Fig. 8. Model for BMP and FGF interaction during interdigital PCD in normal and mutant mouse limb development. During normal development (left), BMP signaling from the interdigit region signals to the BMPR1A in the AER, causing downregulation of Fgf8. FGF8 normally prevents apoptosis in the interdigit region; therefore its downregulation allows interdigit PCD to occur. In Msx2-Cre; BmpR1A^flox/null forelimb buds (right), BMPR1A is absent, Fgf4 and Fgf8 expression persists and abnormal cell survival occurs.