Abstract
Treatment of embryonic chick heart fibroblast cultures with 0.2 M urea reversibly increases cellular overlap. The increase in cellular overlapping over that in control cultures may be quantitated by the overlap ratio (R), the ratio of the number of superimposed nuclei observed, to the number expected to occur when cells are assumed to be distributed randomly over the culture substratum (R = observed/expected overlaps). Reversal of the urea‐induced increase in R is blocked by 0.2 μg/ml cycloheximide. In the presence of cycloheximide, normal (low) overlap ratios are restored to urea‐treated cultures by adding non‐dialyzable material recovered by washing fibroblast monolayers with serum‐free medium. The overlap ratio assay revealed no effect of supernatant material added either to urea‐treated cultures in the continued presence of urea, or to untreated cultures. Although unfiltered supernatants were shown by SDS‐polyacrylamide gel electrophoresis to contain fibronectin (CSP; LETS; MW~appar.~ = 220,000 d) and smaller proteins, the ability to reverse the urea‐induced increase in overlap ratio was present in Diaflo and Millipore filtrates of culture supernatants in which fibronectin was greatly depleted or absent. In contrast, purified fibronectin preparations failed to lower urea‐induced increases in overlap‐ratio. Partially purified, biologically active supernatants, prepared from ^14^C‐leucine or ^125^I‐labeled cultures, contained several macromolecules smaller than fibronectin that were labeled by both radioisotopes. In particular, one band (MW~appar.~ = 58‐60,000 d) was present in polyacrylamide gels of active supernatant and also depleted in gels of homogenates from urea‐treated cultures. These results indicate that external macromolecules other than fibronectin are synthesized by culture fibroblasts and can affect cell social behavior or culture morphology.