Numerous metabolic studies have demonstrated heterogeneity of fibroblast populations in culture, yet little is known about the structure of fibroblast populations in adult tissues in vivo. To determine if populations of both cycling and non-cycling cells are present in gingiva, hamsters were labelled with 3H-thymidine to label cycling cells in vivo, and explanted biopsies were subsequently incubated with bromodeoxyuridine to label cycling cells in vitro. Cycling cells were identified by combined immunohistochemistry and radioautography. Fibroblasts were recognized by the presence of vimentin and the absence of keratin as determined by immuno-fluorescence. The largest proportion of cells were double-labelled with 3H-thymidine and bromodeoxyuridine (43.8) indicating the presence of actively cycling populations that maintained their proliferative status upon explantation. Cultures also exhibited a second population of cells labelled only with bromodeoxyuridine (38.7) that did not cycle in vivo, but retained the capacity for proliferation in vitro. However, limiting dilution analysis of single-cell suspensions revealed only a single class of progenitors capable of forming large colonies in vitro. Approximately 1 in 190 plated cells was capable of colony-formation, indicating that, upon explantation, a subset of the cycling cells in vitro exhibits extensive proliferative capacity. There was also a small population of cells unlabeled with either 3H-thymidine or bromodeoxyuridine (9.4) that appeared to be terminally differentiated. Different substrates, including glass and thin films of gelatin and collagen, did not significantly alter the fraction of cells labelled with 3H-thymidine. These data demonstrate the existence of 2 separate progenitor-cell populations with different capacities for proliferation in vivo and in vitro.
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