part of start stage end stage mesoderm pectoral fin bud zone of polarizing activity pectoral fin bud zebrafish_anatomy A small group of mesodermal cells located at the posterior margin of the developing fin bud. The ZPA is thought to produce retinoic acid and thereby provides positional information to the developing bud. ZFA:0001451 ZPA TAO:0001451 portion of tissue pharyngula:prim-5 pharyngula:prim-15