J. COMP.
PATH.
1975.
VOL.
85.
MICROAUTORADIOGRAPHIC STUDY DISTRIBUTION OF METHYLMERCURIC’20” CHLORIDE IN THE MALLARD DUCK
OF
THE
(AJVAS
PLATYRHYJKHOS) 6)
Dejartment
of PatholoQ,
D. A.
PASS*
Univeui&
of Guelph: Guelph,
Ontario,
Canada
INTRODUCTION
The distribution of mercury within an organ has been studied by dissection of and by histoanatomical substructures of the organ, by autoradiography chemical techniques (Clarkson, 1972). Several studies on the autoradiographic distribution of methyl mercurials have been performed in mammals (Ostlund, 1969), but the 1969; Nordberg, Berlin and Grant, 1971) and birds (Backstrom, sub-gross distribution only has been reported. The distribution of methyl mercury at a cellular level has been examined by a histochemical method (Takeuchi, 1968 ; Backstrom, 1969) and the same method has been used to study the subcellular localization of methyl mercury (Chang and Hartmann, 1972a). The histochemical methods in use are said to lack sensitivity and are subject to interference from other metals (Clarkson, 1972). A microautoradiographic technique, using CH,203HgC1, was employed in the present study in an attempt to gain information on the cellular distribution of methyl mercury in the Mallard duck. Particular attention was paid to the vascular and nervous systems as they have been found to be primarily involved in subacute and chronic methyl mercury poisoning in the Mallard duck (Pass, Little and Karstad, 1975). MATERIALS
AND
METHODS
Six adult male Mallard ducks (numbers 1 to 6) were used. The experiment was carried out in two parts. Birds 1, 2 and 3 were dosed orally with 0.228 mCi CHs203 HgClt and birds 4, 5 and 6 with 0.684 mCi CH, 203HgC1 via a flexible pipette that was inserted down the oesophagus to the proventriculus. The specific activity of CH3203HgC1 given to ducks 1 and 4 was 4.6 mCi/mg. Hg. and that given to birds 2, 3,5 and 6 was 13.5 mCi/mg. Hg. Two days following administration the birds were anaesthetized with sodium pentobarbitone and exsanguinated. A routine necropsy was performed and the following tissues were fixed in 10 per cent. formalin and embedded in paraffin : cervical and lumbar spinal cord, dorsal root ganglia, brachial, sciatic and vagus nerves, pectoralis major muscle, lung, liver, kidney, spleen, testes, adrenals, oesophagus, proventriculus, gizzard, duodenum and pancreas, ileum and skin. Eight transverse from the posterior rhombencephalon to the anterior sections of the brain, telencephalon, were processed in the same way. * Present
address:
Bcndigo, Victoria t ICN, Irvine, A
Regional Veterinary 3550, Australia. California U.S.A.
Laboratory
Bendigo,
Department
of Agriculture,
Box
123,
2
D. A. PASS
A section (6 pm) from each of the above was stained with HE. Two unstained sections (6 pm) were dipped in a 50 per cent. solution of NTBP nuclear tract emulsiont and distilled water (M. A. Fernando-personal communicationS), dried and stored in light-tight plastic boxes at 4 “C. for 7 days. The slides were developed for 4 min. in Kodak D1gR* at room temperature and fixed in Kodak Fixera* for 5 min. The sections were stained with HE. and mounted using DPX$. RESULTS
In all birds there was a diffuse scatter of silver grains over sections of the liver, kidney, spleen, testes, adrenal gland, lung, heart, oesophagus, proventriculus, small intestine, pancreas, skeletal muscle and peripheral nerves. In all cases, apart from the liver, kidney and oesophageal mucosa, the concentration of grains, which was only slightly greater than the background concentration, was most apparent over the liver, oesophageal mucosa and the kidney in that order. Localization to particular cell types in these tissues was not determined.
Fig.
1. Microautoradiograph of ventral horn of lumbar spinal cord of Mallard duck (No. 4) given CHazo3HgC1. Note the concentration of silver grains over astrocytes (arrows). Silver grains are mainly present over the cytoplasm. HE. x 504.
In the brain and spinal cord of the 6 birds the overall concentration of grains, in general, was no greater than that of the background. There was, however, specific localization of grains over certain cell types in these tissues. Throughout the central nervous system there were marked concentrations of * Eastman Kodak 1 M. A. Fernando, 5 BDH Chemicals,
Company, Rochester, New Department of Pathology, Toronto, Canada.
York, U.S.A. University of Guelph,
Guelph,
Canada.
METHYL
MERCURIC
CHLORIDE
IN DUCK
: AUTORADIOGRAPHS
3
silver grains over astrocytes (Figs 1 and 2) in the neuropil and around vessels irrespective of whether the bird received 0.228 or O-684 mCi of CHzzo3HgC1. In general, grains were most obvious over the cytoplasm. This was most noticeable in the ventral horn of the cervical and lumbar spinal cord, throughout the medullary and internal cerebellar nuclei, cerebellar peduncles, mesencephalic and diencephalic nuclei, all the striatal layers and the hippocampus. Astrocytes with over-lying grains were most prominent in the paleostriatum, particularly the paleostriatum augmentatum and the dorsal medullary lamina. Although there were similar findings in the cerebellar folia, optic lobe and the large white matter tracts (quintofrontal, striothalamic, septomesencephalic and others) traversing the brain stem, the cells involved were not as numerous as in those areas mentioned above. Silver grains were concentrated over ependymal cells in some cases, but it was difficult to determine their exact location with respect to the cytoplasm and nucleus of these cells.
Fig. 2. Microautoradiograph ofpaleostriatum augmentatum of Mallard Note the silver grains over astrocytes scattered in the neuropil vascular position (arrows). HE. x 504.
duck (No. 4) given CH,203HgCI. and over similar cells in a peri-
In one bird (No. 4) silver grains were concentrated over the cytoplasm of large neurones in certain nuclear groups. In some cells the grains were diffusely scattered over the cytoplasm, but in others they were concentrated over the axon hillocks and dendritic terminations. Neurones in the following nuclear groups were involved: paragriseal columns in the spinal cord (Fig. 3), large pyramidal ventral horn neurones throughout the medullary nuclei, particularly the trigeminal nucleus (sensory and motor parts), excepting the vagus nucleus,
4
I :ig. 3. Microautoradiograph CI&~W~Cl. Note . .
D. A.
PASS
of lateral funiculus of lumbar spinal cord of Mallard the silver grains overlying the cytoplasm of paragriseal
F ‘ig. 4. Microautoradiograph of the fastigial nucleus (i.e. internal (No. 4) given CHsZo3HgCl. Note the silver grains overlying astrocytes (arrow). HE. x 504
duck (No. 4) given neurones (arrows).
cerebellar nucleus) of Mallard duck the cytoplasm of large neurones and
METHYL
MERCURIC
CHLORIDE
IN DUCK
: AUTORADIOGRAPHS
5
internal cerebellar nuclei (fastigial, interposital and dentate nuclei) (Fig. 4) and the mesencephalic and diencephalic nuclear masses.Silver grains overlying the cytoplasm of large neurones in the nucleus rotundus were very prominent. DISCUSSION
A heavy concentration of exposed silver grains, which indicated a heavy concentration of methyl mercury, was present over the oesophageal mucosa following oral administration of CH, 203HgC1. Similar findings were observed by Backstrom (1969) m ’ J ap anese quail that were given CH3203HgN03 orally and intravenously. It is likely, therefore, that the mercurial was deposited in the oesophageal mucosa following absorption from the proventriculus or small intestine. The presence of a heavy concentration of silver grains over the liver and kidney is consistent with analytical results following administration of methylmercuric salts to Mallard ducks (Passet al., 1975). The heavy concentration of grains over the oesophageal mucosa, liver and kidney frustrated attempts to determine specific cellular localization of CH3203HgC1. A selective concentration of silver grains over the cytoplasm of astrocytes throughout the central nervous system of the Mallard duck was demonstrated. Takeuchi (1968) showed, with a histochemical technique, that mercury was localized in glial cells in the brain in human casesof methyl mercury poisoning in Japan and morphological changes in astrocytes have been described in methyl mercury poisoning in man (Takeuchi, 1968), pigs (Tryphonas and Nielsen, 1973) and Mallard ducks (Pass et al., 1975). The pathogenesis of the neuronal degeneration of methyl mercury poisoning (Takeuchi, 1968; Klein, Herman, Brubaker, Lucier and Krigman, 1972; Tryphonas and Nielsen, 1973 ; Pass et al., 1975) may be associated with functional abnormalities induced in astrocytes as they are considered to be a component of the blood-brain-barrier (Dunn and Wyburn, 1972). Increased permeability of the blood-brain barrier following administration of methyl mercurials has been demonstrated by Steinwall and Olsson (1969) and Chang and Hartmann (1972b). Following the administration of labelled inorganic mercury, certain neurones in the brain and spinal cord tend to accumulate more inorganic mercury than others (Cassano, Viola, Ghetti and Amaducci, 1969; Nordberg and Serenius, 1969). Nordberg and Serenius (1969) refer to these cells as being “mercurophilic” and Cassano et al. (1969) suggest that this may be due to differences in structure and metabolism of certain neurones. It appears that in the Mallard duck certain neurones accumulate more methyl mercury than others as one bird (No. 4), that received O-684 mCi of CH3203HgC1, accumulated methyl mercury in neurones in the paragriseal columns and ventral horn of the spinal cord, and the medullary, cerebellar, mesencephalic and diencephalic nuclei. SUMMARY
Following oral administration of CH, 203HgC1 to adult Mallard ducks, a heavy concentration of labelled mercury was detected in the oesophageal mucosa, liver and kidney. The concentration of exposed silver grains in other
6
D. A.
PASS
tissues was only slightly greater than that of the background. At a cellular level a selective concentration of labelled mercury was seen only in the central nervous system, an accumulation being present in astrocytes in all regions of the brain and spinal cord and in ependymal cells. In one bird that received 0.684 mCi of 203Hg a selective accumulation of mercury was present in the cytoplasm of neurones of several brain nuclei and neurones in the spinal cord. The presence of methyl mercury in astrocytes may be related to functional changes in astrocytes. ACKNOWLEDGMENTS
This work 209. I would
was supported like to thank
by Ontario Provincial Health Research Mrs C. Beggs for technical assistance.
Grant
No. P.R.
REFERENCES
Backstrom, J. (1969). Distribution studies of mercuric pesticides in quail and some fresh-water fishes. Acta pharmacologica et toxicologica, 27 (suppl. 3), l-103. Cassano, G. B., Viola, P. L., Ghetti, B., and Amaducci, L. (1969). The distribution of inhaled mercury (Hgzo3) vapors in the brain of rats and mice. Journal of Neuropathology and Experimental .Neurologv, 28, 30&320. Chang, L. W., and Hartmann, H. A. (1972a). Electron microscopic histochemical
study on the localization and distribution of mercury in the nervous system after mercury intoxication. Experimental neurology, 35, 122-137. Chang, L. W., and Hartmann, H. A. (1972b). Blood-brain barrier dysfunction in experimental mercury intoxication. Acta neurologica, 21, 179-184. Clarkson, T. W. ( 1972). The pharmacology of mercury compounds. Annual Reviewer of Pharmacology, 12, 375-406. Dunn, J. S., and Wyburn, G. M. (1972.) Th e anatomy of the blood-brain barrier. A review. Scottish Medical Journal, 17, 21-36. Klein, R., Herman, S. P., Brubaker, P. E., Lucier, G. W., and Krigman, M. R. (1972). A model of acute methyl mercury intoxication in rats. Archives of Pathology, 93, 408-418. Nordberg, G. F., and Serenius, F. (1969). Distribution of inorganic mercury in the guinea pig brain. Acta pharmacologica et toxicologica, 27, 269-283. Nordberg, G. F., Berlin, M. H., and Grant, C. A. (1971). Methyl mercury in the monkey-autoradiographical distribution and neurotoxicity. XVI. International Congress of Occupational Health, pp. 234-237. Ostlund, K. (1969). Studies on the metabolism of methyl mercury and dimethyl mercury in mice. Acta pharmacologica et toxicologica, 27, (suppl. l), 1-132. Pass, D. A., Little, P. B., and Karstad, L. H. (1975). The pathology of subacute and chronic methyl mercury poisoning of the Mallard duck (Anus platyrhynchos). Journal of Comparative Pathology, 85, 7-21. of the blood-brain barrier in Steinwall, O., and Olsson, Y. (1969). I m p airment mercury poisoning. Acta neurologica scandinavica, 45, 35 l-36 1. Takeuchi, T. (1968). Pathology of Minamata disease. In Minamata Disease. Study Group of Minamata Disease, Kumamoto University, Japan. Tryphonas, L., and Nielsen, N. 0. (1973). Pathology of chronic alkyl-mercurial poisoning in swine. American Journal of Veterinary Research, 34, 379-392. [Receivedfor publication,
December 11 th, 19731
PATH.
1975.
VOL.
85.
MICROAUTORADIOGRAPHIC STUDY DISTRIBUTION OF METHYLMERCURIC’20” CHLORIDE IN THE MALLARD DUCK
OF
THE
(AJVAS
PLATYRHYJKHOS) 6)
Dejartment
of PatholoQ,
D. A.
PASS*
Univeui&
of Guelph: Guelph,
Ontario,
Canada
INTRODUCTION
The distribution of mercury within an organ has been studied by dissection of and by histoanatomical substructures of the organ, by autoradiography chemical techniques (Clarkson, 1972). Several studies on the autoradiographic distribution of methyl mercurials have been performed in mammals (Ostlund, 1969), but the 1969; Nordberg, Berlin and Grant, 1971) and birds (Backstrom, sub-gross distribution only has been reported. The distribution of methyl mercury at a cellular level has been examined by a histochemical method (Takeuchi, 1968 ; Backstrom, 1969) and the same method has been used to study the subcellular localization of methyl mercury (Chang and Hartmann, 1972a). The histochemical methods in use are said to lack sensitivity and are subject to interference from other metals (Clarkson, 1972). A microautoradiographic technique, using CH,203HgC1, was employed in the present study in an attempt to gain information on the cellular distribution of methyl mercury in the Mallard duck. Particular attention was paid to the vascular and nervous systems as they have been found to be primarily involved in subacute and chronic methyl mercury poisoning in the Mallard duck (Pass, Little and Karstad, 1975). MATERIALS
AND
METHODS
Six adult male Mallard ducks (numbers 1 to 6) were used. The experiment was carried out in two parts. Birds 1, 2 and 3 were dosed orally with 0.228 mCi CHs203 HgClt and birds 4, 5 and 6 with 0.684 mCi CH, 203HgC1 via a flexible pipette that was inserted down the oesophagus to the proventriculus. The specific activity of CH3203HgC1 given to ducks 1 and 4 was 4.6 mCi/mg. Hg. and that given to birds 2, 3,5 and 6 was 13.5 mCi/mg. Hg. Two days following administration the birds were anaesthetized with sodium pentobarbitone and exsanguinated. A routine necropsy was performed and the following tissues were fixed in 10 per cent. formalin and embedded in paraffin : cervical and lumbar spinal cord, dorsal root ganglia, brachial, sciatic and vagus nerves, pectoralis major muscle, lung, liver, kidney, spleen, testes, adrenals, oesophagus, proventriculus, gizzard, duodenum and pancreas, ileum and skin. Eight transverse from the posterior rhombencephalon to the anterior sections of the brain, telencephalon, were processed in the same way. * Present
address:
Bcndigo, Victoria t ICN, Irvine, A
Regional Veterinary 3550, Australia. California U.S.A.
Laboratory
Bendigo,
Department
of Agriculture,
Box
123,
2
D. A. PASS
A section (6 pm) from each of the above was stained with HE. Two unstained sections (6 pm) were dipped in a 50 per cent. solution of NTBP nuclear tract emulsiont and distilled water (M. A. Fernando-personal communicationS), dried and stored in light-tight plastic boxes at 4 “C. for 7 days. The slides were developed for 4 min. in Kodak D1gR* at room temperature and fixed in Kodak Fixera* for 5 min. The sections were stained with HE. and mounted using DPX$. RESULTS
In all birds there was a diffuse scatter of silver grains over sections of the liver, kidney, spleen, testes, adrenal gland, lung, heart, oesophagus, proventriculus, small intestine, pancreas, skeletal muscle and peripheral nerves. In all cases, apart from the liver, kidney and oesophageal mucosa, the concentration of grains, which was only slightly greater than the background concentration, was most apparent over the liver, oesophageal mucosa and the kidney in that order. Localization to particular cell types in these tissues was not determined.
Fig.
1. Microautoradiograph of ventral horn of lumbar spinal cord of Mallard duck (No. 4) given CHazo3HgC1. Note the concentration of silver grains over astrocytes (arrows). Silver grains are mainly present over the cytoplasm. HE. x 504.
In the brain and spinal cord of the 6 birds the overall concentration of grains, in general, was no greater than that of the background. There was, however, specific localization of grains over certain cell types in these tissues. Throughout the central nervous system there were marked concentrations of * Eastman Kodak 1 M. A. Fernando, 5 BDH Chemicals,
Company, Rochester, New Department of Pathology, Toronto, Canada.
York, U.S.A. University of Guelph,
Guelph,
Canada.
METHYL
MERCURIC
CHLORIDE
IN DUCK
: AUTORADIOGRAPHS
3
silver grains over astrocytes (Figs 1 and 2) in the neuropil and around vessels irrespective of whether the bird received 0.228 or O-684 mCi of CHzzo3HgC1. In general, grains were most obvious over the cytoplasm. This was most noticeable in the ventral horn of the cervical and lumbar spinal cord, throughout the medullary and internal cerebellar nuclei, cerebellar peduncles, mesencephalic and diencephalic nuclei, all the striatal layers and the hippocampus. Astrocytes with over-lying grains were most prominent in the paleostriatum, particularly the paleostriatum augmentatum and the dorsal medullary lamina. Although there were similar findings in the cerebellar folia, optic lobe and the large white matter tracts (quintofrontal, striothalamic, septomesencephalic and others) traversing the brain stem, the cells involved were not as numerous as in those areas mentioned above. Silver grains were concentrated over ependymal cells in some cases, but it was difficult to determine their exact location with respect to the cytoplasm and nucleus of these cells.
Fig. 2. Microautoradiograph ofpaleostriatum augmentatum of Mallard Note the silver grains over astrocytes scattered in the neuropil vascular position (arrows). HE. x 504.
duck (No. 4) given CH,203HgCI. and over similar cells in a peri-
In one bird (No. 4) silver grains were concentrated over the cytoplasm of large neurones in certain nuclear groups. In some cells the grains were diffusely scattered over the cytoplasm, but in others they were concentrated over the axon hillocks and dendritic terminations. Neurones in the following nuclear groups were involved: paragriseal columns in the spinal cord (Fig. 3), large pyramidal ventral horn neurones throughout the medullary nuclei, particularly the trigeminal nucleus (sensory and motor parts), excepting the vagus nucleus,
4
I :ig. 3. Microautoradiograph CI&~W~Cl. Note . .
D. A.
PASS
of lateral funiculus of lumbar spinal cord of Mallard the silver grains overlying the cytoplasm of paragriseal
F ‘ig. 4. Microautoradiograph of the fastigial nucleus (i.e. internal (No. 4) given CHsZo3HgCl. Note the silver grains overlying astrocytes (arrow). HE. x 504
duck (No. 4) given neurones (arrows).
cerebellar nucleus) of Mallard duck the cytoplasm of large neurones and
METHYL
MERCURIC
CHLORIDE
IN DUCK
: AUTORADIOGRAPHS
5
internal cerebellar nuclei (fastigial, interposital and dentate nuclei) (Fig. 4) and the mesencephalic and diencephalic nuclear masses.Silver grains overlying the cytoplasm of large neurones in the nucleus rotundus were very prominent. DISCUSSION
A heavy concentration of exposed silver grains, which indicated a heavy concentration of methyl mercury, was present over the oesophageal mucosa following oral administration of CH, 203HgC1. Similar findings were observed by Backstrom (1969) m ’ J ap anese quail that were given CH3203HgN03 orally and intravenously. It is likely, therefore, that the mercurial was deposited in the oesophageal mucosa following absorption from the proventriculus or small intestine. The presence of a heavy concentration of silver grains over the liver and kidney is consistent with analytical results following administration of methylmercuric salts to Mallard ducks (Passet al., 1975). The heavy concentration of grains over the oesophageal mucosa, liver and kidney frustrated attempts to determine specific cellular localization of CH3203HgC1. A selective concentration of silver grains over the cytoplasm of astrocytes throughout the central nervous system of the Mallard duck was demonstrated. Takeuchi (1968) showed, with a histochemical technique, that mercury was localized in glial cells in the brain in human casesof methyl mercury poisoning in Japan and morphological changes in astrocytes have been described in methyl mercury poisoning in man (Takeuchi, 1968), pigs (Tryphonas and Nielsen, 1973) and Mallard ducks (Pass et al., 1975). The pathogenesis of the neuronal degeneration of methyl mercury poisoning (Takeuchi, 1968; Klein, Herman, Brubaker, Lucier and Krigman, 1972; Tryphonas and Nielsen, 1973 ; Pass et al., 1975) may be associated with functional abnormalities induced in astrocytes as they are considered to be a component of the blood-brain-barrier (Dunn and Wyburn, 1972). Increased permeability of the blood-brain barrier following administration of methyl mercurials has been demonstrated by Steinwall and Olsson (1969) and Chang and Hartmann (1972b). Following the administration of labelled inorganic mercury, certain neurones in the brain and spinal cord tend to accumulate more inorganic mercury than others (Cassano, Viola, Ghetti and Amaducci, 1969; Nordberg and Serenius, 1969). Nordberg and Serenius (1969) refer to these cells as being “mercurophilic” and Cassano et al. (1969) suggest that this may be due to differences in structure and metabolism of certain neurones. It appears that in the Mallard duck certain neurones accumulate more methyl mercury than others as one bird (No. 4), that received O-684 mCi of CH3203HgC1, accumulated methyl mercury in neurones in the paragriseal columns and ventral horn of the spinal cord, and the medullary, cerebellar, mesencephalic and diencephalic nuclei. SUMMARY
Following oral administration of CH, 203HgC1 to adult Mallard ducks, a heavy concentration of labelled mercury was detected in the oesophageal mucosa, liver and kidney. The concentration of exposed silver grains in other
6
D. A.
PASS
tissues was only slightly greater than that of the background. At a cellular level a selective concentration of labelled mercury was seen only in the central nervous system, an accumulation being present in astrocytes in all regions of the brain and spinal cord and in ependymal cells. In one bird that received 0.684 mCi of 203Hg a selective accumulation of mercury was present in the cytoplasm of neurones of several brain nuclei and neurones in the spinal cord. The presence of methyl mercury in astrocytes may be related to functional changes in astrocytes. ACKNOWLEDGMENTS
This work 209. I would
was supported like to thank
by Ontario Provincial Health Research Mrs C. Beggs for technical assistance.
Grant
No. P.R.
REFERENCES
Backstrom, J. (1969). Distribution studies of mercuric pesticides in quail and some fresh-water fishes. Acta pharmacologica et toxicologica, 27 (suppl. 3), l-103. Cassano, G. B., Viola, P. L., Ghetti, B., and Amaducci, L. (1969). The distribution of inhaled mercury (Hgzo3) vapors in the brain of rats and mice. Journal of Neuropathology and Experimental .Neurologv, 28, 30&320. Chang, L. W., and Hartmann, H. A. (1972a). Electron microscopic histochemical
study on the localization and distribution of mercury in the nervous system after mercury intoxication. Experimental neurology, 35, 122-137. Chang, L. W., and Hartmann, H. A. (1972b). Blood-brain barrier dysfunction in experimental mercury intoxication. Acta neurologica, 21, 179-184. Clarkson, T. W. ( 1972). The pharmacology of mercury compounds. Annual Reviewer of Pharmacology, 12, 375-406. Dunn, J. S., and Wyburn, G. M. (1972.) Th e anatomy of the blood-brain barrier. A review. Scottish Medical Journal, 17, 21-36. Klein, R., Herman, S. P., Brubaker, P. E., Lucier, G. W., and Krigman, M. R. (1972). A model of acute methyl mercury intoxication in rats. Archives of Pathology, 93, 408-418. Nordberg, G. F., and Serenius, F. (1969). Distribution of inorganic mercury in the guinea pig brain. Acta pharmacologica et toxicologica, 27, 269-283. Nordberg, G. F., Berlin, M. H., and Grant, C. A. (1971). Methyl mercury in the monkey-autoradiographical distribution and neurotoxicity. XVI. International Congress of Occupational Health, pp. 234-237. Ostlund, K. (1969). Studies on the metabolism of methyl mercury and dimethyl mercury in mice. Acta pharmacologica et toxicologica, 27, (suppl. l), 1-132. Pass, D. A., Little, P. B., and Karstad, L. H. (1975). The pathology of subacute and chronic methyl mercury poisoning of the Mallard duck (Anus platyrhynchos). Journal of Comparative Pathology, 85, 7-21. of the blood-brain barrier in Steinwall, O., and Olsson, Y. (1969). I m p airment mercury poisoning. Acta neurologica scandinavica, 45, 35 l-36 1. Takeuchi, T. (1968). Pathology of Minamata disease. In Minamata Disease. Study Group of Minamata Disease, Kumamoto University, Japan. Tryphonas, L., and Nielsen, N. 0. (1973). Pathology of chronic alkyl-mercurial poisoning in swine. American Journal of Veterinary Research, 34, 379-392. [Receivedfor publication,
December 11 th, 19731
