Fig. 3. Severe AV cushion defects in Bmp2 CKO mutant embryos. Parasagittal sections through heart of wild type (A,B) and Bmp2 CKO mutant (C,D) at 9.5 dpc, showing that mesenchymal cells are absent in the AV cushion (arrows). Panels B and D are high power images of A and C. (E,F) Parasagittal sections at 10.5 dpc, showing lack of AV cushion mesenchyme in the Bmp2 CKO mutant (arrows). Also note the severely compromised myocardium in the mutant heart. (G-J) In-situ hybridization indicated that Msx2 is expressed in the AV myocardium and migrating mesenchymal cells (arrow, G), but its expression is diminished in the mutant (arrow, H). Panels I and J are parasagittal sections of embryos in G and H, showing lack of Msx2 expression in the mutant (arrows). (K-N) Has2, encoding an extracellular matrix protein, is present in the AV myocardium (arrow) and migrating mesenchyme (arrowhead) in the 9.5 dpc wild-type embryos (K,M) but is reduced in the Bmp2 CKO myocardium (L,N). M and N are parasagittal sections of K and L. a, atrium; ba, branchial arch; v, ventricle.