The apical ectodermal ridge is the most distal rim of the epithelium of the limb bud. The proximal to distal elongation is produced by the apical ectodermal ridge. The apical ectodermal ridge is the first center to appear. It basically directs the longitudinal growth of the limb. Insult to the apical ectodermal ridge may cause congenital amputation. The mesodermal cells that are underlying the apical ectodermal ridge (rapidly diving cells) make the limb elongate. This is called the progress zone. It controls the dorsal/ventral axis growth. The dorsal/ventral growth is done through a non-apical ectodermal ridge limb ectoderm.
The Wnt genes control the dorsal/ventral growth and the Wnt gene is expressed in the dorsal part of the progress zone (responsible for making the finger nails). Enlargement of the limb bud occurs
due to interaction between the apical ectodermal ridge and the progress zone. The zone of polarizing activity (ZPA) is a mesenchyme tissue, just below the apical ectodermal ridge, along the proximal posterior limb bud. It secretes the sonic hedgehog protein (SHH). The zone of polarizing activity (ZPA) is a major signaling center for the regulation of the anterior/posterior patterning. It will give us the ulnoradial limb growth.
Remember! The thumb is up (anterior) and the little finger is posterior, initially. When there is a loss of zone polarizing activity, there will be a loss of the posterior elements or ulnar digits. When there is an up regulation of the zone of polarizing activity, this will result in additional posterior elements, such as polydactyly on the ulnar side, and the concentration of SHH will be high. If there is duplication of the zone of polarizing activity, 
there may be duplication of the posterior element (little finger or on both sides of the thumb).
In general, there is a higher concentration of SHH on the ulnar side (posterior side) for small finger development and there is a lower concentration of the radial side (anterior side) for thumb development.
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