Characterization of isolated fibroblast populations. Isolated breast fibroblast populations were characterized by immunofluorescent staining. All cells exhibited staining for vimentin (a) but were uniformly negative for the luminal epithelial membrane antigen (b), cytokeratin 18 (c), the endothelial-associated CD31 (d), and pericyte-related α1-integrin (e). Approximately 1% of cells stained strongly for cytokeratin 14 (f, inset), and approximately 35% of cells were positive for alpha-smooth muscle actin (g). (h) Negative control.