Zone of polarizing activity

Zone of polarizing activity
The apical ectodermal ridge is a thickened epithelium at the most distal end of the limb bud. The zone of polarizing activity is at the posterior part of the limb bud.
Details
Identifiers
Latin	zona activitatis polarisantis
Acronym(s)	ZPA
TE	of polarizing activity_by_E5.0.3.0.0.1.5 E5.0.3.0.0.1.5
Anatomical terminology [edit on Wikidata]

The zone of polarizing activity (ZPA) is an area of mesenchyme that contains signals which instruct the developing limb bud to form along the anterior/posterior axis. Limb bud is undifferentiated mesenchyme enclosed by an ectoderm covering. Eventually, the limb bud develops into bones, tendons, muscles and joints. Limb bud development relies not only on the ZPA, but also many different genes, signals, and a unique region of ectoderm called the apical ectodermal ridge (AER). Research by Saunders and Gasseling in 1948 identified the AER and its subsequent involvement in proximal distal outgrowth.^[1] Twenty years later, the same group did transplantation studies in chick limb bud and identified the ZPA. It wasn't until 1993 that Todt and Fallon showed that the AER and ZPA are dependent on each other.^[2]