FIBROBLAST BEHAVIOR IN THE EMBRYONIC CHICK HEART

Intracardiac fibroblasts (mesenchymal cells) of Hamburger and Hamilton stage 36 chick heart reside in the epicardium and atrioventricular va lves. The characteristics of the epicardial fibroblasts include segreg ation from the myocytes of the heart wall myocardium, voluminous extra cellular matrix production, and some cell proliferation activity. The atrioventricular fibroblasts intermingle with myocytes at the mutual b order between these tissues, produce smaller amounts of extracellular matrix, and show very active cell proliferation. Is the behavior of ea ch population of fibroblasts predetermined or is each responding in a reversible fashion to local environment? A cell aggregate culture syst em, which permits 3-dimensional cell-cell and cell-matrix interactions , is used to study the behavior of each isolated population of fibrobl asts in vitro. In the presence of serum-free medium, each population p roduces very little extracellular matrix, has relatively low mitotic a ctivity, and does not segregate from myocytes when the aggregate is co mposed of randomly intermixed myocytes and fibroblasts. In the presenc e of chicken serum, each population increases matrix production, incre ases cell proliferation, and sorts from myocytes. Thus, we suggest tha t the two populations of fibroblasts in the developing heart are respo nding to local environments and the differences observed in vivo are n ot the consequence of irreversible states of cellular differentiation. (C) 1993 Wiley-Liss, Inc.