Figure 3. Overlapping expression of Crtl1, versican, and hyaluronan in the embryonic mouse heart at ED13.0.

This figure shows the expression of Crtl1, versican and HA in the respective mesenchymal cell populations in the developing heart at ED13.0. Panels A–D and panels I and J show sections of the OFT. Panels E–H and panels K and L show sections of the AV mesenchymal structures. Panels A and E show Crtl1 expression (green), panels C and G show versican staining (red), and panels D and H show hyaluronan staining (purple). Furthermore, panels B, F, I, J, K and L are sections from Tie2-cre/ROSA26 embryos in which endocardially-derived cells are expressing lacZ (blue). The sections in panels I and K are co-stained immunohistochemically for Crtl1 (brown), while the section in J is stained for sarcomeric actin. Combined, panels A, B, I, and J demonstrate that Crtl1 expression is largely confined to the endocardially-derived mesenchyme in the conal cushions and developing semilunar valves. The (proximal) truncal ridges (arrows in A and B) do not express Crtl1. Panels C and D show that in the conal cushions and semilunar valves, the expression of Crtl1, versican and HA overlap. Additionally, versican and HA are also expressed in those parts of the truncal ridges where Crtl1 is absent. Panels E, F, K, and L show that in the AV mesenchymal structures, Crtl1 expression is also restricted to the mesenchymal structures that are of endocardial origin. Thus, Crtl1 is expressed in the inferior, superior, and lateral AV cushions, but not in the dorsal mesenchymal protrusion, which forms a non-endocardially derived mesenchymal wedge in between the inferior and superior cushions (M). Panels G and H show that the DMP expresses lower levels of versican and HA than surrounding mesenchyme. AVC=atrioventricular cushions, sAVC=superior AVC, iAVC=inferior AVC, llAVC=left lateral AVC, rlAVC=right lateral AVC, PAS=primary atrial septum, DMP=dorsal mesenchymal protrusion.