The cell cycle data of early cleavages of echinoderms

The cell cycle data of before hatching of Hemicentrotus pulcherrimus was analyzed by 3 different methods for different phases of development. (1) During the first 5 cleavages when a high degree of division synchrony exists, chromatin staining is done to determine nuclear stages and autoradiography for measurement of the length of the S phase. At this stage, the cycle time is around 55 min with no G₁ nor G₂ phases. (2) From the 5th to the 10th cleavage (hatching time ) when the synchrony is gradually lost, the so-called “percent labeled mitoses” of Quast 1er and Sherman type of plots are made. Between the 6th and the 10th cleavages, the cycle time lengthens from 65 min to 256 min and a few minutes each of G₁ and G₂ begin to appear. (3) During the final stage before hatching, traverse time of the cell cycle becomes much longer among the micromere progenies so that while the progenies of mesomeres and macromeres divide 5 times before hatching, those of the larger micromeres undergo 3 divisions and those of the smaller micromeres only once more. Moreover, as the size of micromere descendants is small, for easier detection of them, eggs are labeled with BrdU at the 1st cleavage and later when necessary, BrdU is made to shine with anti-BrdU antibody and FITC staining. DNA is made visible with propidium iodide. The bright fluorescence together with the position of the micromeres next to macromeres makes identification quite easy. Owing to these techniques, we now know that the cycle of the 1st cleavage (the 6th of the embryonic cleavage) of smaller micromere lasts for 6 hrs with a long S-phase of 5 hrs and they double their number to 8 by hatching.