A Golgi and electron-microscopic study of cerebellum in methylmercury-poisoned neonatal mice

Developmental exposure to methylmercury and resultant muscle mercury accumulation and adult motor deficits in mice

Developmental methylmercury (MeHg) exposure can have lasting consequences on neural development and motor function across the lifespan. Recent evidence for MeHg targeting of myogenic pathways has drawn attention to the possibility that developing skeletal muscle plays a role in the motor deficits stemming from early life MeHg exposure. In this study we examined a potential role for muscle in influencing MeHg developmental toxicity in offspring of female mice exposed to MeHg via drinking water. Dams had access to 0, 0.5 or 5.0 ppm MeHg chloride in drinking water from two weeks prior to mating through weaning. Blood, brain and muscle tissue was harvested from dams at weaning and pups at postnatal days (PND) 6, 21 and 60 for analysis of total Hg. Muscle tissue sections were examined with histological stains. Behavioral testing of offspring was conducted at PND 60 and included locomotor activity, inverted screen, grip strength and rotarod tests to assess motor function. Total Hg (tHg) levels in dam muscles at weaning were 1.7-3-fold higher than Hg levels in blood or brain. In PND6 male and female pups, muscle and brain tHg levels were 2 to 4-fold higher than blood tHg. Brain tHg levels decreased more rapidly than muscle tHg levels between PND 6 and 21. Premised on modeling of growth dilution, brain tissue demonstrated an elimination of tHg while muscle tissue exhibited a net uptake of tHg between PND 6 and 21. Despite overall elevated Hg levels in developing muscle, no gross morphological or cytological phenotypes were observed in muscle at PND 60. At the higher MeHg dose, grip strength was reduced in both females and males at PND 60, whereas only male specific deficits were observed in locomotor activity and inverted screen tests with marginally significant deficits on rotarod. These findings highlight a potential role for developing skeletal muscle in mediating the neuromuscular insult of early life MeHg exposure.

Neurotoxicological effects of low-dose methylmercury and mercuric chloride in developing offspring mice

Mercury is a well-known toxic metal and potently induces severe neurotoxicological effects, especially in infants and children. The purpose of this study was to explore the underlying mechanisms of neurotoxic effects of mercurial compounds on the different stages of developing mice. Low-doses (the probability of human exposure in mercury-contaminated areas) of methylmercury (MeHg) (M, 0.02mg/kg/day) and mercury chloride (HgCl(2)) (H, 0.5mg/kg/day) were administered to mice of the following groups: (1) treatment with distilled water for 7 consecutive weeks after weaning (control-vehicle (CV)); exposure to mercurial compounds at different stages; (2) for 7 consecutive weeks after weaning (control-MeHg (CM) and control-HgCl(2) (CH)); (3) only during perinatal and weaning stages (MeHg-vehicle (MV) and HgCl-vehicle (HV)); and (4) in all experimental stages (MeHg-MeHg (MM) and HgCl(2)-HgCl(2) (HH)). Results revealed the neurobehavioral defects (increased locomotor activities, motor equilibrium impairment, and auditory dysfunction) that correlated with increasing Hg accumulation in CM and CH groups. However, it revealed a decrease and an increase in locomotor activities in MV and HV groups, respectively; these became more severe in MM and HH groups than in MV and HV groups. Motor equilibrium performance in MV and HV groups remained normal, while that in MM and HH groups was decreased. The most severe auditory defects (altered auditory brainstem response, ABR test) found in MM and HH groups than those in the respective CM and CH, MV and HV, including absolute wave III delays and interwave I-III latencies, which suggested that the irreversible auditory dysfunction caused by mercurial compounds. Furthermore, the alteration of lipid peroxidation (LPO), Na(+)/K(+)-ATPase activities, and nitric oxide (NO(x)) in the brain tissues contributed to the observed neurobehavioral dysfunction and hearing impairment. These findings provide evidence that fetuses were much more susceptible to the effects of mercurial compounds with regard to inducing severely neurotoxicological injuries as that found in human beings. The signaling of ROS/Na(+)-K(+)-ATPase/NO(x) plays a crucial role in the underlying mechanism for mercurial compound-induced toxic effects in offspring.

Neurodevelopmental toxicity of methylmercury: Laboratory animal data and their contribution to human risk assessment

Methylmercury (MeHg) is one of the most significant public health hazards. The clinical findings in the victims of the Japanese and Iraqi outbreaks have disclosed the pronounced susceptibility of the developing brain to MeHg poisoning. This notion has triggered worldwide scientific attention toward the long-term consequences of prenatal exposure on child development in communities with chronic low level dietary exposure. MeHg neurodevelopmental effects have been extensively investigated in laboratory animals under well-controlled exposure conditions. This article provides an updated overview of the main neuromorphological and neurobehavioral changes reported in non-human primates and rodents following developmental exposure to MeHg. Different aspects of MeHg's effects on the immature organism are reported, with particular reference to the delayed onset of symptoms and the persistency of central nervous system (CNS) injury/dysfunction. Particular attention is paid to the comparative toxicity assessment across species, and to the degree of concordance/discordance between human and animal data. The contribution of animal studies to define the role of potential effect modifiers and variables on MeHg dose-response relationships is also addressed. The ultimate goal is to discuss the relevance of laboratory animal results, as a complementary tool to human data, with regard to the human risk assessment process.

Developmental neurotoxicity testing in vitro: models for assessing chemical effects on neurite outgrowth

In vitro models may be useful for the rapid toxicological screening of large numbers of chemicals for their potential to produce toxicity. Such screening could facilitate prioritization of resources needed for in vivo toxicity testing towards those chemicals most likely to result in adverse health effects. Cell cultures derived from nervous system tissue have proven to be powerful tools for elucidating cellular and molecular mechanisms of nervous system development and function, and have been used to understand the mechanism of action of neurotoxic chemicals. Recently, it has been suggested that in vitro models could be used to screen for chemical effects on critical cellular events of neurodevelopment, including differentiation and neurite growth. This review examines the use of neuronal cell cultures as an in vitro model of neurite outgrowth. Examples of the cell culture systems that are commonly used to examine the effects of chemicals on neurite outgrowth are provided, along with a description of the methods used to quantify this neurodevelopmental process in vitro. Issues relating to the relevance of the methods and models currently used to assess neurite outgrowth are discussed in the context of hazard identification and chemical screening. To demonstrate the utility of in vitro models of neurite outgrowth for the evaluation of large numbers of chemicals, efforts should be made to: (1) develop a set of reference chemicals that can be used as positive and negative controls for comparing neurite outgrowth between model systems, (2) focus on cell cultures of human origin, with emphasis on the emerging area of neural progenitor cells, and (3) use high-throughput methods to quantify endpoints of neurite outgrowth.

Cerebellum cholinergic muscarinic receptor (subtype-2 and -3) and cytoarchitecture after developmental exposure to methylmercury: an immunohistochemical study in rat

The developing central nervous system (CNS) is a target of the environmental toxicant methylmercury (MeHg), and the cerebellum seems the most susceptible tissue in response to this neurotoxicant. The cholinergic system is essential for brain development, acting as a modulator of neuronal proliferation, migration and differentiation processes; its muscarinic receptors (MRs) play pivotal roles in regulating important basic physiologic functions. By immunohistochemistry, we investigated the effects of perinatal (GD7-PD21) MeHg (0.5 mg/kg bw/day in drinking water) administration on cerebellum of mature (PD36) and immature (PD21) rats, evaluating the: (i) M2- and M3-MR expression; (ii) presence of gliosis; (iii) cytoarchitecture alterations. Regarding to M2-MRs, we showed that: at PD21, MeHg-treated animals did not display any differences compared to controls, while, at PD36 there was a significant increase of M2-immunopositive Bergmann cells in the molecular layer (ML), suggesting a MeHg-related cytotoxic effect. Similarly to M2-MRs, at PD21 the M3-MRs were not affected by MeHg, while, at PD36 a lacking immunoreactivity of the granular layer (IGL) was observed after MeHg treatment. In MeHg-treated rats, at both developmental points, we showed reactive gliosis, e.g. a significant increase in Bergmann glia of the ML and astrocytes of the IGL, identified by their expression of glial fibrillar acidic protein. No MeHg-related effects on Purkinje cells were detected neither at weaning nor at puberty. These findings suggest: (i) a delayed MeHg exposure-related effect on M2- and M3-MRs, (ii) an overt MeHg-related cytotoxic effect on cerebellar oligodendroglia, e.g. reactive gliosis, (iii) a selective vulnerability of granule cells and Purkinje neurons to MeHg, with the latter that remain unharmed.

Neurobehavioural and molecular changes induced by methylmercury exposure during development

There is an increasing body of evidence on the possible environmental influence on neurodevelopmental and neurodegenerative disorders. Both experimental and epidemiological studies have demonstrated the distinctive susceptibility of the developing brain to environmental factors such as lead, mercury and polychlorinated biphenyls at levels of exposure that have no detectable effects in adults. Methylmercury (MeHg) has long been known to affect neurodevelopment in both humans and experimental animals. Neurobehavioural effects reported include altered motoric function and memory and learning disabilities. In addition, there is evidence from recent experimental neurodevelopmental studies that MeHg can induce depression-like behaviour. Several mechanisms have been suggested from in vivo- and in vitro-studies, such as effects on neurotransmitter systems, induction of oxidative stress and disruption of microtubules and intracellular calcium homeostasis. Recent in vitro data show that very low levels of MeHg can inhibit neuronal differentiation of neural stem cells. This review summarises what is currently known about the neurodevelopmental effects of MeHg and consider the strength of different experimental approaches to study the effects of environmentally relevant exposure in vivo and in vitro.

Motor function following developmental exposure to PCBS and/or MEHG

Recent studies raise concern for combined exposure to polychlorinated biphenyls (PCBs) and methylmercury (MeHg), two environmental contaminants that are found in fish and seafood. Past accidental poisonings in humans show that exposure to high levels of either contaminant is associated with motor impairments, including alterations in cerebellar functions such as balance and coordination. Epidemiological studies of lower level exposures suggest some neuromotor impairment in exposed children, but the majority of these studies have focused on cognitive endpoints rather than examining a full-range of motor function. In particular, the cerebellum could be a sensitive target for combined PCB and MeHg toxicity. MeHg exposure during development damages the cerebellum along with cortical areas, and PCBs may also cause cerebellar damage via thyroid hormone disruption during development. In addition, in vitro studies report interactive effects of PCBs and MeHg on ryanodine-sensitive calcium signaling. Ryanodine receptors are found especially within the cerebellum, and alterations in calcium signaling within the cerebellum could impair long-term depression and subsequent motor learning. This article reviews the motor impairments reported in humans and laboratory animals following exposure to PCBs and/or MeHg during development. There is need for a better understanding of the interactive effects of PCBs and MeHg, especially in regard to motor function.

Motor impairment induced by oral exposure to methylmercury in adult mice

The effects of oral exposure to methylmercury chloride (MeHg) on locomotor control and activity in adult mice were investigated in the present study. MeHg was diluted in drinking water (0, 20 and 40mg/L - as methylmercury chloride) and locomotion (spontaneous locomotor activity) and motor impairment tests (beam walking, footprint and clasping) were performed at 7, 14 and 21 days after the beginning of the treatment. MeHg exposure caused a significant decrease in spontaneous locomotor activity and this effect was dose- and time-dependent. Significant dose- and duration-dependent increases in beam walking latency were observed following chronic MeHg exposure. Furthermore, dose- and duration-dependent locomotor deficits on footprint coordination were also observed. Taken together, these results show that MeHg-induced impairment on locomotor activity is not limited to exposures that take place during neural development. We discuss the possible relationship between our findings and the similar clinical signs observed in adult humans exposed to MeHg.

Neurobehavioral changes in mice chronically exposed to methylmercury during fetal and early postnatal development

Pregnant C57BL/6 mice were chronically treated with 0, 4, 6, or 8 ppm of methylmercury chloride (MeHg) in drinking water during fetal and early postnatal development. Four behavioral functions were analyzed in female and male offspring between the age of 6 and 12 weeks: motor coordination learning on the rotarod; training to spatial alternation in the standard T maze followed by a working memory test with delays; spontaneous locomotion and rearings in the open field; reference and working memory assessment in the modified T maze [Behav. Neurosci. 102 (1988) 635]. Chronic perinatal treatment with MeHg resulted in moderate brain levels of mercury near birth which rapidly decreased during nursing. MeHg exerted an effect on the performance of females, but not of males, on two of the four measurements. All treated females exhibited less locomotion than control mice when the open field was new, but not in the following four sessions when the environment was becoming increasingly familiar. Working memory was impaired in females treated with 6 and 8 ppm of MeHg in the modified T maze, but not on the test with delays in the standard T maze. Taken together, these results show that chronic exposure to MeHg during fetal and postnatal development had sex-dependent effects on horizontal exploration and on working memory in the modified T maze, and no effects on motor coordination learning and reference memory.

Developmental methylmercury administration alters cerebellar PSA-NCAM expression and Golgi sialyltransferase activity

Brain dysmorphogenesis and persistent psychomotor disturbances are hallmarks of developmental methylmercury (MeHg) exposure, but the molecular mechanisms underlying these effects are poorly understood. Targets of developmental MeHg exposure include neural cell adhesion molecules (NCAMs), sialoglycoconjugate molecules whose proper temporal and spatial expression is important at all stages of neurodevelopment and especially during synaptic structuring. To investigate the effects of MeHg on the temporal expression of NCAM during development, rat pups were dosed with 7.0 mg/kg MeHgCl (s.c.) on alternate days from postnatal days (PNDs) 3-13 and killed on PNDs 15, 30 and 60. Brain MeHg concentrations were determined in a subset of litters injected with CH(3)203Hg. Expression of NCAM180 protein and of NCAM180 polysialylation was examined in whole cerebellum homogenates, cerebellar synaptosomes and isolated cerebellar growth cones by Western blotting and immunocytochemical staining. NCAM sialyltransferase activity was assayed in preparations of purified Golgi apparatus from the cerebelli of rats treated in vivo, or following in vitro incubation with 0, 1, 2.5, or 7.5 microM MeHg for 2 h. At PND15, no change in NCAM180 protein expression was observed in any cerebellar preparations, but decreased polysialylation of NCAM180 was observed in cerebellar whole homogenates, synaptosomes and isolated growth cones. At PND30, both NCAM180 protein expression and NCAM180 polysialylation were elevated in whole homogenate preparations but not in synaptosomes. NCAM180 expression in MeHg-treated rats was similar to controls at PND60, 47 days after the last methylmercury administration. In vivo studies of cerebellar Golgi sialyltransferase activity revealed significant reductions in PND15 MeHg-treated rats as compared to controls, but no changes in sialyltransferase activity in PND30 and PND60 animals. In vitro experiments revealed decreasing sensitivity of cerebellar sialyltransferases to MeHg as the developmental age of the rat increased. Toxic perturbation of the developmentally-regulated expression of polysialylated NCAM during brain formation may disturb the stereotypic formation of neuronal contacts and could contribute to the behavioral and morphological disturbances observed following MeHg poisoning.

Gestational exposure to methylmercury alters the developmental pattern of trk-like immunoreactivity in the rat brain and results in cortical dysmorphology

Nerve growth factor signal transduction mediated through the trk receptor has been implicated in neuronal growth, differentiation, and survival. In this study, we examined the effects of gestational exposure to the developmental neurotoxicant methylmercury (CH3Hg) on the ontogeny of trk-immunoreactivity (IR). Long-Evans dams were dosed on gestational days 6-15 (p.o.) with 0, 1, or 2 mg/kg CH3Hg dissolved in saline. Pups were sacrificed and perfused with buffered paraformaldehyde on postnatal days (PND) 1, 4, 10, 21 and 85. The brains were sectioned sagitally, Nissl-stained or stained immunohistochemically for trk receptors or glial fibrillary acidic protein (GFAP), and examined throughout the medial to lateral extent of the brain. The greatest density of IR in neural cell bodies was seen in the olfactory bulb, hippocampus, cerebral, and cerebellar cortex, striatum, septum, nucleus basalis, inferior colliculus, pons, and brain stem nuclei. trk IR was not limited to nerve cell bodies, with prominent axonal and dendritic staining in the brainstem, neocortex, hippocampus, cerebellum, and olfactory tract. The regional pattern of trk IR varied in an age-dependent manner. In controls, trk-like IR appeared to peak in most regions between PND4-10 and decreased dramatically after PND21. This age-related difference in trk IR was supported by western blot analysis of PND10 and adult neocortex. This reduced and more adult-like pattern of trk IR was apparent on PND21 with some persistent trk-like IR in the olfactory bulb, hippocampus, neocortex, cerebellum and basal forebrain. In contrast to the normal regional patterns of trk IR, CH3Hg produced a dose-related decrease in trk-like IR in the absence of overt maternal toxicity or neonatal toxicity. CH3Hg-induced decreases in trk-like IR were especially apparent during the early postnatal period when trk IR was the greatest. The effects of CH3Hg exposure were restricted regionally, with the largest decrease in trk-like IR apparent in cortical regions, basal forebrain nuclei, and brain stem nuclei. Subsequent to the effects of CH3Hg on cortical trk-like IR were alterations in the development of cortical laminae on PND10 and 21 of neocortex. These alterations were characterized by quantifiable decreases in cell density, cell size and the widths of the layers of posterior neocortex. Not all of the CH3Hg-induced effects were characterized by decreased trk-like IR. Robust increases in trk IR in glial cells in the corpus callosum and brain stem were observed coincident with increased GFAP IR in cells of similar morphology. The present results localize the cellular and regional ontogeny of trk and suggest that developmental exposure to CH3Hg alters the normal ontogeny of this trophic factor receptor which may be associated with the developmental neurotoxicity of this chemical.

The early effects of methylmercury on the developing rat brain

The effects of organic mercury compounds on the development of the brain are well known since the exposure of people at a large scale to methylmercury in the Minamata Bay area and in Iraq. The neuropathological examination of the brains of children prenatally exposed revealed dysplasia of the cerebral and cerebellar cortex, neuronal ectopia and several other developmental disturbances. In this experimental study we examined developmental mechanisms involved in methylmercury-induced cerebral anomalies. By examining the fetuses soon after treatment we concentrated in the initial effects of the treatment. The pregnant rats were given 10 mg/kg methylmercury chloride i.p. on day 18. Already at 2 h after administration mitochondrial degeneration occurred in the endothelium of the cerebral capillaries. Subsequently hemorrhages developed interfering with the cellular arrangement in the ventricular zone, with neuronal migration in the intermediate zone and with the development of the cortical cytoarchitecture. Macrophages and cavities appeared in the hemorrhagic areas. It is suggested that the abnormalities seen in the experiments can be considered as the initial methylmercury-induced effects which, in combination with various other toxic effects, ultimately result in the anomalies that have been observed in the brains of children prenatally exposed to methylmercury.

Methylmercury developmental neurotoxicity: a comparison of effects in humans and animals

A qualitative and quantitative comparison of the neuropathological and neurobehavioral effects of early methylmercury (MeHg) exposure is presented. The focus of the qualitative comparison is the examination of how specific end-points (and categories of behavioral functions) compare across species. The focus of the quantitative comparison is the investigation of the relationship between MeHg exposure, target-organ dose and effects in humans and animals. The results of the comparisons are discussed in the context of the adequacy of the proposed EPA neurotoxicity battery to characterize the risk of MeHg to humans. The comparisons reveal several qualitative and quantitative similarities in the neuropathological effects of MeHg on humans and animals at high levels of exposure. Reports of neuropathological effects at lower levels are available for animals only, precluding any comparison. At high levels of exposure, specific neurobehavioral end-points affected across species are also similar. Effects at lower levels of exposure are similar if categories of neurobehavioral functioning are compared. Changes in the EPA test battery consistent with the results of the comparisons are discussed.

Developmental neurotoxicology

Some injuries to the developing nervous system can be detected with traditional evaluation for morphologic pathology, but many early injuries differ in character from those that are produced later in life. Such injuries arise from interference with developmental processes, rather than destruction of tissue. For example, an injury which kills neurons in the mature CNS leads to gliosis and a reduction in neuronal density, but a reduction in the number of neurons produced during development is not likely to lead to gliosis, and typically affects tissue volume rather than cell density. Some effects of developmental insults, such as misplaced and misoriented neurons, are never seen after adult injury. Functional effects reflect the role of the CNS in physiological regulation as well as in behavior. To evaluate CNS for developmental injury, it is necessary to know something about the structural and functional outcomes already recognized to result from teratogens and how these effects are related to time of exposure and time of testing.

Effects of methylmercury on the morphogenesis of the rat cerebellum

Developing rat cerebellums were examined following continuous methylmercury exposure via maternal drinking water at 12.5 ppm during gestation and the suckling period. The continuous exposure resulted in reductions of the total cerebellar cell population and higher mercury tissue burdens than previous acute-dose studies. Cell necrosis was not evident, but rather alterations in the pattern of mitotic figures were observed. A decreased number of cells in the methylmercury exposed cerebellums was associated with an increased number of mitotic figures in the early stages of mitosis and a decrease in the number in the middle and late stages. These in vivo exposure observations are consistent with in vitro cell cycle studies in which the cells were found to have accumulated in G2 and early M phases. Impaired cell proliferation is suggested to be a major mechanism of developmental neurotoxicity following continuous low-dose exposure to methylmercury.

Changes in the molecular structure of mouse fetal astrocyte nucleosomes produced in vitro by methylmercuric chloride

The fluorescent probe N-(3-pyrene)maleimide, which specifically labels the cysteine residues of histone H3 within the nucleosome, was used to monitor changes in the nucleosomal structure of mouse fetal astrocytes exposed to varying concentrations of methylmercuric chloride. Methylmercuric chloride treatment (10 microM) for 6 hr produced a significant decrease in the degree of fluorescence of the probe. The decrease was much smaller following a 4-hr incubation period. These results correlate with recent observations showing that significant changes in the thymidine labeling index occur following 4-6 hr of exposure to methylmercury (MeHg). It is hypothesized that MeHg enters the cells during the growth phase and attaches to the protein moiety of the nucleosome in or near the cysteine groups of histone H3, thus diminishing the binding capacity of the fluorescent probe. Addition of a detergent (sodium dodecyl sulfate) resulted in only a small increase in the degree of fluorescence of the treated nucleosomes as compared to controls, showing that the interaction of MeHg with the nuclear proteins was not dissociated by detergent. In view of the strong interaction between DNA and the histone dimer H3-H4 and the potential importance of the latter in gene regulation, these results suggest an additional means by which MeHg may exert its toxic effects.

Methylmercury poisoning of the developing nervous system in the rat: decreased activity of glutamic acid decarboxylase in cerebral cortex and neostriatum

The specific activities of glutamic acid decarboxylase (GAD) and choline acetyltransferase (ChAT) were measured in 6 regions of the central nervous system in young rats, following chronic postnatal administration of methylmercuric chloride. These rats exhibited signs of neurological impairment which included visual deficits, ataxia, spasticity and myoclonus. At the onset of neurological impairment, there was a significant reduction in GAD activity in the occipital cortex (43%), frontal cortex (37%) and caudate-putamen (42%). Preceding the onset of neurological impairment, diminished GAD activity was detected only in the occipital cortex. In the cerebellum, thalamus and spinal cord, GAD activities were normal throughout the experiment. No significant differences in ChAT activity were detected in any of the 6 regions. These results are consistent with a preferential involvement of GABAergic neurons in methylmercury-induced lesions of the cerebral cortex and neostriatum.

Review of the health effects of methylmercury

The study critically reviews recent data relating to the health effects of methylmercury in man and the attendant dose-response relationships. New data obtained from animal studies, including pre-and postnatal exposure, are also examined. The consumption of fish and fish produce represents the major source of methylmercury exposure in the general population. Reported mercury concentrations in fish throughout the world are examined, particularly in the Mediterranean Sea. Here there is limited knowledge of methylmercury intake in critically exposed populations such as fishermen, employees of the fish industries and their families. The measurement of mercury in hair is now regarded as the most useful indicator of exposure but more experimental data are still required to increase the value of this index. The threshold levels of methylmercury in blood, hair and for dietary intake, as estimated by the World Health Organization, have been largely endorsed. However, new information from Japan and Canada suggests the existence of a latency period for some effects, so that the frequency or probability of their occurrence is inversely related to the duration of exposure. Incorporation of such findings would therefore lead to the designation of lower threshold values than are presently recognized. Pregnant women and the fetus have been identified as groups that are at special risk. The fetal blood mercury level is up to twice that of the mother and the sensitivity of both mother and fetus may be higher than in non-pregnant adults. This should be taken into account when assigning protective threshold concentrations.

The comparative effects of methylmercuric chloride and mercuric chloride upon DNA synthesis in mouse fetal astrocytes in vitro

The relative effects of direct exposure to methylmercuric chloride (MMC) and mercuric chloride (MC) upon [3H]thymidine incorporation were determined using cultured mouse fetal astrocytes. The labeling indices of cells exposed to MMC were significantly lower than those of cells exposed to equimolar concentrations of MC. These results indicate that on a mole-for-mole basis, MMC is much more deleterious upon DNA synthesis of mouse fetal astrocytes than those of MC.

Alterations in the postnatal development of the cerebellar cortex due to zinc deficiency. II. Impaired maturation of Purkinje cells

Zinc deficiency during the first 3 postnatal weeks retarded the maturation of Purkinje cells. The dendrites of the Purkinje cells of 21-day-old zinc-deficient (ZD) rats were reduced in size and had fewer branches. Somatic processes were found in 24% of the Purkinje cells of ZD animals. Only 3% of the Purkinje cells of normal animals had somatic processes. A basal polysomal mass in the Purkinje cells of 21-day-old ZD rats indicated that zinc deficiency impaired the cytoplasmic maturation of Purkinje cells. The development of the glial envestment of the dendrites and the maturation of climbing fibers also were retarded. Pair-fed controls were studied to control for the effects of inanition in the ZD dams. In the pups of pair-fed dams, undernutrition slightly impaired the growth of the dendrites but produced few qualitative changes in the maturation of the soma and climbing fibers. Somatic processes were found on 10% of the Purkinje cells of pair-fed animals. Thus, the findings in the ZD animals were not only caused by the decreased maternal food consumption but by zinc deficiency. The retarded maturation of Purkinje cells was related to the altered metabolism of Purkinje cells and to effects secondary to decreased numbers of parallel fibers.

Methylmercury poisoning of the developing nervous system: morphological changes in neuronal mitochondria

Neonatal rats received s.c. injections of methylmercuric chloride (MeHg) in physiological saline (1.5 mg Hg/kg b.wt.) at 48-h intervals from postnatal day 2 to day 50. Littermate controls were injected with an equivalent volume of saline. All animals were perfused on day 51 and blocks of cerebral cortex were prepared for electron microscopy. Ultrastructural changes in mitochondria were evident in the dendrites, axons and presynaptic terminals of cortical neurones in the MeHg-treated animals. Many mitochondria were condensed with an increased electron density of the inner matrix. Some profiles exhibited regressive alterations, including a disruption of cristae and the inner membrane with an accumulation of electron-opaque material in the matrix. Membranous whorls were found in association with the most degenerate mitochondria. A morphometric analysis of mitochondrial profiles in the neuropil of layer I revealed a 24% decrease in average profile area and a 16% increase in the number of profiles per micrograph in the MeHg-treated animals. These pathological changes in mitochondrial ultrastructure are consistent with an inhibition of mitochondrial respiration. The observed increase in the number of mitochondrial profiles may reflect a compensation by cortical neurons for the reduced efficiency of aerobic metabolism in the individual organelle.