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Figure 2. Characterization of the Cell Cycle, Rate of DNA Synthesis, Rate of RNA Synthesis and Ceil Motility at the MBT (A) Length and synchrony of the cell cycle during early embryogenesis was measured from time-lapse videotapes of dividing dissociated blastomeres made during the first 10 hr after fertilization. The cell cycles of individual cells from different parts of a dissociated embryo were measured over a 4-5 hr period and found to be 35 + 2 min long until cleavage 11. At the MBT (cleavage 12) the cell cycle times of different cells appeared to differ substantially. However, daughter cells originating from a common ancestor at the MBT appeared to remain synchronized for at least 2-3 hr after MET. Lines: average time of these individual cell cycles. Brackets: variation in length of time among individual cell cycles. The rate of DNA synthesis was measured by continuous labeling of embryos during the first 10 hr of development in 0.5 mCi 3H-thymidine in 0.5 ml MMR at 23°C. Eggs (ten) were collected at various stages, and DNA was isolated as described in the Experimental Procedures. As expected for a synchronized, exponentially growing population of cells, the logarithm of the amount of TCA-precipitable cpm increases linearly until the MBT. From the slope of this line, one obtains average values for the cell cycle of 35 min for the period leading up to the MBT, in good agreement with the data collected via time-lapse video recording of dissociated embryos. (open circle) Average cleavage period as a function of cleavage number; (solid circle) DNA synthesis as a function of time. (B) Onset of cell motility and RNA synthesis at the MBT. Dissociated embryos were observed via time-lapse video recording, and cells were counted as motile if they exhibited either cytoplasmic blebbing or pseudopod formation. To quantitate this parameter accurately at stages past the MBT it was necessary to dissociate large clumps of cells by either gently bumping the dish in which they lay or carefully moving the embryo once or twice with a wide bore siliconized Pasteur pipette. During this procedure most of the largest vegetal cells (about 5% of the total cells) were broken, The onset of motility and percentages of cells motile during these stages did not differ when the embryos had been injected with 50 pg/ ml cv-amanitin (final concentration in the egg) at the single-cell stage. The rate of RNA synthesis as a function of cleavage stage was quantitated by counting the number of 3H-dependent grains per nucleus. After autoradiography, 50-l 00 cells from embryos at different stages were counted. The number of grains per nucleus of different nuclei from the same embryo varied as much as threefold. The cleavage stage and elapsed time are given in the abscissa. (open circle) RNA synthesis: (solid circle) cell motility. Image published in: Newport J and Kirschner M (1982) Copyright © 1982. Image reproduced with permission of the Publisher, Elsevier B. V. Larger Image Printer Friendly View

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