EXPRESSION OF SMOOTH-MUSCLE ALPHA-ACTIN IN MESENCHYMAL CELLS DURING FORMATION OF AVIAN ENDOCARDIAL CUSHION TISSUE - A ROLE FOR TRANSFORMING-GROWTH-FACTOR BETA-3

During early cardiac morphogenesis, outflow tract (OT) and atrio-ventr icular (AV) endothelial cells differentiate into mesenchymal cells, wh ich have characteristics of smooth muscle-like myofibroblasts, and whi ch form endocardial cushion tissue, the primordia of valves, and septa in the adult heart. During this embryonic event, transforming growth factor beta 3 (TGF beta 3) is an essential element in the progression of endothelial-transformation into mesenchyme. TGF beta s are known to be a potent inducer for mesodermal differentiation and a promoter for differentiation of endothelial cells into smooth muscle-like cells. U sing a monoclonal antibody against smooth muscle-specific alpha-actin (SMA), me examined the immunohistochemical staining of this form of ac tin in avian endocardial cushion tissue formation. To determine whethe r TGF beta 3 initiates the expression of SMA, the pre-migratory AV end othelial monolayer was cultured with or without chicken recombinant TG F beta 3 and the expression of SMA was examined immunochemically. Migr ating mesenchymal cells expressed SMA beneath the cell surface membran e. These cells showed a reduction of endothelial specific marker antig en, QH1. Stationary endothelial cells did not express SMA. The deposit ion of SMA in the mesenchymal tissue persisted until the end of the fe tal period. Pre-migratory endothelial cells cultured in complete mediu m (CM199) that contained TGF beta 3 expressed SMA, whereas cells cultu red in CM199 alone did not. At the onset of the endothelial-mesenchyma l transformation, migrating mesenchymal cells express SMA and the expr ession of this form of actin is upregulated by TGF beta 3. The inducti on of the expression of SMA by TGF beta 3 is one of the initial events in the cytoskeletal reorganization in endothelial cells which separat e from one another during the initial phenotypic change associated wit h the endothelial-mesenchymal transformation. (C) 1997 Wiley-Liss, Inc .