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.

Studies on Unequal Cleavage in Sea Urchins III. Micromere Formation under Compression: (equal division/unequal division/micromere formation/Hertwig's rule/Balfour's rule).

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.

Studies of Unequal Cleavage in Molluscs: I. Nuclear Behavior and Anchorage of a Spindle Pole to Cortex as Revealed by Isolation Technique: (mollusc/unequal cleavage/MA isolation).

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.

Studies on Unequal Cleavage in Sea Urchins II. Surface Differentiation and the Direction of Nuclear Migration: (sea urchin micromere/cortical differentiation/equal-unequal cleavage).

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.

CELL CYCLE STUDY UP TO THE TIME OF HATCHING IN THE EMBRYOS OF THE SEA URCHIN, HEMICENTROTUS PULCHERRIMUS.

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, T = 55.

STUDIES ON UNEQUAL CLEAVAGE IN SEA URCHINS I. MIGRATION OF THE NUCLEI TO THE VEGETAL POLE.

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.

ON THE SYSTEM CONTROLLING THE TIME OF MICROMERE FORMATION IN SEA URCHIN EMBRYOS.

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.