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. Author manuscript; available in PMC: 2008 Aug 15.
Published in final edited form as: Dev Biol. 2007 May 31;308(2):343–354. doi: 10.1016/j.ydbio.2007.05.030

Figure 7.

Figure 7

Proposed model of digit specification incorporating mouse mutant analysis. Digit specification in wildtype (A) mouse limb buds as previously detailed (see Figure 1). In Prx1-Cre mutant limb buds (B), early diffusion of Shh allows specification of digit 2 while in the posterior, only digit 3 can be specified before Cre activity abolishes all Shh signaling by E11.0. In contrast, continuous but low level Shh signaling observed in Shh-Cre limb buds (C) is not sufficient to specify digit 2; Posterior digits are however specified with time despite significant reduction of Shh levels. Together, mouse mutant analysis supports the proposed model of digit specification whereby digit 2 is most sensitive to the concentration of Shh while for posterior digit specification, the length of Shh exposure is the more critical parameter.