Plxna3 is a candidate receptor for Sema3fb. A, B myh6-expressing atria of control (A) and plxna3 (B) morpholino-injected 48 hpf embryos (N = 1 WT n = 5/5 normal; plxna3 MO, n = 9/15 small atria). White line indicates the morphological constriction between the atrium and ventricle, and arrow points to ectopic ISH label in the ventricle. C, D bmp4a expression in control (C) and plxna3 MO (D) embryos. Ectopic ISH label in the ventricle (arrow) and atria (arrowhead). E tnnt2a shows a sharp reduction at the AVC in a control (n = 8/8) but not a plxna3 MO (n = 7/10 imprecise) embryo. F Mean atrial area is reduced in plxna3 MO embryos (N = 2; p < 0.0001, n = 17) as compared to controls (n = 13). G-I Average heart rate (p = 0.011, G), cardiac output (p = 0.095, H), and ejection fraction (p = 0.18, I) (N = 3, n = 9 controls, n = 10 plxna3 MO). J-L Cell size of DM-GRASP + ventricular cardiomyocytes. A significant reduction is seen in cardiomyocyte area (J, p = 0.019) and perimeter (K, p = 0.015) in plxna3 MO as compared to control embryos, but not circularity (L, p = 0.15). N = 3, n = 6 control and n = 11 plxna3 MO embryos, with 8–10 cells/embryo. Scale bar in A is 50 µm
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