Figure 1. Dose-dependent Gli3-5′Hoxd interaction regulates digit joint formation.

(a–e) Gdf5 RNA expression (E13–13.5) in early interzones in control (Gli3^+/−) and different mutant genotypes (indicated to left); interzones are expanded in Gli3^−/−, absent in 5′Hoxd^−/− digit condensations and more normal width in Gli3^−/− upon 5′Hoxd gene dosage reduction. (d,e) are slightly later stage than panels (a–c). (f–j) E17.5 skeletal stains show digit morphologies in corresponding control and different mutant genotypes with comparable changes in digit joints. Brackets in insets to left show digit joint regions examined in k–m, and sites of joint restoration (arrowheads) in compound mutants in i,j (n=10/10 for 5′Hoxd^−/−;Gli3^−/−; n=16/16 for 5′Hoxd^+/−;Gli3^−/−). Note that 5′Hoxd^−/−;Gli3^−/− digits are all biphalangeal; the internal phalanx is designated ‘P?' to indicate uncertain identity (P1 versus P2). (k–m) Sox9 immunofluorescence (green) on sections of E17.5 digit cartilage elements from control, Gli3^−/− and 5′Hoxd^−/− embryos also expressing Gdf5Cre and RosaTdTomato reporter alleles³⁵ to mark interzone descendant cells (red). Scale bar, 100 μm for k–m. P1, 2, 3, phalanx 1, 2, 3; MC, metacarpal.