Mechanisms of metamorphosis in flatfishes - structural and functional analyses

最典型的变态发生在比目鱼中，与眼睛迁移和不对称的身体转变有关。主要研究了比目鱼（Hirame）在变态过程中的结构和功能变化。在变态的高潮期，所有7种比目鱼的组织甲状腺激素水平均显著升高。在少数物种中，皮质醇水平在甲状腺激素激增之前也会升高，但在其他物种和/或不同条件下并没有表现出明显的升高。在变态过程中，消化系统发生了显著的发育变化：功能性胃分化为胃腺和幽门盲囊，胃蛋白酶样酶活性显著增加。外源性甲状腺激素明显加速了这些变化。小淋巴细胞在变态过程中分化，这表明转移的身体防御机制从幼虫到成人类型。胸腺退化发生在与皮质醇高峰相关的高潮期。在变态过程中，平肢表皮黑化发生背腹分化，其中眼侧成黑素细胞分化显著，盲侧成黑素细胞分化减弱。这一过程可能受眼部促黑因子和盲侧抑制因子的调控。

The most typical metamorphosis occurs in flatfishes, associated with eye migration and asymmetrical body transformation. The structural and functional changes occurring during flatfish metamorphosis were examined mainly in the Japanese flounder (hirame).Marked elevations in tissue thyroid hormone levels were observed at the climax phase of metamorphosis in all seven species examined. Cortisol levels also elevated just prior to thyroid hormone surge in a few species, but it didn't exhibit marked elevation in other species and/or different conditions. During metamorphosis of hirame prominent developmental changes were observed in the digestive system: differentiation of functional stomach with gastric gland and pyloric caeca and marked increase in pepsin-like enzyme activity. These changes were significantly accelerated by exogenous thyroid hormone. Small lymphocytes differentiated during metamorphosis, this suggesting transfer of the body defense mechanism from larval to adult types. Thymus involution occurred at the climax phase related to cortisol surge in hirame. This suggests an intimate relationship between endocrine and immune systems.Dorso-ventral differentiation of melanization in the epidermis of hirame occurs during metamorphosis, at which melanoblast differentiation was significantly promoted in the ocular side while it was degrade in the blind side. This process seemed to be controlled be melanization promoting factors in the ocular and the inhibiting factors in the blind sides.