Neoplasia, aplasia, and metaplasia are problems of great importance in medicine today. The mechanism of development of diseases involving these processes is unknown, and treatment is unsatisfactory. Perhaps this is not surprising since the mechanism of control of normal cell destruction and renewal is also obscure. The development of tritium-labeled thymidine, a specific deoxyriboside precursor of deoxyribonucleic acid (DNA), by W. L. Hughes (1) provided a powerful tool for the in vivo and in vitro study of cell turnover in man and animals. Individual cell histories can be followed easily using histoautoradiographic techniques (2). A review of earlier studies on protection against radiation-induced aplasia of the marrow (3) led to the hypothesis that totipotential primitive mesenchymal cells may be circulating 3.
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