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2.
Concentration gradient.
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As suggested by the transplantation
experiments,
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the
polarizing activity stimulates differentiation of the
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mesenchyme
in a concentration dependent manner.
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This has been verified by using
fibroblasts that
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have
been transfected with Shh. Depending
on the
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number
of cells used to substitute for the ZPA different
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patterns
of digit formation occur.
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