part of part of pectoral appendage bud mesenchyme zone of polarizing activity zone of polarizing activity of pectoral appendage TAO:0001451 A zone of polarizing activity that is part of a pectoral appendage bud mesenchyme. ZFA:0001451 zone of polarizing activity pectoral 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] UBERON:0005415 fore ZPA anterior ZPA pectoral ZPA zone of polarizing activity of pelvic appendage