Fig. 1. Spatiotemporal expression of Hoxa13 correlates with sites of malformation and decreased BMP expression. (A-D) Analysis of Hoxa13 expression by in situ hybridization using an exon 1-specific riboprobe. Hoxa13 expression is present in the distal interdigital mesenchyme and peridigital tissues (A, arrowheads). (C) Hoxa13 localizes to the distal joint/nail bed in E14.5 limbs (arrowheads). (B,D) In homozygous mutants, elevated levels of Hoxa13 exon 1 transcripts were detected throughout the autopod. (E,F) Alcian Blue staining of E15.5 mutant limbs reveals defects in distal digit separation (F, arrows) and chondrogenesis (arrowheads), when compared with wild-type controls. Pi, pisiform carpal element. (G-J) Bmp7 expression is reduced (arrows) in the distal interdigital tissues at E12.5 (G,H) and in the peridigital tissues at E13.5 (I,J) of homozygous Hoxa13 mutants, when compared with age-matched wild-type controls (arrowheads). (K-N) Bmp2 expression is reduced (arrows) in the interdigital tissues of E12.5 Hoxa13 mutant embryos, and in the distal joints/nail beds of E14.5 Hoxa13 mutants, when compared with wild-type controls (arrowheads). (O-R) Msx2, a target of BMP signaling, exhibits reduced interdigital expression in E12.5-E13.5 mutant limbs (arrows) compared with age-matched controls (arrowheads). (S,U) BMP7 (red, arrowhead) and HOXA13-GFP (green) co-localize (yellow cells in U and V) in the interdigital tissues of E12.5 limbs. (T,V) Age-matched homozygous mutants exhibit reduced numbers of BMP7-positive cells in the same interdigital regions (arrow), a finding consistent with the reduced Bmp7 transcripts in these same tissues (compare H and T). Scale bar: 50 µm.