Fig. 2. Expression of FGFs and FGF receptors in the developing autopod during the stages of interdigital cell death. (A) Expression of Fgf8 in the AER at stage 31. At this stage interdigital apoptosis is present in the second and third interdigits. (B) Expression of Fgf12 at stage 29. This member of the FGF family is expressed in the progress zone mesoderm and in the interdigital regions showing a predominant distribution in the posterior part of the autopod. (C,D) Expression of FGFR1 at stages 30 (C) and 29 (D). This receptor is predominantly expressed in the surface of the condensing mesenchyme of the digital rays and in the developing tendons, but a low level of expression is also present in the undifferentiated mesenchyme. (E-G) Expression of FGFR2 in the developing autopod. (E) Whole-mount in situ hybridization after short digestion with proteinase K (20 minutes) in a stage 30 autopod showing the ectodermal expression of FGFR2 in the ectoderm. (F) Tranverse and (G) longitudinal sections of the autopod at stage 31. Note the intense expression of this receptor in the interdigital mesoderm. (H-M) Expression and regulation of FGFR3 in the developing autopod. (H) Longitudinal and (I) transverse sections of the autopod at stage 30 showing the intense expression of this receptor in the diferentiating cartilage and in the interdigital regions. (J-M) Whole-mount in situ hybridization showing the regulation of FGFR3 after interdigital application of BMP7 (K) and FGF2 (L,M). (J) Control autopod at stage 30. (K) Experimental autopod 6 hours after implantation of a BMP-bead. Note the intense up-regulation of this receptor in the treated interdigit. (L,M) Experimental autopodes 10 hours (L) and 20 hours (M) after implantation of an FGF-bead (arrow). Note that FGFR3 is initially down-regulated (L) and then up-regulated (M) by the application of FGFs.