Figure 3.

Models and mechanisms for SHH-mediated AP limb axis patterning (A) The early limb bud is already prepatterned by an antagonistic interaction between HAND2 (orange) and the repressor form of GLI3 (GLI3R, dark blue) transcription factors. Nested expression of 5′Hoxd genes and HAND2 participate in activation of Shh expression. (B) Skeletal preparation of a Shh deficient mouse limb at birth. (C) Spatial gradient model. Diffusion of the SHH peptide secreted by the ZPA generates a GLI3R gradient across the limb bud (graded blue) by inhibiting processing of full-length GLI3. The red line indicates the threshold values predicted by Wolpert's French flag model (Fig. 1E). (D) Temporal gradient model. Descendants of Shh expressing ZPA cells contribute to the progenitor domains of digit 3 to 5. Cells having expressed Shh for a short time contribute to digit 3, whereas the progenitor domains of digits 4 and 5 contain cells having expressed Shh for progressively longer times. Progenitors forming digit 2 and parts of digit 3 are specified by long-range SHH signaling. (E) Genetic analysis of the temporal requirement of SHH in the mouse shows digit identities are specified early. Subsequently, SHH is required for proliferative expansion of progenitor pools and determination of specified identities. Determination of digit identities in the mouse occurs in the following sequence: digit 4 (first), 2, 5, and 3 (last).