Organization of cytoskeleton and chromatin is related to the timing of thefirst zygotic cleavage and early developmental competence.

Timing of the first zygotic cleavage is a reliable predictor of embryo quality. Embryos that cleave early have higher developmental viability compared to their late counterparts. It is hypothesized that differences in viability is attributed to cytoskeletal and chromatin organizations. This study investigated cytoskeletal and chromatin structures, and distributions in earlycleaving (EC) versus late-cleaving (LC) embryos. Embryos were retrieved from superovulated ICR mice, 28 hours after hCG injection. Two-cell stage embryos were categorized as EC, while zygotes with two pronuclei as LC embryos. After overnight culture in M16 medium, embryos were fixed and immunostained to visualize cytoskeletal and chromatin distributions, and intensities. EC embryos were observed to have significantly higher actin and chromatin fluorescence intensities compared to LC embryos [(14.68 + 14.07) x 105 versus (1.50 + 1.20) x 105 pixels (p<0.05) and (11.43 + 3.48) x 105 versus (7.98 + 3.08) x 105 pixels (p<0.05)], respectively. There was no significant difference in tubulin intensity between EC and LC embryos. This suggests that higher densities of actin and chromatin in EC embryos appreciably contributed to more efficient cell division and therefore, greater developmental competence.