Thyroid hormone stimulates gastric development in flounder larvae during metamorphosis

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Abstract
To elucidate the hormonal regulatory mechanisms of gastric development of the flounder, Paralichthys olivaceus, premetamorphic flounder larvae were treated with either 100 ppb of L‐thyroxine sodium salt (T4), or 30 ppm of thiourea (TU, a goitrogen) by dissolving drugs in ambient seawater. Control fish were reared in untreated seawater. Measurement of gastric gland volume with an image analyzing system revealed that the T4 treatment markedly increased the volume of the glands. Immunocytochemistry using an antiserum raised against purified quail pepsinogen as the primary antibody showed precocious appearance of pepsinogen‐like immunoreactivity in the gastric glands of the larvae treated with T4. In contrast, the gastric glands of the larvae treated with TU remained undifferentiated throughout the experiment. The volume of the glands of TU‐treated larvae was very small and immunoreactivity for antiquail pepsinogen was very weak even at the end of the experiment (4 weeks after the start of the experiment) when control fish had well‐developed gastric glands. These results strongly suggest that thyroid hormone stimulates differentiation of gastric glands during metamorphosis of the flounder.