Autoregulation of Shh expression and Shh induction of cell death suggest a mechanism for modulating polarising activity during chick limb development.

The polarising region expresses the signalling molecule sonic hedgehog (Shh), and is an embryonic signalling centre essential for outgrowth and patterning of the vertebrate limb. Previous work has suggested that there is a buffering mechanism that regulates polarising activity. Little is known about how the number of Shh-expressing cells is controlled but, paradoxically, the polarising region appears to overlap with the posterior necrotic zone, a region of programmed cell death. We have investigated how Shh expression and cell death respond when levels of polarising activity are altered, and show an autoregulatory effect of Shh on Shh expression and that Shh affects cell death in the posterior necrotic zone. When we increased Shh signalling, by grafting polarising region cells or applying Shh protein beads, this led to a reduction in the endogenous Shh domain and an increase in posterior cell death. In contrast, cells in other necrotic regions of the limb bud, including the interdigital areas, were rescued from death by Shh protein. Application of Shh protein to late limb buds also caused alterations in digit morphogenesis. When we reduced the number of Shh-expressing cells in the polarising region by surgery or drug-induced killing, this led to an expansion of the Shh domain and a decrease in the number of dead cells. Furthermore, direct prevention of cell death using a retroviral vector expressing Bcl2 led to an increase in Shh expression. Finally, we provide evidence that the fate of some of the Shh-expressing cells in the polarising region is to undergo apoptosis and contribute to the posterior necrotic zone during normal limb development. Taken together, these results show that there is a buffering system that regulates the number of Shh-expressing cells and thus polarising activity during limb development. They also suggest that cell death induced by Shh could be the cellular mechanism involved. Such an autoregulatory process based on cell death could represent a general way for regulating patterning signals in embryos.