Hormonal Control of Chloride Cell Differentiation in Fish Gills

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The aim of this study is to clarify endocrine control of differentiation of chloride cells in fish gills in response to the changes in environmental salinity. The results obtained during the past two years are as follows.1. A cDNA of 453 bp was cloned from liver mRNA of tilapia with primers designed for the hormone-binding domain of cortisol receptor of the rainbow trout. The receptor message by Northern blot analysis was stronger in fish in fresh water than in those in seawater. The receptor mRNA was already present in the yolk just after fertilization, indicating its importance in cell and tissue differentiation during the early development.2. Two types of chloride cells were identified in the gill filament of chum salmon. Lamellar chloride cells were mainly present in the fish in fresh water and disappeared during seawater adaptation, suggesting their role as a site for ion uptake in fresh water. On the other hand, filament chloride cells were developed during seawater adaptation, i … More ndicating their role as a site of salt excretion in seawater.3. Cortisol receptor gene expression, examined by in situ hybridization, were found in the two types of chloride cells in the chum salmon fry, indicating that cortisol is one of the important factors regulating chloride cell functions.Cortisol receptors were also present in undifferentiated cells, also suggesting its involvement in the functional differentiation of chloride cells.4. Prolactin receptor gene was cloned from the liver mRNA of tilapia with primers for the extracellular domain of the receptors of mammalian and stenohaline tilapia. Competitive PCR revealed the presence of the receptor in the embryos 2 days after fertilization, and the gene expression was stronger in the embryos developing in fresh water than those in seawater.5. It has becaome increasingly, clear that various hormones such as cortisol and thyroid hormones are contained in the yolk of various fish species, possibly affecting the cell and tissue differentiation. Further studies are called for to examine whether prolactin and thyroid hormones are involved specifically in the differentiation of the gill chloride cells. Less