Cell-cell contacts accelerate cell colony growth in sparse cultures of chick embryo fibroblasts.

There is no cell proliferation in very sparcely plated chick embryo cell cultures. Substituting conditioned medium or adding of ethanol-fixed homologous cells to the cultures accelerates cell colony growth. The mechanism for the mitogenic action of fixed cells is considered to be the contact stimulation of cell proliferation, and addition of extra cells to sparse culture is believed to mimic the cell micro-environment existing in subconfluent cultures. The role of diverse cell-cell contacts in cultured cell growth regulation is discussed. The procedure used (addition of ethanol-fixed cells) may improve normal cell cloning techniques.