Abstract
Contractions were observed in blastocysts developed from 2-cell embryos collected from mice of different ages, in order to evaluate the influence of aging of dams on the quality of blastocysts, i.e. their ability to hatch, based on contractions. The zona pellucida of cultured blastocysts collected from mice of different ages was ultrastructurally observed to examine the relationship between hatching and contractions or zonae pellucidae. Although the rate of development from 2-cell embryos to blastocysts did not differ by dams’ ages, the hatching rate was significantly lower in the blastocysts from mice aged 30 days (31.3%) and 360 days (25.0%) than in those from mice aged 60 to 90 days (53.8%). The number of weak contractions was significantly fewer in the blastocysts from mice aged 360 days (4.23 times), compared with those in the blastocysts from mice aged 30 days (5.53 times) and 60 to 90 days (5.50 times). The number of strong contractions was significantly larger in the blastocysts from mice aged 30 days (2.03 times) than in those from mice aged 60 to 90 days (1.37 times) and 360 days (1.50 times). The zona pellucida showed a dense and homogeneous fiber-network in the blastocysts from mice aged 30 and 60 to 90 days, but showed a rough and irregular fiber-network in the blastocysts from mice aged 360 days. The thickness of zonae pellucidae significantly differed among blastocysts collected from mice of different ages, and the zona pellucida was particularly thin in the blastocysts from mice aged 360 days (0.79 μm). From these results, it was confirmed that 2-cell embryos collected from young or aged mice can develop to blastocysts at a high rate, but their ability to hatch is poor. A low hatching ability is thought to be due to frequent strong contractions observed in the blastocysts from mice aged 30 days, and to a small number of weak contractions seen in the blastocysts from mice aged 360 days. In addition, it was inferred that blastocyst hatching is closely related to the number of weak contractions, rather than the structure or thickness of the zona pellucida.
