Aging changes in the alignment of chromosomes after human chorionic gonadotropin stimulation may be a possible cause of decreased fertility in mice.

The relation between the alignment of chromosomes in the aged oocyte and its capacities of fertilization and development of oocytes was examined. More embryos in the condition of insemination at 12 and 16 h after human chorionic gonadotropin (hCG) however grew into two and more cell stages than those in the insemination of 20, 24 and 28 h. Some embryos in the group of insemination at 12, 16 and 20 h after hCG injection had grown into the blastocyst stage. The embryos inseminated 24 h after hCG and later had shown no further development. Twelve hours after hCG injection, 67% of oocytes have chromosomes on a straight line (A), and 29% have chromosomes scattered in one group (B). Three percent of oocytes have them separated toward both spindle ends (C) and 2% of oocytes have no chromosomes (D). At 16 h after hCG, the alignments of chromosomes are almost the same as that of 12 h after hCG injection. At 20 h after hCG, the most common alignment of chromosomes was scattered in a small group. The alignment of chromosomes of most oocytes was splitting at 24 and 28 h after hCG injection. Thus the oocytes with the chromosome alignment of A or B can be fertilized and developed into the blastocyst stage. The alignment of A and B lasts only about 8 h after ovulation, and the time was extremely restricted for oocytes to develop normally.