nkx genes limit atrial cell number during heart tube assembly. (A,B) Immunofluorescence indicates nuclear localization of Mef2 (green) throughout the heart tube, facilitating counting of differentiated cardiomyocytes at 26 hpf. Atrial cells are indicated by the anti-Amhc antibody, S46 (red). Hearts are flattened with a cover slip to improve visualization of cardiomyocyte nuclei. (C) Quantification of cardiomyocyte nuclei in wild-type (n = 25) and nkx-deficient (n = 11) embryos reveals a statistically significant increase in atrial cell number in nkx-deficient embryos (p < 0.001, Student's t-test) and no significant difference in ventricular cell number. Bar graph indicates mean and standard error of each data set; asterisk indicates statistically significant difference from wild-type.
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