EXPRESSION OF HOMEOBOX GENES MSX-1 (HOX-7) AND MSX-2 (HOX-8) DURING CARDIAC DEVELOPMENT IN THE CHICK

The vertebrate homeobox genes Msx-1 and Msx-2 are related to the Droso phila msh gene and are expressed in a variety of tissues during embryo genesis. We have examined their expression by in situ hybridisation du ring critical stages of cardiac development in the chick from stages 1 5 + to 37. Msx-1 expression is apparent in a number of non-myocardial cell populations, including cells undergoing an epithelial to mesenchy mal transformation in the atrioventricular and the outflow tract regio ns that play an integral role in heart septation and valve formation. Msx-2 expression is restricted to a distinct subpopulation of myocardi al cells that, in later stages, coincides morphologically with the car diac conduction system. The timing of Msx-2 expression suggests that i t plays a role in conduction system tissue formation and that it ident ifies precursor cells of this specialised myocardium. The pattern of M sx-2 expression is discussed with reference to current models of condu ction tissue development. (C) 1993 Wiley-Liss, Inc.