Polarizing Region Tissue Grafting in the Chick Embryo Limb Bud.

The polarizing region of the developing limb bud is an important organizing center that is involved in anteroposterior (thumb to little finger) patterning and has three main functions that are now considered to depend on the secreted protein Sonic hedgehog (Shh). These are (1) specifying anteroposterior positional values by autocrine and graded paracrine signaling; (2) promoting growth in adjacent mesenchyme; (3) maintaining the distal epithelium that is essential for limb outgrowth by induction of a factor in adjacent mesenchyme. The polarizing region was identified using classical tissue grafting techniques in chicken embryos. Here we describe this procedure using tissue from transgenic Green Fluorescent Protein-expressing chicken embryos that allows the long-term fate of the polarizing region to be determined. This technique provides a highly useful and effective method to understand how the polarizing region patterns the limb and has implications for other organizing centers.