The effect of divalent cations on cell-to-cell and cell-to-substrate adhesions was studied using two different types of chicken embryonic cells (fibroblastic and neural retina cells). Mn
2+, Co
2+, Mg
2+ and Ca
2+ were effective in promoting the aggregation of fibroblastic cells, while Sr
2+ and Ba
2+ were not effective. Among these ions, Ca
2+ showed some unique effects on cell aggregation. Aggregation was initiated immediately after the addition of Mn
2+, Co
2+ or Mg
2+ whereas with Ca
2+ a lag period was always observed. When the medium contained both Ca
2+ and Mg
2+, the aggregate size formed was bigger than that formed with either ion at an equimolar concentration. The divalent cation dependence of fibroblastic cell adhesion to non-cellular substrate was essentially the same as that of cell-to-cell adhesion. The aggregation of neural retina cells specifically depended on the presence of Ca
2+. For adhesion of neural retina cells to non-cellular substrate, however, Ca
2+ was not effective, and only Mn
2+ or Co
2+ was effective. These results suggest two different mechanisms in cell adhesion.
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