Editorial Volume 4, Issue 11 pp 723—725

Figure 1. Premature separation of sister chromatids (PSSC) during oocyte maturation. During each estrous cycle (menstrual cycle in women), the oocyte undergoes two asymmetric cell divisions (meiosis I and meiosis II), each expelling half of the chromosomes to a non-viable polar body (pb). A chromosome bivalent, consisting of four chromatids, is depicted with the centromeres attached to spindle microtubules (blue lines) at metaphase I. During oocyte maturation (meiosis I), cohesin complexes distal to the cross-over are removed but those close to the centromeres are retained, segregating the sisters together to the egg or the polar body (left scenario). Sister chromatids inadvertently separated in meiosis I could both end up in the egg (middle scenario) or partitioned between the egg and 1^st polar body (right scenario). Either way, the egg is considered PSSC. Sister chromatids are separated during fertilization (meiosis II) when chromosome cohesin is completely removed. The schematic representation of mouse chromosomes is adapted from Kudo et al. 2006. Cell. 126:135-146.