Page 850 - The Toxicology of Fishes

830 The Toxicology of Fishes

2005). Rearing TCDD-treated embryos in an isosmotic mannitol solution prevents edema but does not
prevent cardiac toxicity (Antkiewicz et al., 2005; Hill et al., 2004a). TCDD also increases vascular
permeability in the dorsal midbrain of the zebrafish embryo (Dong et al., 2004). Thus, several factors
may lead to edema in TCDD exposed zebrafish larvae, including heart failure and an increase in
permeability of the skin and/or vasculature.

Cardiovascular Toxicity
In zebrafish embryos exposed to TCDD, both the heart and vasculature are target organs, and the ensuing
cardiovascular toxicity is characterized by ischemia and impaired growth and development of the heart
and vasculature culminating in heart failure and mortality (Antkiewicz et al., 2005; Bello et al., 2004;
Carney et al., 2006b; Henry et al., 1997; Teraoka et al., 2002). Embryos of zebrafish exposed to a lethal
concentration of TCDD shortly after fertilization initially develop a normal circulation and a functional
beating heart; however, at 48 hpf the first endpoints of cardiac toxicity emerge as subtle changes in heart
morphology followed by reduced blood flow in certain vascular beds and a slight accumulation of edema
fluid in the pericardial sac. As TCDD developmental toxicity evolves over the next 72 hours, the endpoints
of cardiovascular toxicity become strikingly more evident. At 120 hpf, failure of an abnormally small
malformed heart, associated with virtually no peripheral blood flow, severe pericardial edema, heart
malformation, and ventricular standstill, occurs (Antkiewicz et al., 2005; Belair et al., 2001; Carney et al.,
2006b; Prasch et al., 2003). Until recently, the earliest occurring adverse cardiovascular response to TCDD
observed in the zebrafish embryo was decreased blood flow in certain peripheral vascular beds; for example,
a transient decrease in blood flow was detected in the mesencephalic vein of the dorsal midbrain at 50 hpf
(Dong et al., 2002), but decreases in blood flow in other major vascular beds were not seen until about
72 hpf (Belair et al., 2001; Prasch et al., 2003; Teraoka et al., 2002). This characteristic, later occurring,
profound peripheral ischemia that is caused by TCDD exposure in zebrafish larvae is secondary to decreased
cardiac output. The drop in cardiac output is caused by a decrease in stroke volume; reduced heart rate
does not play a role (Antkiewiez et al., 2005; Carney et al., 2006b). Reductions in peripheral blood flow
precede reductions in heart rate by at least 24 hours (Antkiewicz et al., 2005; Henry et al., 1997). The
number of myocytes in the heart is reduced by about 20% in TCDD-treated embryos at 48 hpf, and the
percent reduction in cardiac myocyte number at 96 hpf is even greater (24%) (Antkiewicz et al., 2005).
This decrease in cardiomyocyte number causes the overall size of the heart to be smaller and stroke volume
to be less, leading to the term small heart syndrome (Antkiewicz et al., 2005, 2006; Carney et al., 2006b).

Anemia
The formation of red blood cells is reduced by TCDD exposure in zebrafish. The zebrafish embryo
proceeds through early hematopoiesis 18 to 96 hpf (Davidson et al., 2004); however, TCDD blocks the
switch from primitive to definitive erythropoiesis, resulting in larvae that lack circulating definitive
erythrocytes (Belair et al., 2001). The mechanism by which TCDD disrupts this developmental eryth-
ropoietic process is unknown.

Impaired Chondrogenesis
Exposure of the zebrafish embryo to TCDD also impairs development of cartilage structures in the lower
and upper jaw and the cranium (Henry et al., 1997; Hill et al., 2004b; Teraoka et al., 2002). More
specifically, TCDD inhibits jaw cartilage growth and orientation but not initial formation of the jaw
cartilage structures. All cartilaginous structures are present in the jaw of the TCDD-treated zebrafish
embryo, but they are reduced in size and altered in shape (Hill et al., 2004b; Teraoka et al., 2002).
Because the adverse effects of TCDD on jaw development precede reductions in jaw blood flow, a
primary effect of TCDD is to decrease jaw cartilage growth (Carney et al., 2005; Teraoka et al., 2002).
Furthermore, the decrease in jaw cartilage length in TCDD-treated embryos is greater than the reduction
in body length, suggesting it is unlikely for the inhibitory effect of TCDD on jaw growth to be secondary
to stunted growth of the whole fish (Teraoka et al., 2002; Hill et al., 2004b). A final point, the inhibitory
effect of TCDD on cartilage growth in the zebrafish differs from its disruptive effects on osmoregulation,