium red blocked the stimulatory effect of MeHg on MEPP frequency while uncouplers of oxidative phosphorylation and a K+ ionophore did not. The results with ruthenium red are consistent with the proposal that MeHg may block mitochondrial uptake of Ca2+ or promote its release, leading to an increased free cytosolic Ca2+ concentration which in turn stimulates spontaneous release of acetylcholine.

Protective effects of selenium against methylmercury neurotoxicity: a morphological and biochemical study

The protective effect of selenium (as sodium selenite) against methylmercury neurotoxicity was investigated morphologically and biochemically. Young adult Charles River rats were divided into 4 groups and were treated (i. p. injection) with methylmercury chloride (2 mg MeHg/kg b.w.), sodium selenite (2 mg Se/kg b.w.), both MeHg and Se, or with saline solution for 8 weeks. Four animals from each group were randomly selected for pathological investigation. These animals were sacrificed by intracardiac perfusion. The dorsal root ganglia (DRG) were dissected out for both light and electron microscopic examination. Six animals from each group were injected with 14 C-leucine 3 hours prior to sacrifice. The brains (cerebrum and cerebellum) of these animals were removed, weighed, homogenized and subjected to liquid scintillation analysis. Morphological investigation revealed extensive degenerations of the dorsal root fibers and destructions of neuronal Nissl substances in animals treated with MeHg alone. In MeHg/Se-treated animals, no neuronal change was observed and the nerve fiber damage was also significantly reduced. Proliferative thickenings of the myelin membrane, however, were observed in some dorsal root fibers. These proliferative membrane thickenings tend to protrude into the axonal compartment of the nerve fibers causing compression and shrinkage of the axons. It is believed that this proliferative, membraneous whirling phenomenon may represent a simultaneous destruction/repair condition of the myelin sheaths. Biochemical investigation also demonstrated a significant suppression of amino acid uptake (protein synthesis) in the CNS under the toxic influence of MeHg. However, such suppression effect was largely blocked or alleviated by selenium. Therefore, we have provided both morphological and biochemical evidences on the protective effect of selenium on methylmercury neurotoxicity.

Effects of Methylmercuric chloride on sympathetic preganglionic nerves

The effect of methylmercuric chloride was investigated on the isolated cervical sympathetic nerve of the rabbit. All concentrations of methylmercury tested decreased both major spikes, S1 and S2, of the electrically evoked compound action potential. The blocking effect is temperature-dependent and not reversible on washing. Despite severe inhibition of the spike, conduction velocities of the fiber groups involved were not changed. Threefold increases in the Ca2+ concentration in the Locke's bathing solution before exposure to methylmercury markedly slowed the progress of inhibition of both major spikes of the fiber groups in the cervical sympathetic nerve. The results suggest that methylmercury may act by interfering with the normal function of Ca2+ in the nerve.

In vivo effect of methylmercury on protein synthesis in peripheral nervous tissues of the rat

The in vivo rates of protein synthesis in the peripheral nervous tissues of methylmercury-treated rats (10 mg/kg/day, for 7 days) have been estimated with improved methods by the injection of a large amount of [1-14C]valine of low specific activity. Protein synthesis activity in the dorsal root ganglia was inhibited to the extent of 60% of the control as early as day 5 and this continued to the symptomatic period (day 15) on which crossing of hind limbs, a typical sign of organomercurial poisoning, was observed in the animals. The sciatic nerves and dorsal roots increased protein synthesis by 56% at the symptomatic period. These increases in protein synthesis may be due to the stimulation of reactivity of Schwann's cells. On the contrary, the protein synthesis in the ventral roots showed a gradual decrease as the intoxication proceeded and decreased to 73% of the control at the symptomatic period, being similar to the case of brain. The double-labeling studies with sodium dodecyl sulfate/polyacrylamide gel electrophoresis exhibited that methylmercury inhibited the synthesis of the dorsal root ganglion proteins non-uniformly in various apparent molecular sizes, especially on day 10.

Effects of methyl mercury on protein synthesis in vitro

Methyl mercury has been shown to interact with protein synthesis in vivo and in vitro. In the present paper a brain postmitochondrial supernatant was used for studies in vitro. Inorganic mercury (Hg2+) was shown to be more potent inhibitor of protein synthesis than methyl mercury, puromycin or cycloheximide. The inhibitory effect of methyl mercury was potentiated by puromycin. It is thus possible that methyl mercury causes disintegration of polysomes in brain cells.

Effect of methylmercury and inorganic mercury on the nerve growth factor-induced neurite outgrowth in chick embryonic sensory ganglia

Methylmercury inhibited the fiber outgrowth in chick embryonic sensory ganglia in the presence of nerve growth factor (NGF) by 50% at 2 x 10(-6) M and completely depressed it at 5 x 10(-6) M. Whereas inorganic mercury completely inhibited the fiber outgrowth at 10(-4) M. The possible role of microtubules is discussed.

Peripheral neuropathy following intraneural injection of mercury compounds

Between 0.25-25 micrograms of an aqueous solution of either mercuric chloride or methyl mercuric acetate was injected directly into the sciatic nerve of 28 adult Wistar rats. The resultant pathological changes in the nerve were examined by teasing individual fibres and by light and electron microscopy. In most respects mercuric chloride was more toxic than methyl mercuric acetate. The large doses of mercuric chloride produced a hind limb paresis within 24 h but no clinical signs followed injection of organic mercury. The predominant effect of mercuric chloride was on Schwann cells which showed cytoplasmic swelling and necrosis, associated with extensive segmental demyelination. In contrast methyl mercuric acetate caused axonal degeneration in many of the large myelinated fibres but only minor alterations were observed in Schwann cells.

Changes in the activities of some digestive enzymes of Channa punctatus, exposed chronically to mercuric chloride

The effect of a chronic exposure to sublethal concentration of mercuric chloride (0.3 mg/l) on the activities of some enzymes in the digestive system of the teleost fish Channa punctatus was examined after 15 and 30 days of treatment. Glucose-6-phosphatase was significantly inhibited in the intestine and pyloric caeca. No marked alterations were observed in the activities of maltase and lactase except for elevation in maltase activity and inhibition in lactase activity in the intestine and pyloric caeca after 15 days of treatment. Three peptidases (aminotripeptidase, glycylglycine dipeptidase and glycyl-1-leucine dipeptidase) showed decreased activities in all parts of the digestive system. A decrease was also observed in the activity of lipase except for the stomach where inhibition after 15 days was insignificant. The results indicate that the activities of all the enzymes examined are inhibited in intestine and pyloric caeca and digestion of proteins and lipids may be more affected by mercury than the digestion of some carbohydrates.

Chronic mercuric chloride intoxication in digestive system of Channa punctatus

The effect of exposure to a sublethal concentration (0.30 mg/l) of mercuric chloride on the activities of alkaline phosphatase, acid phosphatase, amylase, trypsin, and pepsin has been examined at intervals of 7, 15, and 30 d in the digestive system of a teleost fish, Channa punctatus. Inhibition of the activities of all these enzymes was noted after the first week of treatment. Treatment of the fish for 15 d resulted in marked increases in the activities of all the enzymes. A slight fall in enzyme activity was recorded after 30 d, but the overall activity was higher than in control fish.

Hereditary "draft lamb" disease of Border Leicester sheep: the ultrastructural pathology of the skeletal muscles

A hereditary and congenital disease of Border Leicester lambs clinically indistinguishable from the "draft lambs" of Innes et al. (1949) but with no cerebellar lesion, is defined in terms of the ultrastructural pathology of the muscle, intramuscular nerves and neuromuscular junctions. The pathognomonic feature of these congenitally affected lambs was the occurrence of large intrafascicular muscle cells. On the basis of the mitochondrial content and Z disc thickness these were considered to be one of two subtypes of type I cell. There was evidence of muscle cell growth in all cell types between birth and 3 mth of age, particularly in the form of myofibrillar splitting. The myofibrils of the large cells were very large and incompletely split. The mitochondria in these cells were of two kinds, roughly rounded and tubular. The latter showed conspicuous branching. A few muscle cells appeared immature but no significant degenerative changes were detected. In some neck muscles intramuscular nerves were seen and in 28 per cent. of the axons lamellated electron-dense bodies occurred. The periodicity of the lamellations was 40--60 nm. No other defects were observed in their vicinity but some axon terminals contained distended mitochondria which themselves contained membranous bodies. The axon terminals also contained morphologically normal synaptic vesicles and there was evidence that these were transmitting their transmitter substance. The lesions in hereditary "draft" lamb disease of Border Leicester sheep may result from functional defects of the nervous system which have a delaying effect on the development of some muscle cells, whilst others capable of normal growth hypertrophy. Alternatively, there may be a primary defect in the muscle cells themselves either alone or together with an unrelated neuropathy.

An ultrastructural study of chronic cadmium chloride-induced neuropathy

After the long-term exposure to cadmium chloride in drinking water, the Wistar rats developed peripheral polyneuropathy. The main lesion was of myelin degeneration. Ultrastructural examination of the roots and sciatic nerves revealed segmental demyelination beginning from the node of Ranvier. There was the active autophagocytosis of Schwann cells which contained a number of myelin remnants and dense bodies. There was, on the other hand, the evidence of remyelination with toxic damage, in which the thinner myelin sheaths and abnormal myelinations were observed with increase of Schwann cells containing rich ribosomes. Axoplasmic changes were minimal, but consisted of accumulation of glycogen particles which very often produced glycogenosomes in characteristic appearance with axoplasmal dysfunction.

The effects of mercurial poisoning on the vestibular system

The sensory epithelium with adjacent nerve endings and the secretory epithelium may both become damaged following mercury chloride intoxication. Peripheral myelinated nerve fibres in the crista ampullaris showed signs of degeneration following chronic poisonins. Ultrastructural alterations of the vestibular hair cells initially occurred in animals free from clinical signs of intoxication. The sensory epithelium and occasionally the secretory cells were affected before signs of ultrastructural damage could be detected in the myelinated nerves.

Pathological changes of human sural nerves in Minamato disease (methylmercury poisoning). Light and electron microscopic studies

Biopsy of the sural nerve was performed on six patients with relatively mild Minamata disease of 10-year or longer duration. All of the six patients presented characteristic pathologic changes. Light microscopy disclosed the formation of irregularly shaped myelin sheaths and fine axons, an increase in them, which is suggestive of incomplete regeneration, cicatrization following the loss of nerve fibers, increase in Schwann's nuclei, and formation of Büngner's bands. Electron microscopy revealed incomplete myelinated fibers and ultrafine unmyelinated fibers associated with incomplete regeneration, formation of regeneration units, and collagen increase. The laminar encapsulation with the processes of Schwann's cells were often observed in ultrafine fibers. In view of the fact that small quantities of mercury-contaminated fishes are still being caught in the Minamata district, myelin degeneration, glycogen deposits and appearance of dense bodies in axons, and vesiculation and fragmentation of endoplasmic reticulum were observed as degenerative changes due to the effects of mercury accumulation.

Toxicity of methylmercury chloride in rats. III. Long-term toxicity study

Four groups, each of 25 male and 25 female weanling rats, were given dietary levels of 0, 0.1, 0.5 and 2.5 ppm MeHgCl for 2 years. Observations were made on behaviour, growth, food intake, haematology, serum enzymes, urinalysis, microsomal liver enzymes, organ weights and histology with special reference to the nervous system, histochemistry of the kidneys and cerebellum and on tissue Hg concentrations. Significant findings included a slight growth reduction in females at 2.5 ppm, increased relative kidney weight at 2.5 ppm and histochemical changes in kidney enzymes at 2.5 ppm. No effect was seen on the nature or incidence of pathological lesions or tumours at any level. From the results obtained in the short-term, reproduction and long-term studies, the no-toxic effect level for rats appears to be between 0.1 and 0.5 ppm MeHgCl in the diet. Exposure of the Dutch population does not appear to present a health hazard at the moment because the mean intake of total Hg is still far below the intake deemed to be safe.

Autoradiographic localisation of 3H-uridine in spinal ganglion neurones of the rat and the effects of methyl mercury poisoning

The uptake of 3H-uridine into rat spinal ganglion neurones has been followed by autoradiography for up to 48 h after its intravenous injection. Labelling of nucleoli and of nuclei reached a peak within 1 h and then declined. Nuclear labelling returned to background levels by 24 h, but nucleolar labelling was still significant after 48 h. Animals dosed with methyl mercury chloride (7.5 mg/kg daily) showed no change in labelling rate in nucleolus or nucleus after 1, 2, or 4 doses. After 8 doses there was severe reduction in labelling in both nucleolus and nucleus; this amount causes extensive loss of axons, loss of some cell bodies and a marked reduction in amino acid incorporation into proteins. On recovery after a further 8 days, labelling levels returned to normal. It is concluded that at the time when loss of ribosomes occurs from the cytoplasm methyl mercury is more likely to be directly disturbing ribosomal structure than RNA synthesis, for methyl mercury causes marked changes in ribosomal organisation after 4 doses, but disturbances to RNA synthesis do not occur until the 8th dose.