Zebrafish periostin is required for the adhesion of muscle fiber bundles to the myoseptum and for the differentiation of muscle fibers.

The myoseptum of fishes, composed of dense collagen, is a connective tissue layer that forms in the embryo, dividing somites from the trunk, and its structure and function are similar to those of the mammalian tendon. Both the myoseptum and tendon serve as the transmitter of muscular contractility to bones and adjoining muscles, and their structure is indispensable for movement of vertebrate animals. We cloned the zebrafish periostin gene and examined its expression and function in the myoseptum. The expression in embryos started in the rostral part of each segmented somite in the early segmentation stage; and consequently, metameric stripes were observed. At the end of segmentation, the expression region shifted to the transverse myoseptum and the myotome-epidermis boundary, and each myotome was surrounded by periostin. Using a polyclonal antibody, we found that the periostin protein was localized to the transverse myoseptum. Consistently, periostin morpholino antisense oligonucleotide led to defects in myoseptum formation, a delay in the differentiation of myofibers, and disorder of connection between myofibrils and myoseptum. We demonstrated here that periostin is the first molecule involved in myoseptum formation and propose that periostin secretion on the surface of the myoseptum is required for the adhesion of muscle fiber bundles to the myoseptum and the differentiation of muscle fibers.