Study of the Lineage and Cell Cycle of Small Micromeres in Embryos of the Sea Urchin, Hemicentrotus pulcherrimus: (small micromeres/cell cycle/cell lineage/unequal cleavage/sea urchin).

A study was made of 1st cell cycle of small micromeres, segregated at the 5th cleavage cycle, in the sea urchin embryos of Hemicentrotus pulcherrimus. For identification of small micromeres, the embryos were pulse labeled with 5-bromodeoxyuridine (BrdU) at the 1st cleavage. Using multiparametric microfluorometry equipped with a scanning stage (Tanaka, 1990), DNA content, extent of BrdU incorporation, protein content and the extent of H-thymidine labeling were measured on identical individual cells dissociated from an embryo. The findings of the present study are as follows. There is a short period of time between the telophase and onset of DNA replication. The period of DNA replication is 5 hr and after which, asynchronous mitosis takes place to produce 8 cells before hatching. The long S period is 83% the total 6 hr of the cell cycle. The rate of DNA accumulation is quite small during the initial one third of S but increases later in this phase. The degree of chromatin condensation remains high even during the S phase but it is low in large micromeres. The cell cycle may possibly be related causally to the development of small micromeres. The developmental significance of cell cycle duration, particularly that of DNA replication is discussed.