Mechanical properties of the endoplasm in starfish oocytes

The mechanical properties of the endoplasm were determined in oocytes and mature eggs of the starfish, Asterina pectinifera as follows. The cell was first deformed into a dumbbell shape by aspirating it through a circular hole of 35 or 50 μm radius formed in an agar plate of about 150 μm thickness. The movement of endoplasm in the cylindrical part of the cell was determined when a definite pressure was applied between both sides of the plate. Mechanical properties were practically represented by a viscoelastic model (fig. 5 a) consisting of a Voigt element and a viscous element connected in series. The strain was proportional to the 0.60 ± 0.17th power of the stress in mature eggs and to the 0.76 ± 0.17th power of the stress in primary oocytes. Viscoelastic coefficients (G, η 1 and η 2 shown in fig. 5) of endoplasm changed in parallel to one another during maturation of the oocyte. They decreased with the breakdown of the germinal vesicle, increased before the extrusion of the first polar body, decreased during and after the first polar-body formation, increased before the extrusion of the second polar body, and decreased during and after the second polar-body formation.