Effects of thyroid hormones on growth, survival, early development, and epithelial cell proliferation of larval teleosts
These studies were carried out to test the effects of thyroid hormones on growth, survival, early development, and epithelial cell proliferation in teleosts, using striped bass (Morone saxatilis) and summer flounder (Paralichthys dentatus) as models.^ Experiments were first set up to test the effect of triiodothyronine (T$\sb3$) on the growth, survival and early development of larval striped bass. T$\sb3$ dissolved in the 5 ppt seawater was taken up by larval striped bass in a dose-dependent manner. At 5 days after hatching, T$\sb3$ at 100 ng/ml and 50 ng/ml retarded the growth of the larvae. At 16 days after hatching, T$\sb3$ at 100 ng/ml retarded the growth of the fish. The stomachs of fish treated with 100 ng T$\sb3$/ml were significantly more muscular than those of other groups, and had shorter fundic stomachs with fewer pepsinogen-immunostaining gastric glands than stomachs of fish treated with 1 ng T$\sb3$/ml or the control. T$\sb3$ treatments did not affect survival or body form, but accelerated somatic melanogenesis. Body content of T$\sb3$ returned to control levels within days following the end of treatments. The results indicate that exogenous T$\sb3$ can cause abnormal development of the stomach and may thus retard the growth of the fish.^ The effects of thyroid hormone on the histogenesis of the stomach of summer flounder was different from the effect on striped bass. Thyroxine (T$\sb4$) at 100 ppb accelerated formation of the gastric glands of summer flounder larvae by at least one week and production of pepsinogen by two weeks compared to the controls. Four weeks after the initiation of the treatment, T$\sb4$-treated and the control fish had well developed stomachs with well differentiated gastric glands, whereas the stomachs of fish treated with thiourea (TU, a thyroid hormone synthesis inhibitor) only had poorly differentiated gastric glands. TU totally suppressed the production of pepsinogen. The results indicated that T$\sb4$ stimulates the formation of gastric glands and is necessary for the production of pepsinogen of metamorphosing summer flounder.^ Bromodeoxyuridine (BrdU)-immunocytochemistry (ICC) technique was used to localize proliferating cells in the digestive tract and other tissues in larval striped bass. BrdU at 500 mM was absorbed and circulated to internal organs such as intestine and brain within 4 hr and resulted in visible immunostaining of proliferating nuclei.^ Finally the effect of T$\sb4$ on epithelial cell proliferation in the digestive tract of premetamorphic and metamorphosing summer flounder was investigated, using BrdU-ICC technique. One week after the treatments, the control and T$\sb4$-treated fish had some proliferating epithelial cells, but TU-treated fish had no proliferating cells in the digestive tract. Three weeks after the treatments, proliferating cells in the control and T$\sb4$-treated fish concentrated in the antero-median intestine, whereas in the TU-treated fish proliferating cells occurred at all lengths of the intestine and immunostained cells were more abundant than in the control and T$\sb4$-treated fish. No proliferation was found in the gastric glands of fish from any group. The results indicate alteration of thyroidal status affects cell proliferation in the digestive tract of summer flounder. ^
Biology, Animal Physiology|Biology, Zoology|Agriculture, Fisheries and Aquaculture
"Effects of thyroid hormones on growth, survival, early development, and epithelial cell proliferation of larval teleosts"
Dissertations and Master's Theses (Campus Access).