Figure 4.

The role of BMP signaling from the interdigital mesenchyme in determination of digit identities. Graded BMP signaling from the interdigital (ID) mesenchyme (blue) in the chicken foot primordia is involved in determining the identities of digits 1 to 4 at late developmental stages. The distal phalanx of individual digits form from the sub-AER mesenchyme, which is therefore called phalanx forming region (PFR). The activity of phosphorylated SMAD (pSMAD) proteins, which are the intracellular mediators of BMP signal transduction, is graded within the PFR (green), such that each digit has its characteristic pSMAD activity signature. Note that the pSMAD activity in the PFR of the posterior-most digit 4 is lower than the one of digit 3. AER is indicated in red.