Page 851 - The Toxicology of Fishes

Reproductive Impairment of Great Lakes Lake Trout by Dioxin-Like Chemicals 831

cardiovascular function, and hematopoiesis. This is because there is not a critical developmental window
for eliciting the inhibitory effect of TCDD on cartilage growth like there is for endpoints of developmental
toxicity such as the smaller malformed heart, edema, or anemia (Belair et al., 2001; Carney et al., 2005;
Heideman et al., 2001). This difference is illustrated by the ability of TCDD to stunt the growth of lower
jaw cartilage, not only in zebrafish exposed to TCDD as embryos and larvae but also in zebrafish exposed
to TCDD as juveniles and adults.

Neurotoxicity
The developing zebrafish brain is adversely affected by TCDD. At approximately 50 hpf, a small increase
in apoptosis is observed in the dorsal midbrain (Dong et al., 2001, 2002). In addition, more prominent
effects are seen at later stages of brain development; for example, TCDD causes a 29% reduction in
brain volume at 168 hpf that is associated with a decrease in the total number of neurons in the brain
(Hill et al., 2003). It is unclear if TCDD affects the zebrafish brain directly or if the decrease in number
of neurons is secondary to a decrease in brain blood flow. In support of the latter, an increase in apoptosis
in the midbrain of TCDD-exposed zebrafish embryos has been associated with decreased blood flow to
the dorsal midbrain region (Dong et al., 2002).

Female Reproductive Toxicity of TCDD in Fish

Although much has been learned regarding the impacts of TCDD on early embryonic development of
zebrafish, relatively little is understood about the impacts of TCDD on the reproductive system of adult
female zebrafish. Wannamacher et al. (1992) showed that acute dietary exposure of 5 to 20 ng TCDD
induces overt toxicity associated with a dose-dependent reduction in egg production and complete
suppression of spawning activity, corresponding with arrested gonadal development and oocyte atresia.
Unfortunately, the small sample size of this study made reproductive toxicity difficult to evaluate, and
a dose–response relationship for TCDD-induced reproductive toxicity could not be determined because
levels of TCDD were not measured in females or eggs. More recently, King Heiden et al. (2005, 2006)
found that the reproductive success of female zebrafish was impaired when exposed to concentrations
of TCDD that do not induce acute toxicity. They demonstrated that sublethal concentrations of TCDD
are capable of modulating reproductive success of female zebrafish even when spawning activity and
overall egg production are not decreased and that subtle physiological changes produced by TCDD can
lead to attenuated follicular development and ovarian steroidogenesis. Reproductive effort measured by
the ovarian somatic index is significantly reduced following the accumulation of 0.6 ng TCDD per g
female (King Heiden et al., 2005); following an estimated accumulation of 4.0 ng TCDD per g female,
egg production and spawning success are decreased (King Heiden et al., 2006). Following the accumu-
lation of 3.0 ng TCDD per g female (0.3 ng TCDD per g egg), maternal transfer reduces offspring
survival, while maternal transfer of as little as 0.094 ng TCDD per g egg induces the typical signs of
larval toxicity (King Heiden et al., 2005). Even when overall egg production is not impacted, subtle
physiological changes induced by TCDD can lead to altered follicular development and decreased serum
17β-estradiol and vitellogenin concentrations (King Heiden et al., 2006). Histopathological analyses
suggest that, although liver toxicity may contribute to observed impacts on follicular development and
vitellogenesis, reproductive toxicity of TCDD likely results from direct action at the ovary by inhibiting
follicular development, in addition to inducing follicular atresia. Taken together, these findings support
the hypothesis that maternal transfer of low concentrations of TCDD can have profound effects on
offspring health and survival and potentially impact recruitment in natural populations and that long-
term exposure to very low concentrations of TCDD could potentially impact fecundity.
The mechanisms by which TCDD induces these reproductive alterations have not been fully charac-
terized. In an effort to identify the transcriptional changes that precede observed ovarian toxicities, King
Heiden et al. (2007) used quantitative reverse-transcription polymerase chain reaction (RT–PCR) to
assess the effect of TCDD on the expression of several candidate genes important in the regulation of
follicular development and steroidogenesis. Additionally, global changes in gene expression in the ovary
caused by TCDD exposure were identified using microarray analysis. Their data suggest that suppression