Bacterial and fungal infections of the swimbladder are occasionally reported in various fish species. Having an open swimbladder that connects to the gastro-intestinal tract, the zebrafish swimbladder is more AbMole Cetylpyridinium chloride monohydrate vulnerable to infection than physoclistous fishes. Our observation indicated that the swimbladder had its own defensive mechanism by expressing high levels of surface recognition molecules. The epithelium is the inner most layer of the swimbladder and is in direct contact with the gas inside. It has been shown by transmitted electron microscopy that the swimbladder epithelial cells are polarized even prior to inflation. Tight junctions serve to form seals between epithelial cells, creating a selectively permeable barrier to intercellular diffusion. Consistent with this, our KEGG pathway analysis indicated that the tight junction pathway genes were indeed enriched in the swimbladder. Claudins are transmembrane proteins which act in concert with other transmembrane and peripheral proteins to form the physical basis for tight junction. There are roughly two dozens of different claudins. In human airways, both bronchi and bronchioles express Claudin 1, 3, 4, 5 and 7. Particularly, CLDN3/4/5 have been found to be co-expressed by type II alveolar epithelial cells. It has been revealed by immunofluorescence staining that CLDN4 is increasingly localized to the apical tight junction region, but with lower expression at the lateral region. In contrast, CLDN3 and 5 are localized exclusively in the apical-most region of the tight junctions. Altered Claudin expression pattern can AbMole D-Pantothenic acid sodium change the paracellular permeability characteristics of the epithelium. For example, CLDN3 overexpression decreases solute permeability, whereas CLDN5 increases permeability. 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.