Masking of extracellular collagen and the co-distribution of collagen and fibronectin during matrix formation by cultured embryonic fibroblasts.

Embryonic chicken heart fibroblasts (CHF) were examined by double immunofluorescence in order to study the interactions of collagen and fibronectin. Double-labelling studies of permeabilized whole mounts of CHF, cultured for 15 min to 2 days after plating, detected extracellular collagen at very early times (15 min to 1 h) but not at times later than 3 h. Abundant fibronectin was seen at all times. These observations suggested that the collagen/procollagen antigen was being "masked' by matrix components at the later times. This possibility was tested in two ways: (1) immunolabelling of frozen sections of multilayered 4-day CHF cultures, which showed extensive extracellular collagen superimposable with fibronectin. (2) Limited trypsinization of 4-day CHF cultures, which allowed collagen to be immunolabelled concomitantly with a partial decrease in the degree of fibronectin labelling. The results are discussed in terms of the secretion and assembly of collagen and fibronectin in cultures of embryonic fibroblasts.