ABSTRACT
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 3 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.
ABSTRACT
When sea urchin embryos at 2-cell stage are flattered between agar plates, the direction of cleavage is rotated by 90° in each division in reference to the preceding cleavage and no micromere is formed. But under this condition, micromeres are formed in 2 cases; 1) When the egg axis is parallel to the plane of flattening, 2 micromeres are formed on one side of a square 16-cell stage. 2) when the egg axis is perpendicular to the plane, 4 micromeres are formed at the center of the square. When put into a groove, a string of 4 cells is formed showing that the spindle direction is further deflected by the groove. In the following 16-cell stage in the groove, which consists of 2 layers of 8 cells, cases with 2 micromeres on one side and 4 micromeres at the center are still found. If the 2-cell stage is introduced into a groove after the formation of mitotic apparatus, the spindle direction can no longer be changed and the 4-cell stage becomes like 4 pancakes stuck in 2 layers, indicating that 2 asters are holding the ends of a spindle in fixed positions.
ABSTRACT
Unequal division of the eggs of Spisula solidissima was studied by isolating the nuclear elements. The isolation technique consists mainly of the use of Na-lauryl sulphate with occasional inverting of the container of alcohol-fixed eggs. The procedure was applied to the following stages. (1) germinal vesicles, (2) 1st maturation division, showing anchorage of a spindle pole to the cortex, (3) 2nd meiosis, (4) pronuclear encouter, (5) metaphase, (6) shifting of the metaphase spindle to one side of the cell and a contact with the cortex, (7) ana- and telophase of the 1st division. Migration and "anchorage" of the spindle to the cortex by one pole is a common phenomenon in unequal divisions of polar body formation and of egg cleavage.
ABSTRACT
Cortical features of the meso- and macromeres differ from those of the micromeres in sea urchins. At the end of the 8-cell stage, the four animal cells have a continuous row of vesicles lining the free surface of the cell by transmission electron microscopy (TEM) and the nuclei and the resulting mitotic apparatuses (MA) remain at the cell centers and eventually divide equally into eight mesomeres. In the four vegetal cells, narrow gaps can be seen in the vesicular rows near the vegetal pole. The resting nuclei migrate to these gaps and on forming the spindles, they point directly to the gaps. The result is formation of vesicle-free micromeres and vesicle-covered macromeres by unequal divisions.
ABSTRACT
Cell cycles have been analyzed in 10 divisions up to the time of hatching in the embryos of the sea urchin, Hemicentrotus pulcherrimus. In the first 5 cleavages, division synchrony is very high. On the average, TGC = 55.4 min, TG1 = 0 min, Ts = 12 min, TG2 =±0 min, TM = 42 min. In the remaining 5 cleavages, TGC becomes longer: 70 min for the 7th to 246 min for the 10th cleavage. G1 and G2 become definitely recognizable and become longer along with Ts . TM stays more or less constant. Plots of the changing lengths of the four compartments (G1 , S, G2 , M) on the Y-axis against TGC (X-axis) can be fitted to the following 4 regression equations; TG1 = 0.28TGC - 19.7, Ts = 0.609TGC - 15.2, TG2 = 0.104TGC - 4.72 and TM = 0.007TGC + 39.6.
ABSTRACT
It has been known for nearly a century that at the 16-cell stage of sea urchin embryos, the animal 4 cells divide equally and horizontally, whereas the vegetal 4 cells cleave unequally and practically vertically into macromeres and micromeres. Recently, more careful observations were made on the process of micromere formation and it has been revealed that a primary cause for the inequality lies in the migration of the 4 vegetal nuclei to the vegetal pole of the embryo which brings about excentricity of the mitotic apparatus. Records of this phenomenon are given in the present paper.
ABSTRACT
The previously reported observation that micromere formation after cleavage suppression is not linked with the number of blastomeres present but rather with the time schedule of the fourth cleavage of the normal embryos has been confirmed. A hypothesis is advanced that a rhythmical fluctuation of the sulfhydryl contents of the egg proteins is the clock system, and micromere formation is connected with the fourth SH cycle after fertilization. The hypothesis was tested under 3 conditions: (i) Conditions which stop the nuclear activities but preserve the SH cycle, followed by a release from these conditions. (ii) Conditions which "freeze" both nuclear and cytoplasmic rhythms, and later removal of the conditions. (iii) Conditions which leave nuclear activities intact but prevent the cytoplasmic rhythms, followed by normal culturing. The results came out as anticipated by the hypothesis.
