A requirement for reversible binding between aggregating embryonic cells before stable adhesion.

Chick embryonic liver and neural retina cells aggregate in a two-step process. Initially, cells formed a loose association in a step that apparently did not require metabolic energy. Cells bound in this manner were dissociable by mild shear forces or by simple dilution. The results of the dilution experiments suggest a readily reversible binding of single cells to form these types of aggregates. In a second step, which required metabolic energy, the cells became firmly, or stably attached. The formation of both types of bond was temperature-dependent. Kinetic studies indicated that the formation of reversible bonds between cells was required before the cells could become stably attached, and that reversibly bound cells were converted directly into stably bound cells.