part of part of pelvic appendage bud mesenchyme zone of polarizing activity zone of polarizing activity of pelvic appendage 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] hind ZPA ZFA:0001452 zone of polarizing activity pelvic fin bud TAO:0001452 A zone of polarizing activity that is part of a pelvic appendage bud mesenchyme. pelvic ZPA UBERON:0005416 posterior ZPA