Role of the extracellular matrix in the control of cell motility in a model morphogenetic system.

One goal of students of embryonic morphogenesis is to provide an explanation at cellular and molecular levels of the factors that regulate the active migration of cells and tissues during embryogenesis. A useful model for investigation of this problem involves study of the factors controlling morphogenetic cell movement in cell aggregates in organ culture. Migratory behavior involves sorting out of tissues in random reaggregates and tissue spreading when two aggregates are cultured in apposition. The present study involves chick embryo heart tissue, the two principal cell types of which are the myocyte (70-80% of the cells) and the mesenchyme (20-30%). In cultured aggregates, the associative properties of heart tissue is dominated by the mesenchyme, apparently mediated by the ability of the mesenchyme, but not the myocytes, to deposit fibronectin in the extracellular matrix of the cultured aggregate. In paired combinations of aggregates, the aggregate type that contains less fibronectin, or that establishes a fibronectin-containing matrix slowly, spreads over the aggregate that establishes a fibronectin-rich matrix rapidly. In random reaggregates of myocytes and mesenchyme, sorting out of the two cell types occurs, but only under culture conditions that stimulate the deposition of a fibronectin-rich matrix by the mesenchyme. Contrary to the usual situation (Steinberg, 1970), the pattern of sorting (mesenchyme sorts external to the myocytes) differs from the pattern of spreading of apposed aggregates (the myocyte aggregate spreads over the mesenchyme aggregate). This has been explained as a consequence of a requirement for access to a component in the serum fraction of the culture medium for fibronectin deposition in the extracellular matrix of cultured cardiac mesenchyme.