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Code = [[Terminologia Embryologica|TE]] E5.0.3.0.0.1.5 |
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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 zone of polarizing activity, 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.<ref name=pmid9846378>{{cite journal |author=Saunders JW |title=The proximo-distal sequence of origin of the parts of the chick wing and the role of the ectoderm. 1948 |journal=The Journal of Experimental Zoology |volume=282 |issue=6 |pages=628–68 |year=1998 |month=December |pmid=9846378 |doi=10.1002/(SICI)1097-010X(19981215)282:6<628::AID-JEZ2>3.0.CO;2-N |issn=0022-104X}}</ref> Twenty years later, the same group did transplantation studies in chick limb bud and identified the ZPA. It wasn’t until 1987 that Todt and Fallon showed that the AER and ZPA are dependent on each other.<ref>{{cite journal |author=Todt WL, Fallon JF |title=Posterior apical ectodermal ridge removal in the chick wing bud triggers a series of events resulting in defective anterior pattern formation |journal=Development |volume=101 |issue=3 |pages=501–15 |date=1 November 1987|pmid=3502993 |url=http://dev.biologists.org/cgi/pmidlookup?view=long&pmid=3502993 |issn=0950-1991}}</ref>
== Patterning==
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