Page 392 - The Toxicology of Fishes

372 The Toxicology of Fishes

Hepatocytes Under Toxic Exposure
Changes in hepatocytes do not necessarily signify overt toxicity and may represent sublethal cellular
change as seen during adaptation to toxicants or due to an altered equilibrium state in injured tissues or
cells. A detailed consideration of ultrastructural hepatic changes has been provided recently by Braunbeck
(1998) and Au et al. (2004), so this review is brief. The examples are arranged by inclusion body and
by specific organelle.

Lipopigments
Lipofuscin and ceroid share very similar morphology and physicochemical properties. Lipofuscin is
regarded as the classical age or stress pigment, and the accumulation of ceroid is indicative of cell
pathology. Lipopigments are widely regarded as the end products of lipid peroxidation, which is a free-
radical chain reaction. Such a reaction is initiated when some unidentified free radical abstracts a
methylene H-atom from the unsaturated fatty acid and then reacts with oxygen to yield a hydroperoxy
radical. Termination of the chain reaction is often associated with polymer formation, and copolymer-
ization of oxidized lipids with protein and other compounds would form lipopigments (Dayan and
Wolman, 1993; Donato, 1981; Wolke et al., 1985). Lipopigments tend to accumulate in the lysosomal
compartment as residual bodies (Holtzman, 1976; Park and Chi, 1992; Yin, 1996). This is exemplified
by the fact that secondary lysosomes have often been found in the vicinity of lipopigments in hepatocytes
of fishes. Early ultrastructural studies of Elbe flounder and dab collected from sites highly contaminated
with organochlorines and heavy metals showed an unusual extent of transformation of lysosomes into
lipofuscins and were in close association with the large lipid droplets in hepatocytes (Köhler, 1990;
Köhler et al., 1992). Likewise, livers of mummichog inhabiting a PAH-contaminated (creosote) envi-
ronment often exhibited sublethal cellular damage, including the accumulation of ceriod and lipofuscin
in non-neoplastic hepatocytes bordering carcinomas (Vogelbein, 1993). Increased lipofuscin was also
reported in the liver of juvenile grey mullets after exposure to the algicide atrazine (Biagianti-Risbourg
and Bastide, 1995). A recent survey in the San Diego Bay in California (Exponent, 2003) reported that
the prevalence of lipofuscin in hepatocytes of the spotted sand bass (Paralabrax maculatofasciatus) was
most strongly associated with shipyard sites that were characterized by high levels of organic contam-
inants (e.g., PCBs, PAHs) in sediments. It is important to be able to quantify hepatic lipopigment
accumulation with levels of pollutants. Stereological analysis was employed to quantify cytological
changes in the livers of immature sole (Solea ovata) and juvenile orange-spotted grouper (Epinephelus
coioides) exposed to benzo(a)pyrene (BaP). A dose–response relationship was generally demonstrated
between volume density, absolute volume, numerical density, and absolute number of lipopigment
accumulation and levels of BaP under both laboratory and field conditions, regardless of the route of
PAH uptake (Au, 2004; Au and Wu, 2001; Au et al., 1999). The accumulation of lipopigment in mammals
(ceroid-lipofuscinosis) is a pathological symptom known as Batten’s disease. The resulting deleterious
effects include blindness, seizures, dementia, and premature death (Martinus et al., 1991; Palmer et al.,
1986; Yin 1996). Furthermore, large amounts of undegradable lipofuscin and ceroid in human fibroblasts
were shown to increase the susceptibility of cells to oxidative stress, disturbance of lysosomal function,
and cell death (Terman et al., 1999a,b). Condensation of hepatocellular content was often observed in
the vicinity of lipopigments in Solea ovata exposed to BaP (10 mg/kg BaP), which may suggest severe
lipid peroxidation in hepatocytes (Au et al., 1999). It is therefore clear that intracellular lipopigment
accumulation is indicative of adverse cytological effects. Conceivably, an excessive amount of lipopig-
ment interferes with vital cellular functions and decreases the survival of affected cells, which may
eventually decrease the fitness of fish in the contaminated environment.

Lysosomes

These membrane-delimited organelles contain hydrolytic enzymes for breaking down substances
within a cell (autophagy) or substances that have been taken in from outside the cell (heterophagy)