Analysis of Cldn9 mutant mice shows that Cldn9 is a paracellular ion permeability barrier for Na+and K+, and loss of Cldn9 expression in the inner ear disrupts the Na+/K+barrier and causes deafness. In contrast, a mutant zebrafish line with K+channel defect shows both hearing defect and Publications Using Abomle Etoposide swimbladder over-inflation, suggesting that K+channel plays a very important role in regulating swimbladder volume. In the zebrafish, the larvae surface and swallow a bolus of air, which is passed down through the esophagus and into the swimbladder via the pneumatic duct, to inflate their swimbladders. However, how the larvae and adult fish maintain and regulate the swimbladder volume is unclear and seems to be independent of surface contact. Based on these findings, we speculate that cldn9 is likely to be involved in forming a Na+/K+barrier in the swimbladder and to regulate swimbladder volume. It is also interesting to note that swimbladder has long been recognized to function for sound production and hearing. It has been previously revealed by phalloidin labeling of muscle fibers revealed that smooth muscles are the major muscle constitution in the swimbladder and myocytes form thick bands along the ventral surface of the anterior chamber and bilaterally along the posterior chamber. In contrast, striated muscle fibers constitute a sphincter at the junction of the esophagus with the pneumatic duct. The abundance of muscle-related genes identified in the swimbladder transcriptome correlates with this feature. Besides, KEGG pathway and GSEA analysis showed critical role of interaction between the cells and surrounding extracellular matrix. The viscoelasticity of smooth muscle is contributed by a complex extracellular matrix. The ECM is not only a supporting structure of the smooth muscles, but also a dynamic structure constantly turning over its contents. This explains the abundant ECM-relating transcripts and the active protein transportation process. The major protein constituting ECM are collagens, glycoproteins and proteoglycans. In our transcriptome data, we also observed these transcripts expressing at high levels in the swimbladder. Collagen I is the only type of collagen identified in the swimbladder transcriptome, and it is also the most abundant collagen in the human body.