part of start stage end stage mesoderm pelvic fin bud zone of polarizing activity pelvic fin bud ZFA:0001452 TAO:0001452 zebrafish_anatomy ZPA 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. portion of tissue unknown larval:days 14-20