Patterning the embryonic heart: Identification of five mouse Iroquois homeobox genes in the developing heart

We isolated cDNAs of mouse Iroquois-related homeobox genes Irx1, -2 -3, -4, and -5 and characterized their patterns of expression in the developing he art. Irx1 and Irx2 were found to be expressed specifically in the ventricul ar septum from the onset of its formation onward. In fetal stages, the expr ession of both genes appeared to gradually become confined to the myocardiu m of the atrioventricular bundle and bundle branches of the forming ventric ular conduction system. Irx3 was found to be expressed specifically in the trabeculated myocardium of the ventricles. Irx4 expression was observed in a segment of the linear heart tube and the atrioventricular canal and ventr icular myocardium including the inner curvature after looping, resembling t he pattern of MLC2V. Transcripts for Irx5 were detected specifically in the endocardium lining the ventricular and atrial working myocardium that also expressed von Willebrand factor, but were absent from the endocardium of t he endocardial cushions, i.e., the atrioventricular canal, inner curvature, and outflow tract. The spatiodevelopmental pattern of Irx5 matched that of ANF, a marker for the forming working myocardium of the chambers. Taken to gether, all members of the Irx gene family were found to be expressed in hi ghly specific patterns in the developing mouse heart, suggesting a critical role in the specification of the distinct components of the four-chambered heart. (C) 2000 Academic Press.