The bipolar spindle is an integral part of the cell division machinery required for faithful chromosome segregation. During meiosis in mammalian females, this machinery must function sequentially during a reductive (meiosis I) and equational (meiosis II) cell division to ultimately produce a haploid egg that can be fertilized and give rise to the next generation. Errors in this process can result in aneuploidy (incorrect chromosome number), which can have adverse reproductive outcomes such as infertility, miscarriages, and birth defects.
In human eggs arrested at metaphase of meiosis II (MII), the bipolar meiotic spindle is dependent on the function of EG5, a molecular motor protein in the kinesin family. Treatment of a human MII egg with monastrol, a pharmacological inhibitor of EG5, results in the formation of a monopolar spindle (green: α-tubulin) and spreading of sister chromatids (blue: DNA; red: CREST). Such chromosome spreading occurs in the context of an intact cell. Therefore treatment of eggs with monastrol has recently become a robust tool for obtaining accurate chromosome counts in several mammalian species, including mouse and human.
The spindle in this image, which is approximately 12 microns in diameter, comes from a human egg from a 19.3 year-old subject. This egg was obtained following in vitro maturation of a cumulusoocyte-complex isolated directly from ovarian tissue from a small antral follicle (< 5 mm in diameter). This ovarian tissue was surgically removed for ovarian tissue cryopreservation in the context of fertility preservation, and a portion was donated to research according to IRB-approved protocols.