2 DISTINCT ADHESION MECHANISMS IN EMBRYONIC NEURAL RETINA CELLS .1. A KINETIC-ANALYSIS

The reaggregation kinetics of embryonic chick neural retina cells, prepared using several different dissociation procedures were monitored through decreases in the small-angle light scattering of aggregating samples. Distinct modes [2] of aggregation were revealed, one Ca2+ independent, the other Ca2+ dependent, suggesting the existence of 2 separate adhesion mechanisms. By varying the concentrations of Ca2+ and trypsin in the dissociation medium, cells which exhibited both, either or neither mode of aggregation were obtained. The Ca2+-independent adhesiveness is active in the absence of proteolysis, is resistant to low levels of trypsin (0.001%) but is readily inactivated at higher trypsin concentrations in the presence or absence of Ca2+. It is relatively temperature independent. The Ca2+-dependent adhesiveness is not detected before exposure of cells to proteolysis. It is expressed after tryptic proteolysis in the presence of Ca2+ and is then highly temperature dependent. It is resistant to further digestion by trypsin in the continued presence of Ca2+, but is lost when Ca2+ is subsequently removed, apparently through the expression of tryptic cleavage incurred earlier. Its increased activity may result, at least in part, from the clustering of surface components into adhesive patches. A provisional model correlating these data is presented.