Live chromosome identifying and tracking reveals size-based spatial pathway of meiotic errors in oocytes.

Meiotic errors of relatively small chromosomes in oocytes result in egg aneuploidies that cause miscarriages and congenital diseases. Unlike somatic cells, which preferentially mis-segregate larger chromosomes, aged oocytes preferentially mis-segregate smaller chromosomes through unclear processes. Here, we provide a comprehensive three-dimensional chromosome identifying-and-tracking dataset throughout meiosis I in live mouse oocytes. This analysis reveals a prometaphase pathway that actively moves smaller chromosomes to the inner region of the metaphase plate. In the inner region, chromosomes are pulled by stronger bipolar microtubule forces, which facilitates premature chromosome separation, a major cause of segregation errors in aged oocytes. This study reveals a spatial pathway that facilitates aneuploidy of small chromosomes preferentially in aged eggs and implicates the role of the M phase in creating a chromosome size-based spatial arrangement.