| The overarching theme of this thesis work is the development of the stomach in summer flounder, which occurs during metamorphosis. During normal development, proliferation of the gastric epithelium increases in early metamorphosis when incipient gastric glands first appear, and declines at late metamorphosis when gastric glands are completely developed. Inhibiting thyroid hormone prevents the increase in proliferation and the differentiation of glands, while high levels of thyroid hormone accelerate gastric gland differentiation but also inhibit proliferation. The hormone is necessary to cause cell proliferation, but at high doses it appears to selectively stimulate differentiation, suggesting that the steady increase in hormone levels that occur during metamorphosis might be necessary to produce the normal patterns.; Developmentally arrested larvae exposed to thyroid hormone at different ages showed that the ability of the epithelium to proliferate decreases around 50 days post hatch, the age when the stomach becomes functional in normal development. Differentiation of gastric glands, pepsinogen expression, and mucus-producing cells did not show a similar decline in response, indicating that there is a critical period for stimulation of cell proliferation, but not for differentiation. This suggests that the response of the epithelium to thyroid hormone might require other factors or signals.; Summer flounder larvae exposed to the dioxin-like PCB 126 express cytochrome P-4501A in the liver, kidney, stomach, and intestine in a dose-dependent manner, but unexpectedly develop no morphological anomalies nor show alterations in the gastric pattern of proliferation and differentiation. Exposure to sublethal doses of PCB 126 caused significant delays in metamorphic rate and growth. In terms of mortality, these fish are as sensitive as the embryos of the most vulnerable freshwater fish tested (e.g. trout).; The changes in glycoconjugates of the gut, skin, and gills epithelia were studied using lectin histochemistry. In the stomach and skin, the glycoconjugate structural diversity increased from larva to juvenile, while the intestine exhibited both increases and decreases in specific structures. The gills underwent few changes. The developmental changes in glycoconjugates of stomach and intestine might be important for the establishment of the bacterial community of the juvenile gut. |
