A New Method of Labeling Cells

A root meristem proves, on observation, to consist of a large number of cells with their nuclei in interphase and a smaller number of other cells in process of division. The division phases of the latter represent every possible stage in the course of mitosis, and the nuclear volumes of the former every possibility between telophase and prophase. In other words, a meristem is composed of cells at different stages of development, with the most obvious lack of any synchronization among them. And yet, there are a certain number of cells which happen, at any given moment, to be at the same stage of mitosis. If these cells could be labeled, we should have a group of cells showing a high degree of synchronization picked out among the whole meristematic colony; and, in fact, some such way of labeling cells was adopted to some extent by Van't Hof, Wilson, and Colon (11) by means of 30-minute treatments with colchicine. This labeling is done by polyploidizing a certain proportion of the cells and watching to see when they come round again to the same step in their metaphase as in the previous division. The observation that thymidine is a highly selective nucleoside for D N A (2) has led several authors to use tritiated thymidine for labeling cells in an asynchronous population during inter-phase (9, 12). Another form of labeling, in the wide sense, is afforded by the experimental synchronization of divisions, since all the cells initiate the process simultaneously. In practice, experimental synchronization of division began with the study, based on sublethal thermic shocks, carried out by Scherbaum and Zeuthen (7) on the ciliate Tetra-hymena. The thermic shocks, one after another at appropriate moments, reduce the time-differences, and eventually bring about the synchronized division of a large proportion of the cells composing the population. Methods similar to the foregoing have led to the partial synchronization of the development of mammalian cells when cultures of these were subjected to temperatures of 4 ° for 1 hour (5). Thermic shocks and/or changes of light have likewise brought about the synchronization of cell divisions in the case of Chlorella and other photo-synthetic organisms (1, 4, 6, 8). In a former communication (3), we reported the formation and development of binucleate cells in roots, pointing out that the effect of caffeine on cell division is highly specific as regards the inhibition of cytokinesis. Treatments …