A composite of images taken from figures in Snarr et al. Upper Left: Histological demonstration of endocardial (lacZ) and non-endocardial derived cells in the atrioventricular (AV) mesenchyme of a Tie2-Cre/R26R mouse heart at ED11.5. Upper Right: Superior view of an AMIRA 3D reconstruction of an ED11.5, Tie2-Cre/R26R heart. Colors distinguish the different mesenchymal components in the AV region. Center: Left lateral view of the ED11.5 reconstruction illustrating the relationship of the different mesenchymal components to the primary atrial septum (PAS). Bottom Left: Histological demonstration of endocardial (lacZ) and non-endocardial derived cells in the AV mesenchyme at ED13. Bottom Right: A 3D reconstruction demonstrating the spatial relationship of the different AV mesenchymal tissues at ED 13. See Snarr et al., Developmental Dynamics 236:1287–1294.