Fertilization and developmental competence of <i>in vitro</i> fertilized embryos from C57BL/6J mice of different ages and the impact of vitrification

Prepubertal animals are often preferred as sources of oocytes for assisted reproductive technologies (ARTs) in laboratory mice, but the normality and developmental competence of these oocytes remain controversial. This study systematically examined in vitro fertilization competence, embryo development, and fetal development after embryo transfer (ET) using oocytes from C57BL/6J mice aged 3 to 10 weeks. Oocytes were collected from superovulated females, fertilized, and cultured in vitro for 96 h or transferred at 2-cell stage to recipient females. Additionally, fetal development was compared between unfrozen and frozen-thawed in vitro-fertilized 2-cell embryos across different age groups. The number of ovulated oocytes per animal decreased with age, while the percentage of morphologically normal oocytes was highest in 3-week-old mice (99%) compared to older ages (70-86%, P < 0.05). Although fertilization percentages were consistently high (≥ 97%), blastocyst development in vitro, the nuclear counts of blastocysts and fetal development after ET were lowest for embryos from 3-week-old mice. Development of frozen-thawed embryos to fetuses was significantly reduced compared to unfrozen embryos in all age groups, except for those from 10-week-old mice. These findings suggest that oocytes from prepubertal mice, particularly from 3-week-old mice, are less developmentally competent than those from older mice. Therefore, the age of animals for oocyte source should be carefully considered based on the specific requirements of the research or ART applications.