EXPERIMENTAL FIGURE 21-32 A web-footed chicken. During the development of many vertebrate limbs, cells in the soft tissue between the embryonic digits undergo programmed cell death. In the chicken foot, this process leads to the formation of four separate toes (left). During chicken foot development, bone morphogenetic proteins (BMPs) (members of the TGF-β superfamily of hormones; see Figure 16-3) are expressed by interdigital cells and induce apoptosis. In this experiment, a dominant-negative type I BMP receptor was expressed in a developing chicken foot, blocking BMP signaling and preventing the programmed cell death that normally occurs. This manipulation allowed the survival of cells that then divided and differentiated into a web (right). The similarity of this webbing to webbed duck feet led to studies showing that BMPs are not expressed in duck interdigital cells. These results indicate that BMP signaling actively mediates cell death in the embryonic limb.

[Republished with permission of AAAS, from Zou, H. and Niswander, L., “Requirement for BMP signaling in interdigital apoptosis and scale formation,” Science, 1996, 3;272(5262):738–41; permission conveyed through Copyright Clearance Center Inc.]