In normal cultures, the transformation of monocytes into fibroblasts generally occurred when cells became packed together through some mechanical factors that prevented their free migration and determined their accumulation. Various modifications of the medium, the addition of dead tissue, and of trypsin or the products of trypsin digestion, failed to bring about the transformation. The inoculation af cultures of monocytes with filtered extract of Rous sarcoma frequently determined the appearance of fibroblasts. The first change undergone by the monocytes cultivated in vitro was a large increase in their size. Later, the giant monocytes became transformed into cells that did not differ essentially from those that grow from a fragment of adult connective tissue.

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