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Research
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Do Organohalogen Contaminants Contribute to Histopathology in Liver from East Greenland Polar Bears (Ursus maritimus)? Christian Sonne,1,2 Rune Dietz,1 Pall S. Leifsson,3 Erik
W. Born,4 Robert J. Letcher,5 Maja Kirkegaard,1 Derek
C. G. Muir,6 Frank F. Riget,1 and Lars Hyldstrup7 1Department of Arctic Environment, National Environmental Research
Institute, Roskilde, Denmark; 2Department of Veterinary Basic Sciences,
and 3Department of Veterinary Pathobiology, Royal Veterinary and
Agricultural University, Frederiksberg, Denmark; 4Greenland Institute
of Natural Resources, Nuuk, Greenland; 5National Wildlife Research
Centre, Canadian Wildlife Service, Environment Canada, Carleton University,
Ottawa, Ontario, Canada; 6National Water Research Institute, Environment
Canada, Burlington, Ontario, Canada; 7University Hospital of Hvidovre,
Hvidovre, Denmark Abstract In East Greenland polar bears (Ursus maritimus) , anthropogenic organohalogen compounds (OHCs) (e.g., polychlorinated biphenyls, dichlorodiphenyltrichloroethane, and polybrominated diphenyl ethers) contributed to renal lesions and are believed to reduce bone mineral density. Because OHCs are also hepatotoxic, we investigated liver histology of 32 subadult, 24 adult female, and 23 adult male East Greenland polar bears sampled during 1999-2002. Light microscopic changes consisted of nuclear displacement from the normal central cytoplasmic location in parenchymal cells, mononuclear cell infiltrations (mainly portally and as lipid granulomas) , mild bile duct proliferation accompanied by fibrosis, and fat accumulation in hepatocytes and pluripotent Ito cells. Lipid accumulation in Ito cells and bile duct hyperplasia accompanied by portal fibrosis were correlated to age, whereas no changes were associated with either sex or season (summer vs. winter) . For adult females, hepatocytic intracellular fat increased significantly with concentrations of the sum of hexachlorocyclohexanes, as was the case for lipid granulomas and hexachlorobenzene in adult males. Based on these relationships and the nature of the chronic inflammation, we suggest that these findings were caused by aging and long-term exposure to OHCs. Therefore, these changes may be used as biomarkers for OHC exposure in wildlife and humans. To our knowledge, this is the first time liver histology has been evaluated in relation to OHC concentrations in a mammalian wildlife species, and the information is important to future polar bear conservation strategies and health assessments of humans relying on OHC-contaminated food resources. Key words: bile duct proliferation, chlordanes, dichlorodiphenyltrichloroethane, dieldrin, East Greenland, HCB, hexacyclohexanes, Ito cells, lipid granulomas, liver, mononuclear cell infiltrations, polar bear, polybrominated diphenyl ethers, polychlorinated biphenyls, portal fibrosis, DDT, HCH, PBDE, PCB, Ursus maritimus. Environ Health Perspect 113:1569-1574 (2005) . doi:10.1289/ehp.8038 available via http://dx.doi.org/ [Online 5 July 2005] Address correspondence to C. Sonne, Arctic Wildlife Research Veterinarian, National Environmental Research Institute, Department of Arctic Environment, Frederiksborgvej 399, P.O. Box 358, DK-4000 Roskilde, Denmark. Telephone: 45-46-30-19-54. Fax: 45-46-30-19-14. E-mail: csh@dmu.dk We thank H. Tuborg, B. Sandell, J. Brønlund, and local hunters for organizing sampling in East Greenland, and E. Heier for sharing digital images. Financial support was provided by the Danish Cooperation for Environment in the Arctic, the Commission for Scientific Research in Greenland, and the Canada Research Chairs Program. The authors declare they have no competing financial interests. Received 24 February 2005 ; accepted 5 July 2005. |
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Introduction
In rats and mink, several acute studies
of polychlorinated biphenyls (PCBs) have
associated these compounds with hepatotoxicity
(Bergman et al. 1992; Bruckner et al. 1974;
Chu et al. 1994; Jonsson et al. 1981; Kelly
1993; Kimbrough et al. 1971; MacLachlan
and Cullen 1995; Parkinson 1996). Specifically
in the liver, acute organohalogen compound
(OHC) toxicity is mediated through subcellular
toxicity, leading to impaired ATP, protein
synthesis, and other changes (Kelly 1993;
Parkinson 1996), and chronic exposure also
may affect endocrine homeostasis via up-regulation
of cytochrome P450 isozymes (e.g., CYP1A
and CYP1B) (Boon et al. 1992; Lin et al.
2003; van Duursen et al. 2003; Wong et
al. 1992).
In marine wildlife, chronic exposure
to organohalogen compounds [OHCs; e.g.,
PCBs, dichlorodiphenyltrichloroethane (DDT),
and polybrominated diphenyl ethers (PBDEs)]
has been associated with toxic effects
on several organ systems (Bergman 1999;
Bergman and Olsson 1985; Bergman et al.
2001; Schumacher et al. 1993). However,
histologic liver changes associated with
high environmental levels of OHCs in wildlife
have been investigated only in birds, such
as cormorants (Phalacrocorax carbo)
(Fabczak et al. 2000), and fish, such as
common bream (Abramis brama) (Koponen
et al. 2001), but never in marine or terrestrial
mammals.
Polar bears are the most OHC-contaminated
species in the Arctic, and those from East
Greenland and Svalbard (Norway) carry the
most contaminants because of their reliance
on OHC-polluted blubber, mainly from ringed
seal (Phoca hispida) and bearded
seal (Erignathus barbatus), contaminanted
by OHCs originating from lower-latitude
airborne pollution [Arctic Monitoring and
Assessment Programme (AMAP) 2004; de March
et al. 1998; Ramsay and Stirling 1988].
At Svalbard, recent studies of PCBs and
organochlorine (OC) pesticides in polar
bears have indicated negative associations
with plasma testosterone (males), progesterone
(females), cortisol (both sexes), retinol
(both sexes), and thyroxine hormone (both
sexes) (Braathen et al. 2004; Haave et
al. 2003; Oskam et al. 2003, 2004; Skaare
et al. 2001). Additionally, high levels
of PCBs/OC pesticides were associated with
low levels of IgG in the Svalbard bears,
suggesting possible immunotoxic effects
(Bernhoft et al. 2000; Lie et al. 2004,
2005). In East Greenland polar bears, OHCs
are believed to reduce bone mineral density
(BMD) and to be a cofactor in the development
of renal lesions and splenic changes (Kirkegaard
et al. 2005; Sonne et al. 2004, in press).
To determine if OHCs are also a cofactor
in hepatotoxicity, liver tissue histology
was examined in 79 East Greenland polar
bears sampled during the subsistence hunt
from 1999 to 2002, and liver histology
was compared with individual OHC adipose
tissue levels in 65 of the bears. These
new results are intended to fill part of
the existing knowledge gap in understanding
the significance, nature, and effects of
chronic environmental OHC exposure.
Sampling. All polar bear
samples were collected from January through
September by local subsistence hunters
in the Scoresby Sound area in central East
Greenland (69°00´N to 74°00´N)
during 1999-2002. A tissue subsample
from the periphery of a randomly chosen
liver lobe was collected from 79 individuals
and fixed in a phosphate-buffered formaldehyde/alcohol
solution (3.5% formaldehyde, 86% ethanol,
and 10.5% H2O), which prevented
freeze damage. In addition, sternal subcutaneous
adipose tissue was sampled from 65 of the
individuals for OHC analyses and stored
in separate polyethylene plastic bags until
arrival at the laboratory in Roskilde,
where they were transferred into rinsed
[acetone (Supra solv. 1.00012), n-hexane
(Uni-solv 1.04369) both from Merck, KGaA,
Darmstadt, Germany] glass containers, and
covered with aluminum foil in between the
sample and the plastic lid. All samples
were taken < 12 hr postmortem and preserved
frozen during the hunt and later kept at -20°C
before preparation and examination at the
veterinary pathology laboratory in Copenhagen,
Denmark (histology); GLIER, Windsor, Ontario,
Canada (organochlorines); and NWRI, Burlington,
Ontario, Canada (PBDEs).
Age estimation. The age
determination was carried out by counting
the cementum growth layer groups of the
lower left incisor (I3) after
decalcification, thin sectioning (14 µm),
and staining (toluidine blue) using the
method described by Dietz et al. (1991)
and Hensel and Sorensen (1980). When necessary,
the individuals were categorized as adult
males (≥ 6
years of age), adult females (≥ 5
years of age), and subadults (those remaining)
(Rosing-Asvid et al. 2002). In the evaluation
of sex difference in the prevalence of
histologic liver changes, bears were categorized
as old at ≥ 15
years of age based on Derocher and Stirling
(1994).
Histology. The liver tissue
was trimmed, processed conventionally,
embedded in paraffin, sectioned at about
4 µm, and stained with hematoxylin
(aluminum-hematein) and eosin (H&E)
and periodic acid-Schiff for routine diagnostics;
Van Gieson and Masson Trichrome to detect
fibrous tissue (collagen); Best’s
carmine to demonstrate glycogen storage;
Sudan III to detect lipid (frozen tissue);
and Perls’ Prussian blue reaction
and Schmorl technique for detecting hemosiderin
and lipofuscin pigments, respectively (Bancroft
and Stevens 1996; Lyon et al. 1991).
We evaluated six histologic changes and
grouped them semiquantitatively as follows:
- Portal mononuclear cell infiltrations:
absent, unifocally, multifocally, or
diffuse
- Random mononuclear cell infiltrations:
absent, unifocally, multifocally, or
diffuse
- Lipid granulomas: average number in
five fields at 10 magnification
- Hepatocytic intracellular fat: absent,
foamy, multifocal macrovesiculary, or
diffuse macrovesiculary
- Visible Ito cells: average number
in five fields at 20 magnification
- Mild multifocal bile duct hyperplasia
accompanied by portal fibrosis: absent
or present.
For each histologic change, the degree
of change was measured as follows:
- Portal mononuclear cell infiltrations:
mild (unifocally), moderate (multifocally),
severe (diffuse)
- Random cell infiltrations: mild (< 1),
moderate (1-3), severe (> 3)
- Lipid granulomas: mild (< 1), moderate
(1 to < 2), severe (2-5)
- Hepatocytic intracellular fat: mild
(foamy), moderate (multifocal macrovesiculary),
severe (diffuse macrovesiculary)
- Ito cells: mild (< 10), moderate
(10 to < 50), severe (50-200).
Analyses of OHCs. Polar
bear subcutaneous adipose tissue samples
(n = 65) were analyzed for PCBs,
DDTs, chlordanes (CHLs), dieldrin, hexacyclohexanes
(HCHs), and hexachlorobenzene (HCB) according
to Dietz et al. (2004) and Sandala et al.
(2004) at the Great Lakes Institute for
Environmental Research (University of Windsor,
Windsor, Ontario, Canada). An external
standard quantification approach used for
PCBs and OC pesticides in the subcutaneous
adipose tissues was based on peak area
of the gas chromatography-electron capture
detection response, which is described
in detail by Luross et al. (2002).
Briefly, PCB
is the sum of the concentrations of the
51 individual or coeluting PCB congeners
(if detected), given by International Union
of Pure and Applied Chemistry (IUPAC) number:
31/28, 52, 49, 44, 42, 64/71, 74, 70, 66/95,
60, 101/84, 99, 97, 87, 110, 151, 149,
118, 146, 153, 105, 141, 179, 138, 158,
129/178, 182/187, 183, 128, 174, 177, 171/202/156,
200, 172, 180, 170/190, 201, 203/ 196,
195, 194, and 206. DDT
is the sum of 4,4´-DDT, 4,4´-dichlorodiphenyldichloroethane
(DDD), and 4,4´-dichlorodiphenyldichloroethylene
(DDE). HCH
is the sum of the -, β-,
and -hexachlorocyclohexane. CHL
is the sum of oxychlordane, trans-chlordane, cis-chlordane, trans-nonachlor, cis-nonachlor,
and heptachlor epoxide. OHC fractions were
subsequently sent to the National Water
Research Institute for determination of
brominated diphenyl ether (PBDE) flame
retardants. PBDEs (n = 65) were
determined by electron capture negative
ion (low resolution) mass spectroscopy
using an external standard. Briefly, PBDE
is the sum of the concentrations of the
35 individual or coeluting congeners (if
detected), given by IUPAC number: 10, 7,
11, 8, 12/13, 15, 30, 32, 28/33, 35, 37,
75, 71, 66, 47, 49, 77, 100, 119, 99, 116,
85, 155/126, 105, 154, 153, 140, 138, 166,
183, 181, and 190. Gas chromatographic
conditions for the PBDEs were as described
by Luross et al. (2002).
Statistics. The statistical
analyses were performed with SAS statistical
software (version 8, and Enterprise Guide,
version 1; SAS Institute, Cary, NC, USA);
the level of significance was set at p ≤ 0.05,
and levels of significance at 0.05 < p ≤ 0.10
were considered a trend. The OHC data were
log-transformed (base e) before
the analyses in order to meet the assumption
of normality and homogeneity of the variance.
For each specific histologic liver change,
we performed a one-way analysis of variance
(ANOVA) to test for differences in mean
age between individuals with and without
that specific histologic liver change (Table
1). In the case of hepatocytic lipid, we
compared foamy cytoplasm with macrovesicular
lipid. Furthermore, we tested whether there
was a relationship between sex or season
(summer, 1 June through 30 September; winter,
1 October through 31 May), and histologic
liver changes using a chi-square test.
In the case of Ito cells and bile duct
hyperplasia accompanied by portal fibrosis,
we performed the chi-square test within
subadult, adult, and old bears to determine
age dependency. The chi-square test was
also used to test the relationship between
Ito cells and fatty granulomas.
We then performed a one-way ANOVA to
test for differences in mean concentrations
of each group of OHCs (PCBs, DDTs, CHLs,
dieldrin, HCHs, HCB, and PBDEs) between
subadults, adult females, and adult males
(Table 2). The results were then evaluated
from Tukey’s post hoc test. In order
to test the relationship between concentrations
of OHCs and age, we used a linear regression
model for subadults, adult females, and
adult males.
Finally, we tested the relationship between
the concentrations of each group of OHCs
(PCBs, DDTs, CHLs, dieldrin, HCHs, HCB,
and PBDEs, respectively) and each histologic
liver change (absent vs. present) by an
analysis of covariance (Table 3). This
was conducted for each of the three age/sex
groups using OHC concentration as the dependent
variable, age as the covariable, and histologic
liver change as the class variable, including
their first-order interaction links (age histologic
liver change). The statistical analyses
were performed separately on subadults,
adult females, and adult males in cases
of CHLs, dieldrin, HCHs, and HCB, because
the age relationships and/or concentrations
differed among these three age/sex groups.
In the case of lipid granulomas, the relationship
to OHCs was analyzed based on the presence
or absence of Ito cells. After a successive
reduction of nonsignificant interactions,
judged from the type III sum of squares
(p≤ 0.05),
the significance of each of the remaining
factors was evaluated from the final model
least-square mean.
Table
1
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Figure 1. Liver tissue stained with
H&E showing portal mononuclear
cell infiltration in a 3.5-year-old
(subadult) female (A; 10),
random mononuclear cell infiltration
in a 20-year-old female (B; 20),
and lipid granulomas in a 16-year-old
female (C; 40) in liver tissue
stained with H&E. Note the
abnormal localization of the hepatocytic
nuclei in (C). Bars = 50 µm. |
Figure 2. Lipid accumulation in liver
tissue stained with H&E. (A) Zone
2-3 hepatocytic macrovesicular
lipid (vacuoles; 2.5) in a 4-year-old
(subadult) female; inset, taken from
(A; 10). (B) Ito cell lipid accumulation
in a 20-year-old female; 10. Bars
= 25 µm. |
Figure 3. Mild bile duct proliferation
accompanied by portal fibrosis (H&E;
20). Bar = 50 µm. |
Table
2
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Table
3
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We studied a total of 79 free-ranging
East Greenland polar bears (24 subadults,
24 adult females, 22 adult males, 4 old
females, and 5 old males), collected from
1999 through 2002 (Table 1). No background
data describing the general liver histology
of free-ranging polar bears were available
in the scientific literature. The morphology
of the liver tissue was similar to other
carnivorous species; however, interlobular
fibrous septa were lacking as in other
ursid species (Frappier 1998; Heier et
al. 2003, in press; Kelly 1993; Leighton
et al. 1988; MacLachlan and Cullen 1995;
Prunescu et al. 2003). Kupffer cells, located
in the space of Disse, tested positive
for hemosiderin (iron pigments) (Lyon et
al. 1991), and hepatocytes tested positive
for deposits compatible with glycogen (Bancroft
and Stevens 1996). In all individuals,
parenchymal cells exhibited nuclear displacement
toward the cell membrane (Figure 1) (Sato
et al. 2001).
Mononuclear cell infiltrations
and lipid granulomas. We found
portal mononuclear cell infiltrations
(lymphocytes, macrophages, and neutrophils),
as described by Kelly (1993) and MacLachlan
and Cullen (1995), in 18% of the animals
and multifocally mononuclear cell infiltrations
in 12% of the bears examined (Table
1, Figure 1). Additionally, we detected
lipid granulomas, also described by
these authors, in 76% of the animals.
None of these three cell infiltration
types was related to age, sex, or season
(all, p > 0.05) (Table 1).
Finally, we found a trend of livers
with visible Ito cells showing a larger
frequency of fatty granulomas, compared
with livers without visible Ito cells
(p < 0.06).
In addition, we found one case of unifocal
necrosis and a single case of fibrin exudation,
described by Kelly (1993) and MacLachlan
and Cullen (1995), but we did not investigate
the significance further.
Lipids. All animals showed
hepatocytic microvesicular lipid accumulation
(foamy cytoplasm), and 84% showed sharply
demarcated macrovesicular lipid vacuoles
in mainly periacinar (zones 2-3)
hepatocytes (Table 1, Figure 2). In addition,
we found nonparenchymal lipid vacuoles
of diverging size and numbers in centroacinary
Ito cells--located in the narrow space
of Disse, between hepatocytes--mainly
periacinary (zones 2-3) (Table 1,
Figure 2) (Kelly 1993; Leighton et al.
1988; MacLachlan and Cullen 1995; Senoo
et al. 1999, 2001). Intrahepatocytic lipid
accumulation was not related to age (p > 0.05),
whereas Ito cell lipid accumulation was
highly related to age (p < 0.01)
(Table 1). None of the lipid changes was
related to sex or season (summer vs. winter)
(Table 1).
Bile duct proliferation and portal
fibrosis. Mild bile duct proliferation
accompanied by portal fibrosis was
found in 8% of the animals (Table 1,
Figure 3). These changes were associated
with age (both, p < 0.01);
no relationships were found to sex
or season (Table 1).
OHCs and histologic changes. Levels
of PCB, CHL, DDT,
dieldrin, HCH,
HCB, and PBDE
in 65 of the examined polar bears are presented
in Table 2. CHL, PCB, DDT,
dieldrin, HCH,
and PBDE
did not differ significantly among age/sex
groups, but HCB was higher in subadults
when compared with adult males (p ≤ 0.05)
(Table 2). We found a significant negative
relationship between age and HCHs, HCB,
and dieldrin (all, p < 0.05)
for adult females, and between age and CHL
and dieldrin in adult males (both, p < 0.01)
(Table 2). Further information about age
and sex variation of OHCs in the present
East Greenland polar bears has been published
by Dietz et al. (2004) and Sandala et al.
(2004).
The statistical analyses were performed
separately on subadults, adult females,
and adult males in cases of CHL,
dieldrin, HCH,
and HCB because concentrations and/or age
relationships differed between the three
groups of individuals (Table 2). We tested
whether the concentrations of each OHC
group differed between the degree of histologic
liver changes (absent vs. present); for
adult females we found a significant relationship
between HCH
and hepatocytic macrovesicular lipids (vacuoles),
and for adult males we found a significant
relationship between HCB and lipid granulomas
(both, p < 0.05) (Table 3).
We found nuclear displacement toward
the cell membrane in all individuals. In
studies of polar bears from Svalbard, Sato
et al. (2001) revealed the same findings.
It has been proposed that this displacement
is related to the high vitamin A accumulation
(natural storage) in Ito cell cytoplasmic
lipid droplets and hepatocytes, accumulated
through the extensive feeding on blubber
from ringed seal and bearded seal (Käkelä et
al. 1997; Ramsay and Stirling 1988). In
general, such a displacement is associated
with hepatitis, carcinomas, hyperplasia
(adenomatous), or regeneration (Sato et
al. 2001). However, such changes were not
found in the Svalbard study (Sato et al.
2001), and in only two cases were unifocal
hepatitis and regeneration found in the
present study. We could not evaluate whether
there was a relation between nuclear displacement
and OHCs or hepatocytic lipid accumulation
because we found the displacement in nearly
all individuals. Therefore, we hypothesize
that displacement may be a natural phenomenon
in free-ranging polar bears, probably related
to vitamin A intake and/or a result of
lipid/OHCs accumulation (cytoskeletal displacement).
Mononuclear cell infiltrations
and lipid granulomas. Mononuclear
cell infiltrates--accompanied
by fibrosis--is a reaction to
local depositioning of microorganisms
and/or injury of local blood vessels
from, for example, toxic compounds
(Kelly 1993; MacLachlan and Cullen
1995). These cell infiltrates are therefore
a nonspecific inflammatory reaction
that can be linked to even minor tissue
damage (Kelly 1993; MacLachlan and
Cullen 1995). The fact that liver tissue,
rich in visible Ito cells, had a higher
number of lipid granulomas indicates
that microorganisms (originating from
the blood supply) play a role in the
random multifocal necrosis (rupture
of Ito cells) observed (Kelly 1993;
MacLachlan and Cullen 1995). However,
if the lipophilic toxic OHCs accumulate
in the lipid rich Ito cells, we hypothesize
that OHCs may play a role in the burst
of Ito cells, as well.
Lipids. In the present
study, we found macrovesicular lipid in
periacinar hepatocytes. Because polar bears
are hyperphagic from April to July, they
build up their fat deposits during this
period (Messier et al. 1992; Ramsay and
Stirling 1988), and a seasonal pattern
in Ito cell numbers may be expected as
was the case for the fatty tissue lipid
percentage (Dietz et al. 2004). Intrahepatocytic
accumulated lipid vacuoles showed a zonary
pattern similar to that found in individuals
exposed to toxic substances, which produce
a characteristic periacinar injury due
to the low oxygen gradient (hypoxia and
high concentrations of, for example, cytochrome
P450). This could sensitize the liver parenchyma
in this zone to metabolic disorders, resulting
in lipid accumulation (Kelly 1993; MacLachlan
and Cullen 1995; Parkinson 1996).
We also found lipid accumulation in periacinary
Ito cells. In polar bears, the Ito cells
are one of the major accumulation and storage
sites for lipophilic vitamin A (Leighton
et al. 1988; Senoo et al. 1999, 2001) and
probably also lipophilic OHCs, as mentioned
above. As for hepatocytic lipid accumulation,
we did not find a seasonal pattern in the
number of Ito cells, but we did find that
the number of Ito cells is related to age.
If the Ito cell number reflects the vitamin
A exposure through marine prey species,
mainly ringed seal and bearded seal (Ramsay
and Stirling 1988), young bears would have
lower numbers of Ito cells because they
do not start eating prey rich in vitamin
A until they are weaned at approximately
2 years of age (Derocher and Stirling 1994).
This may then explain the age difference
in the number of Ito cells in the liver.
Bile duct proliferation and portal
fibrosis. Bile duct proliferation
has been associated with toxic injury,
parasitism, or periductular fibrosis
in terrestrial animals (Kelly 1993;
MacLachlan and Cullen 1995) and is
therefore a nonspecific reaction to
chronic extrinsic and/or environmental
factors. Specifically in arctic mammals,
bile duct proliferations have been
reported in arctic beluga whale (Delphinapterus
leucas), but the pathogenesis of
this could not be determined (Woshner
et al. 2002).
Age-related portal fibrosis, due to chronic
infections (cholangitis and biliary obstruction),
is a common nonspecific histologic diagnosis
in mammals (Kelly 1993; MacLachlan and
Cullen 1995), and it has been reported
in the Romanian brown bear (Ursus arctos)
(Prunescu et al. 2003) and arctic beluga
whale (Woshner et al. 2002). Prunescu et
al. (2003) showed seasonal liver fibrosis
(highest in spring) of the hepatic venous
system, possibly due to prehibernation
physiologic adaptations. Our findings were
not in agreement with such a seasonal fibrosis
pattern, however, because portal fibrosis
was present with bile duct proliferations
in all individuals.
Liver changes and OHCs. To
our knowledge, liver histology in relation
to environmental levels of OHCs has been
studied only in birds, such as cormorants
(Fabczak et al. 2000), and fish, such as
common bream (Koponen et al. 2001), but
never in marine or terrestrial mammals.
Therefore, it is difficult to evaluate
the relationship between liver histology
and chronic exposure to environmental levels
of OHCs in the East Greenland polar bear
because basic knowledge in this field is
extremely sparse.
Mononuclear cell infiltrates (lymphocytes
and neutrophils) randomly distributed (lipid
granulomas) or portally (around triads)
have been associated with subacute PCB
exposure in mink (Mustela vison)
(Bergman et al. 1992). We found the same
pattern in polar bears, which supports
the hypothesis that OHCs could be a cofactor
in the liver changes of the East Greenland
polar bears in the present study. However,
this could also be a result of microorganisms.
Although the results from the laboratory
studies are nonspecific reactions, parallels
to our results are obvious.
Hepatotoxic substances (e.g., copper,
pyrrolizidine alkaloids, carbon tetrachloride,
and phytotoxins) usually produce a periacinar
zone 2-3 injury due to the low oxygen
gradient (hypoxia) and high concentrations
of, for example, cytochrome P450 isozymes
(activation of reactive metabolites) of
this zone (Kelly 1993; MacLachlan and Cullen
1995; Parkinson 1996). We found such a
zonary appearance in hepatocytic accumulation
in the polar bears in the present study.
Abnormal amounts of fat are known to be
accumulated in the liver during high lipid
ingestion, starvation, abnormal hepatocytic
function, excessive dietary intake of carbohydrates,
and decreased synthesis of apoproteins
(lipoproteins) (Kelly 1993; MacLachlan
and Cullen 1995; Parkinson 1996). Hence,
the large content of lipids in polar bear
livers could be a function of hyperphagia
and starvation due to seasonal changes
in food resources, as discussed above,
although we did not find a seasonal pattern.
However, acute toxic investigations of
PCBs, DDTs, and dieldrin in laboratory
rats have shown to induce high lipid accumulation--probably
due to decreased production of lipoproteins
through impaired ATP synthesis and protein
synthesis--in periacinary hepatocytes
(accumulated as foamy cytoplasm or large
vacuoles) (Bergman et al. 1992; Bruckner
et al. 1974; Kelly 1993; Kimbrough et al.
1971, 1972; MacLachlan and Cullen 1995;
Parkinson 1996). Therefore, OHCs may be
a cofactor in the development of lipid
accumulation in the present study, although
significant differences in OHC concentrations
were not found.
The signs of chronic inflammation, also
in relation to Glisson’s triads (bile
duct proliferation accompanied by portal
fibrosis), as well as the hepatocytic lipid
accumulation, could possibly indicate long-term
exposure to liver toxic substances (OHCs)
in the East Greenland polar bear, as well.
However, other than the OHC considerations
and age, liver histology in free-ranging
Atlantic bottlenose dolphin (Tursiops
truncatus) (Rawson et al. 1993) and
arctic beluga whale (Woshner et al. 2002),
in relation to mercury exposure, have shown
changes similar to those in the present
study. The East Greenland polar bears in
the present study have also accumulated
considerable amounts of mercury in the
liver tissue (2.13-13.4 µg/g
wet weight) (Dietz et al. 1990, 2000),
which are in the range of adverse toxic
effect levels for terrestrial mammals (Thompson
1996).
In the present study, we found the following
histologic changes in liver tissue from
79 East Greenland polar bears: nuclear
displacement, mononuclear cell infiltrations,
mild bile duct proliferation accompanied
by portal fibrosis, and fat accumulation.
Two of the changes (Ito cells and bile
duct hyperplasia accompanied by portal
fibrosis) were related to age, whereas
none were related to sex or season. The
signs and type of chronic inflammation,
and the zonary lipid accumulation in hepatocytes,
may indicate chronic exposure to environmental
levels of OHCs. In addition, we found significant
relationships for HCH
and hepatocytic lipid accumulation in adult
females and between HCB and lipid granulomas
in adult males. We therefore suggest that
the histologic changes were a result of
aging and long-term exposure to OHCs, but
other environmental factors, such as microorganisms
and mercury, cannot be excluded. |
|
|
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Last Updated: October 20, 2005 |
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