An in vitro analysis of murine thymic reticular-fibroblasts.

We describe a clonal culture technique for assay of thymic reticular-fibroblast progenitors. The adherent stromal cells in the colonies were pleomorphic fibroblast-like cells with oval nuclei containing several nucleoli. These cells were also nonphagocytic and negative for 4 of 5 epithelial markers employed. A minor population (less than 5%) were positive for the fifth epithelial marker ER-TR4. The majority of cells in all colonies were positive for the connective tissue marker ER-TR7. Treatment of mice with bacterial lipopolysaccharide, a mobiliser of splenic and bone marrow and reticular-fibroblast progenitors into circulation, failed to alter thymic clonogenic stromal cell numbers. These observations suggest that the colonies formed by thymic stromal progenitors consist of reticular-fibroblasts and that these progenitors are isolated from the fibroblast progenitor populations of other hemopoietic organs.