Normal human leukocytes were cultivated in millipore diffusion chambers which had been implanted subcutaneously in autologous and homologous subjects. The observations were made over periods of a few days to six weeks. It was found that mature granulocytes underwent disintegration within one to two weeks. Mononuclear leukocytes underwent differentiation into macrophages and "polyblasts" within a few days and by three weeks had assumed the morphologic appearance characteristic of histiocytes and fibroblast-like cells. By four to six weeks extensive fibroblastic proliferation and marked collagen formation was found. In several chambers numerous fat cells were seen.

These in vivo studies demonstrate the mesenchymal potential for differentiation possessed by circulating mononuclear leukocytes of adult blood.

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