Page 367 - The Toxicology of Fishes

Liver Toxicity 347

Hn

TL
TL

Sr
Sr

(A) 10 µm (B) 10 µm

FIGURE 7.13 (See color insert following page 492.) In vivo microscopy of dechorionated medaka embryos 5 days after
fertilization; tubular architecture in developing liver of medaka. (A) Autofluorescence of hepatic parenchyma (widefield
fluorescence microscopy). Six to eight hepatocytes can be seen to, in transverse section, form the hepatic tubule. The apical
membranes of hepatocytes form the tubule lumen, a central biliary passageway. Hn, hepatocyte nucleus; Sr, sinusoid with red
blood cells; TL, tubule lumen. (B) Same image as A but showing TRITC fluorescence of 7-benzyloxyresofufin (7-BR) (red).
7-BR-exposed medaka embryo shows fluorescence of resorufin in lumens of hepatic tubules and in canaliculi, thus providing
in vivo evidence for CYP3A metabolic activity and concentrative transport of fluorophore from the sinusoid to the tubule lumen.

GB
GB

(A) 100 µm (B) 100 µm

FIGURE 7.14 (See color insert following page 492.) In vivo microscopy (brightfield illumination) of dechorionated medaka
embryo. (A) Yellow/green pigment in the gallbladder (GB) is seen. This signifies bile synthesis and export. The liver is in
left upper abdominal cavity immediately dorsal to lipid droplet. The circuitous left duct of Cuvier is shown, and a portion
of the media yolk vein is at the caudal margin of the yolk sac. (B) Same orientation as in A, demonstrating autofluorescence
in the gallbladder and to a lesser extent in the intrahepatic passageways.

resorufin, resulting in resorufin fluorescence. Resorufin can then be detected in the canalicular network,
tubule lumens, larger bile passageways of liver, gallbladder, common bile duct, and intestinal lumen
(Figure 7.14). Iwamatsu et al. (2003) reported that the medaka anal aperture is not open until the fish
reaches approximately 5.3 mm total length (stage 40, after hatching). This may explain the retained
fluorescence of the intestinal lumen late in embryogenesis.
Another important feature of L5 is its blood supply, microvasculature, and venous drainage. In vivo
observations (Hinton et al., 2004) have shown that a single vein (hepatic vein) drains L5, with blood
flowing from the liver to the left duct of Cuvier. There is no afferent venous supply to L5; the apparent
sole source of hepatic blood supply, at this time, is arterial from the dorsal aorta (Hinton et al., unpublished
observations). We have observed erythrocytes tumbling through the hepatic microvasculature and passing
through the hepatic vein to the left duct of Cuvier.
During the second week of larval life, an apparent metamorphosis occurs when the yolk sac is
apparently completely absorbed, the gut elongates (Iwamatsu et al., 2003), and the liver (L5) assumes