Abstract
We have studied the behaviour of normal animal cells cultivated in vitro in the presence of Myleran for variable periods ranging from a few hours to 18 days. The cytological modifications produced by this substance have already been described previously. The present report deals essentially with the study of DNA which we have carried out by means of quantitative cytophotometry of individual cells after Feulgen reaction as well as by histoautoradiography after the incorporation of tritiated thymidine. The Myleran profoundly modifies the metabolism of DNA. It provokes an increase of positive Feulgen material in the interkinetic nuclei. This effect becomes more and more marked with time: nuclei with tetraploid value are more frequent, octoploid and even hexadecaploid nuclei appear and become progressively more numerous. Nevertheless, it does not appear that the number of genoms increases, in spite of the possibility of eventual fragmentation, The frequency of mitosis is very low. Under the influence of Myleran there is thus a dissociation between the frequency of tetraploid values of DNA and that of the mitoses. In cultures having been treated for a sufficient time, the cytophotometric estimations of mitotic figures (two wavelength method) show that a certain proportion of mitoses: prophases, metaphases, etc., reach octoploid DNA values, contrary to what happens in the controls. Fibroblasts having this peculiarity are thus capable of undergoing mitosis and to complete it. The histoautoradiographies clearly show that tritiated thymidine is incorporated by the nuclei even if the mitotic activity has been reduced. The level of this incorporation is related to the increase of frequency of nuclei with high DNA content. The cells therefore remain able to synthesize DNA even after long action of Myleran. A hypothesis is proposed to explain the observed facts. According to this, the cell after each mitosis, synthesizes a fresh quantity of deoxyribonucleic acids to compensate for those which have been affected by the Myleran. It will then undertake the preprophase synthesis in readiness for the subsequent mitosis by doubling the quantity of DNA then present in the nucleus. In this way the amount of DNA in the nuclei will increase in time. In addition, under certain aspects, our experiments suggest that the synthesis of DNA does not take place following classical concepts (i.e. from a “model”, by duplication of existing material) but following a different process, from a “plan”.
Published Version
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