During the evolution of vertebrates, for example, the chordate body plan was retained while novel structures such as the cranium, paired sensory organs, and paired appendages were added ( 13, 14). Compartments in a broad sense-of body plans, genomes, and developmental programs-allowed variation of individual modules while protecting others from the potentially deleterious effects of mutations. However, tight junctions also have been found in the blood–brain and blood–testis barriers of adult arthropods ( 3) and septate junctions occur at the nodes of Ranvier in the vertebrate nervous system ( 4).Ĭompartmentalization is not only a hallmark of metazoans, but also may have facilitated the increase in diversity and complexity during their evolution ( 11, 12). Tight junctions are in general characteristic of chordates, whereas septate junctions are found in nonchordates ( 1). At tight junctions the intercellular space is wholly obliterated, whereas at septate junctions the apposed cell membranes are separated by distinct septa and remain 12–18 nm apart.
The two types of junctions are distinguished by their ultrastructure and their distribution among metazoan phyla. Both are linked by intracellular membrane-associated proteins to signaling pathways and the cytoskeleton. Both are selective and dynamic rather than impermeable and static barriers. Both contain matching strands of intramembrane particles in the apposed cell membranes. To form effective diffusional barriers, epithelia use two types of intercellular seals, tight junctions and septate junctions ( 1, 2). Compartmentalization overcomes the limitations imposed by diffusion and facilitates the control of cellular environments, the division of labor among differentiated cell types, and the efficient transport of nutrients and information over long distances. We propose that tight junctions not only form barriers in mature epithelia, but also participate in vertebrate morphogenesis.Ĭompartments bounded by epithelia are a fundamental feature of any metazoan. We also show that the claudin gene family may have expanded along the chordate stem lineage from urochordates to gnathostomes, in parallel with the elaboration of vertebrate characters.
Related claudins in amphibians and mammals are expressed in a similar manner in vertebrate primordia such as sensory placodes, branchial arches, and limb buds. Here we report that two paralogs among 15 claudin genes in the zebrafish, Danio rerio, are expressed in the otic and lateral-line placodes at their earliest stages of development.
Their functional importance is demonstrated by mutations in claudin genes that eliminate tight junctions in myelin and the testis, abolish Mg 2+ resorption in the kidney, and cause autosomal recessive deafness. Claudins, the major transmembrane proteins of tight junctions, are members of the tetraspanin superfamily of proteins that mediate cellular adhesion and migration.
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