sea urchin egg
Life and Medical Sciences
Cell & Developmental Biology
Wiley InterScience Backfile Collection 1832-2000
Birefringence (BR) at the cell surface of fertilized eggs of the sand-dollar, Clypeaster japonicus, during mitosis and cleavage was determined with a photoelectric BR detection apparatus [Hiramoto et al, 1981a]. The cortex of about 2 μm thickness is birefringent positive with respect to the normal to the cell surface. The hyaline layer is negatively birefringent. The halo-layer consisting of a row of microvilli surrounding the egg is positively birefringent in normal Ca-free sea water, while it is negatively birefringent in Ca-free sea water with high refractive index. The BR of the cortex gradually increases over the entire surface during mitosis until the onset of cleavage. The BR of the cortex at the polar region reaches a maximum shortly after the onset of cleavage and then decreases, while the BR of the cortex at the equatorial region begins to decrease shortly before the onset of cleavage, reaches a minimum shortly after the cleavage starts, and then increases again as the cleavage furrow advances. The coefficient of birefringence of the cortex is about 2.5 × 10-5 at the maximum. The BR change of the cortex during mitosis and cleavage is interpreted as a passive deformation caused by the constriction of the contractile ring as well as an active structural change of the cortex occurring in the dividing cell.
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