Fig. 7. Myocardial differentiation in cultured secondary heart field is inhibited by noggin. All the panels show secondary heart fields cultured for 24 hours, incubated with BrdU for 1 hour and then doubly stained to show fluorescein-labeled antiBrdU (green) and rhodamine-labeled MF-20 (red). Medium was conditioned as indicated above the panels: A,E, unconditioned medium; B,F secondary heart field-conditioned medium; C,G, pharyngeal endoderm/ectoderm-conditioned medium; and D,H, myocardium-conditioned medium. The heart fields shown in the lower panels were cultured in the conditioned medium indicated with 15 µg/ml noggin protein added. Noggin appears to have decreased myocardial differentiation and increased proliferation, except in the secondary heart field cultured in secondary heart field conditioned medium (B,F). The endoderm/ectoderm conditioned medium caused increased proliferation in both the absence (C) and presence (G) of noggin.