国家自然科学基金资助项目湖南省卫生计生委科研计划课题湖南省自然科学基金重点资助项目(项目编号:13JJ2014)This study was supported by the National Natural Science Foundation of ChinaScientific Research Plan Project of Hu'nan Health and Family Planning CommissionMajor Project of Natural Science Foundation of Hu'nan Province,China