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*Proliferation of the lateral plate mesoderm cannot sustain entire limb growth; instead the '''apical ectodermal ridge (AER)''' maintains growth.
 
*Proliferation of the lateral plate mesoderm cannot sustain entire limb growth; instead the '''apical ectodermal ridge (AER)''' maintains growth.
 
**The AER develops as mesenchymal cells cause cells of the overlying ectoderm to change their morphology from cuboidal to columnar.
 
**The AER develops as mesenchymal cells cause cells of the overlying ectoderm to change their morphology from cuboidal to columnar.
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**It divides the limb into dorsal and ventral halves.
 
**The ridge can be seen when looking at the limb bud in a distal to poximal direction.
 
**The ridge can be seen when looking at the limb bud in a distal to poximal direction.
 
*The AER maintains an area of rapidly proliferating cells beneath it, called the '''progress zone (PZ)'''.
 
*The AER maintains an area of rapidly proliferating cells beneath it, called the '''progress zone (PZ)'''.
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