Pattern formation: Old models out on a limb

The vertebrate limb is an excellent model for studying fundamental aspects of embryonic development. Cell proliferation, death and movement, and the assignment and interpretation of positional information, must be coordinated if an exquisitely patterned limb is to form. Recent results from gene targeting in mice and from experimental manipulation of the chick embryonic limb have significantly altered the way in which developmental biologists have conceptualized limb patterning.