Molecular regulation of atrioventricular valvuloseptal morphogenesis.

The majority of congenital heart defects arise from abnormal development of valvuloseptal tissue. The primordia of the valve leaflets and membranous septa of the heart are the cardiac cushions. Remodeling of the cushions is associated with a transitional extracellular matrix that includes sulfated proteoglycans and the microfibrillar proteins fibulin and fibrillin. Cushion formation is restricted to the AV canal and ventricular outflow tract regions of the primary heart tube. The proper placement of the cushions may be the result of the development of the primary heart tube as a segmented organ, as well as the subsequent looping of the heart. Segmentation of the heart tube may be demonstrated by the alternating molecular expression pattern along the longitudinal axis. In support of this hypothesis is the restricted expression of BMP-4 and msx-2 to the AV canal and ventricular outflow tract. The importance of looping for cushion positioning may imply that the iv and inv genes and retinoic acid are important for the proper patterning of the heart. The cells of the cushions evolve from endocardial cells that undergo an epithelial-to-mesenchymal transformation. This developmental event is regulated by the myocardium and is probably due to the production of protein complexes, present within the cardiac jelly of the cushion-forming regions, that consist of fibronectin and the ES proteins. Both the cushion mesenchyme and its endocardial cell antecedents express JB3, an ECM protein. JB3 expression is also featured within the heart-forming fields of the primary mesoderm, from which the endocardial progenitors of the cushion cells originate.(ABSTRACT TRUNCATED AT 250 WORDS)