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that cell maturation occurs among circulating hemocytes. These cells are capable of per-
forming several immune functions such as phagocytosis (Lopez et al. 1997; Beaven and
Paynter 1999; Canesi et al. 2002a, 2002b) and cytotoxicity through the production of reac-
tive oxygen intermediates (Anderson et al. 1992; Pipe 1992). They can also secrete high
molecular weight antibacterial substances as well as short peptides also presenting anti-
bacterial activity (mytilins, myticins, defensins, and mytomycins). Hemocytes can also
produce antibacterial enzymes (proteinases, glycosidases, sulfatases, lysozymes, etc.) and
bacterial enzyme inhibitors involved in blocking enzymes produced by bacteria to help
the infectious process. Finally, hemocytes have been shown to interact with several pro-
teins circulating in hemolymph and be responsible for the opsonization and aggregation
of bacteria (gigalins, galectins, integrins, and ficolins) (reviewed by Auffret 2005).
6.2.2 Hemocytes, Front Line of Immunocompetence
The involvement of circulating hemocytes as the first line of defense against invading
microorganisms is well established (Cheng
et al. 1977). There are several types of hemo-
cytes based on their morphology and size. They represent a heterogeneous population
consisting of at least two subpopulations as determined by enzymatic and morphological
characterization in most species of bivalves, except in scallops (Auffret 1988; Brousseau
et al. 2000; Fournier et al. 2001; Pampanin et al. 2002). Granulocytes are granular cells
with a small nucleus. As for hyalinocytes, they are agranular, with a central nucleus sur-
rounded by little cytoplasm (Pampanin et al. 2002). Indeed, hyalinocytes are relatively
heterogeneous in size with diameters ranging from 4 to 17 μm (Bachère et al. 1988). The
granulocyte/hyalinocyte ratio was found to be dependent on the species and flow cyto-
metric measurements have revealed great interindividual variations. A parallel evaluation
of leukocyte count and phenotyping was considered a good indicator for immunotoxic
potential in other species such as humans, monkeys, and frogs (Luster et al. 1992a, 1992b,
1993). Indeed, an increased number of hemocytes was observed after
in vivo
exposure of
bivalves to heat stress (Renwrantz 1990), pathogens (Oubella et al. 1993; Parisi et al. 2008),
and xenobiotics (Renwrantz 1990; Anderson et al. 1992; Coles et al. 1994). Sami et al. (1992)
also showed that the percentage and the relative size of small hemocytes of the oyster
Crassostrea virginica
could be modulated
in situ
by exposure to polycyclic aromatic hydro-
carbons (PAHs). We believe that the profile of hemocytes by flow cytometry has good
potential for use as an effective biomarker of exposure.
6.2.3 Phagocytosis, Marker of Effect
The sensitivity of phagocytosis to chemicals has been the object of numerous studies. It
was observed that exposure of the bivalve
Mercenaria mercenaria
to phenol reduced the
phagocytic activity of hemocytes (Fries and Tripp 1980). Long-term exposures to benzo(
a
)
pyrene, pentachlorophenol, and hexachlorobenzene (HCB) weakly stimulate phagocytosis
(Anderson et al. 1981; Frouin et al. 2007). Bouchard et al. (1999) showed that phagocytosis
decreased with increasing doses of tributyltin (TBT) and dibutyltin (DBT). The toxicity
of butyltin on the hemocytes of the four species of bivalves (
Mytilus edulis, Mya arenaria,
Spisula polynima, Mactromeris polynyma
) was: MBT < TBT < DBT. The comparison of the
relative sensitivity of species has shown that the blue mussel (
M. edulis
) was more tolerant
of butyltin compounds than the other species studied.
Research studies on phagocytosis in several species of freshwater and marine bivalves
have demonstrated a classic curve for the
in vitro
toxicity of mercury: (1) lack of cytotoxicity
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