OBO_REL:part_of (UNDECLARED) mesoderm pelvic fin bud zone of polarizing activity pelvic fin bud 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. Gibert et al., 2006. TAO:0001452 ZPA teleost_anatomy