Fig. 2. Endocardial expression of GATA4 and GATA5 in the developing heart. Immunocytochemical staining of staged mouse embryo sections with anti-GATA4 and -GATA5 polyclonal antibodies. (A) Low magnification of E10.5 stained embryos reveals abundant expression of GATA4 in atrial (A) and ventricular (V) myocytes, in endocardial cells, and in some cells of the endocardial cushion (EC). GATA5 labeling is restricted to endocardial (E) and endocardial cushion cells both in the atrio-ventricular cushion and in the truncus arteriosis (TA), which will give rise to the outflow tract. Staining is absent in the myocardium (M). (B) Expression of GATA5 is transient in endocardial cells. Note how GATA5 staining is undetectable at E12.5 whereas GATA4 is still present both in endocardial and myocardial cells. Counterstaining with Eosin and Hematoxylin.