Effect Of Heat Stress On The Cytoplasmic Maturation Of Mouse Oocyte During In Vitro Maturation

Elevated ambient temperature has long been recognized as a major factor responsible for the reduced fertility or conception rates in farm animals during hot seasons. In cattle, fertilization rate and embryo viability are lower in hot seasons than in cool seasons; conception rates can decrease from 50 to 55% in cooler seasons to 10–20% in the hot summer.It's reporteded that ,when female mice were heat-stressed at 35℃for 10 h after hCG injection (oocyte maturation stage), resulted in a significant decrease in the overall development rate after fertizalition.But there is little information available on in vitro parthenogenetic development of mouse oocytes that were exposed to heat stress during in vitro maturation.In this study, KunMing mouse between 18-23 days old were used as a modle. The embryo development after activation were compared among mouse oocytes that had been matured under different temperature(37-40℃). To investigate the effects of heat stress on nuclear and cytoplasmic components during in vitro maturation using a number of experiments involving a reciprocal transfer of the nucleus(chromosome spindle apparatus) between 37℃in vitro-matured oocytes and 40℃in vitro-matured oocytes. The results are summarized as follows:1. Oocytes matured at a physiologically relevant elevated temperature(38.5-40℃) had no effects on nuclear maturation.2. Heat stress at 38.5℃had no effects on embryo development, exposured oocytes to both 39 and 39.5℃did not alter the number of embryos that cleaved but reduced the number of embryos that developed to the blastocyst stage.In contrast,when exposured to 40℃all the cleaved embryos blocked at 2-cell stage. 3. To investigate whether this poor development of heat stress oocytes is due to a nuclear or a cytoplasmatic factor, we designed an experiment in which the MⅡchromosome spindle was exchanged between 37℃and 40℃matured oocytes by nuclear transfer. Transfer of the spindle apparatus from 40℃matured oocytes to the 37℃cytoplasmic environment resulted in a development rate as the same with reciprocal transfer of the chromosome spindle between 37℃matured oocytes. These results indicate that the low developmental competence of 40℃matured oocytes is caused by the cytoplasmic component rather than the nuclear component.That is to say HS mainly influences the cytoplasmic maturation of oocytes .4. When oocytes were matured at 37-39℃,the appearance of cumulus cell had protective effect on cleavage and blastocyst rate,the development rate of DOs significantly decreased compared to COCs.In contrast , when oocytes matured at 40℃the cumulus cell had no effects on development as all the embryos blocked at 2-cell stage with the same rate.5. when compared the parameters of cytoplasmic maturity of oocytes which matured at 37℃and 40℃,we found that:HS had no effect on spindle organization,as both group of oocytes can form normal spindle shape with the same rate; Partial polarized mitochondrial distribution of HS oocytes were more than that of control.The HS oocytes showed increased partial migration of CGs type.