Mouse zona pellucida dynamically changes its elasticity during oocyte maturation, fertilization and early embryo development

A change in the elasticity of mouse zona pellucidawas quantitatively evaluated during oocyte maturation, fertilization and early embryo development. Young’s modulusof zona pellucidaof germinal vesicle (GV), metaphase-II (MII), pronuclear (PN), 2cell, 4cell, 8cell, morulae (M) and early blastocyst (EB) stages was measured using a micro tactile sensor(MTS) and a chamber exclusively designed for the measurement. The MTS has very high sensitivity and a deformation of only 5 μm was sufficient to calculate the Young’s modulusand the oocyte/embryo maintained its original spherical shape during the measurement. The Young’s modulusof GV, MII, PN, 2cell, 4cell, 8cell, M and EB was 22.8 ± 10.4 kPa (n = 30), 8.26 ± 5.22 kPa (n = 74), 22.3 ± 10.5 kPa (n = 66), 13.8 ± 3.54 kPa (n = 41), 12.6 ± 3.34 kPa (n = 19), 5.97 ± 4.97 kPa (n = 6), 1.88 ± 1.34 kPa (n = 8) and 3.39 ± 1.86 kPa (n = 4), respectively. Experimental results clearly demonstrated that the mouse zona pellucidahardened following fertilization. Interestingly, once the zona pellucidahardened at the PN stage, it gradually softened as the embryo developed (i.e. it was found that the zonahardening is a transient phenomenon). Furthermore, the zona pellucidaof the GV oocyte was as hard as that of the PN embryo and became soft as it matured to the MII stage. In addition, the safety of the MTS measurement for oocytes and embryos was discussed both theoretically and experimentally.