Effects Of Cadmium On Maturation Of Mouse Oocytes And The Early Development Of Mouse Embryos

In recent years,cadmium toxicity research has been widespread concern.Excessive intake of cadmium not only damage a variety of tissues and organs including the liver,kidneys and bones,but also damage the reproductive system,such as the damage to sperm,the changed morphology of the ovary,the growth and development failure of follicles.However,it is not entirely clear how the environmental cadmium harm to oocytes and offspring.Drinking water with the high concentration cadmium is a major way for the body to consume excess cadmium.In this study,ICR mice continuous drinking of cadmium water(64 mg/L CdCl2·2.5 H2O)for 35 days were used as a models to study the effect of cadmium on mouse oocyte maturation and early embryonic development.1.Effects of cadmium on mouse oocyte maturationIn order to eliminate the effect of cadmium in tap water,the control group of female mice drink Milli-Q ultrapure water,the experimental group of female mice drink Milli-Q water containing cadmium water.The superovulation experiments were used to detect follicular development and maturation.The maturity of oocytes were also tested by in vivo and in vitro.The morphology and function of chromosomes,spindle,actin cap and mitochondria during oocyte maturation were detected by immunofluorescence assay.The ATP content in oocytes were detected by detecting immunohistochemistry.The levels of histone H3K9 methylation and H4K12 acetylation levels were tested to reflect the effect of cadmium on apparent genetic modification.The results showed that cadmium treatment resulted in a significant decrease in the number of oocytes in female mice after superovulation(p<0.05).In vivo and in vitro experiments showed that there was no change in germinal vesicle breakdown(GVBD),but most of the oocytes stagnated in the first meiotic phase(MⅠ),and the proportion of mature oocytes(MⅡ)was significantly lower than that of the control group.The results suggest that cadmium treatment does not inhibit the resume of oocyte meiosis,but disturbs the nonnal completion of the first meiosis.The results of immunofluorescence showed that cadmium treatment caused abnormalities in chromosome,spindle and cell polarity of MⅡ oocytes,and the chromosomes of MⅡ oocytes were not neatly arranged after cadmium treatment.The spindle morphology did not show typical spindle shape,At the same time,Actin cap was lost.It was concluded that cadmium treatment resulted in a decrease in the quality of MⅡ oocytes.We further examined the number and function of mitochondria.The results showed that cadmium treatment resulted in a significant decrease in the number of mitochondria in the MⅡ oocytes,although the mitochondrial function was enhanced but it was failed to save the decrease in ATP content.The results of histone modification showed that cadmium treatment significantly enhanced the methylation level of H3K9 and the acetylation of H4K12 in MⅡ oocytes.The effect of cadmiun on the second meiotic division of oocytes was observed by the parthenogenetic activation experiment.The results showed that although cadmium treatment delayed the extrusion of the second polar body and prokaryotic nucleus formation,the number of prokaryotic nuclei was not changed.It was indicated that cadmiun treatment did not significantly affect the final completion of the second meiosis.2.Effects of Cadmium on Early Embryonic Development in MiceAfter mating the cadmium-treated female mice with normal male mice,the fertilized eggs were collected for in vitro culture to observe the effect of cadmium on early embryonic development.The results showed that cadmium treatment significantly reduced the development of 4-cell embryos and blastocysts during the same period.In further experiments,the oocytes of cadmium-treated females were collected for in vitro fertilization,compared with the control group,blastocyst development rate also decreased.This suggests that cadmium treatment reduces the quality of mature oocytes and advances that affect the normal development of early embryos after fertilization.In summary,drinking water chronic cadmium poisoning will not only reduce the ovulation rate of female mouse oocytes,but also by affecting the meiosis process,mitochondrial function and histone modification to reduce the quality of oocytes,and ultimately lead to a decline in embryonic developmental capacity.