Meiosis II

During prophase II, dense, X-shaped chromosomes appear in the haploid cell's nucleus. Each X consists of a centromere and joined sister chromatids containing unique combinations of maternal and paternal material from crossing over. Simultaneously, centrosomes travel to opposite poles and the microtubule-based meiotic spindle apparatus develops between them.

Kinetochores materialize on the centromeres during prometaphase II and serve as anchors for extending microtubules. These structures secure sister chromatids in pairs to opposing centrosomes, positioning them for separation during anaphase II when microtubules shorten and pull the chromatids apart.

In female egg precursor cells, the meiotic spindle apparatus forms off to one side near the periphery, creating asymmetry. This positioning allows two cells of unequal sizes to form: a large egg containing nutrients for embryo support and a smaller polar body that dissolves, ensuring the egg has sufficient resources.

Egg cells arrest in metaphase II, with the meiotic spindle apparatus formed and associated with chromosomes but unable to complete sister chromatid separation. Meiosis only resumes if fertilization occurs. This arrest poses challenges for egg freezing in assisted reproductive technologies, as procedures must minimize damage to the spindle structure.

Meiosis II results in haploid cells, each containing only one copy of all 23 chromosomes. These cells develop into mature haploid cells and gametes capable of fertilization. Each gamete carries a distinct mixture of parental information, enabling the creation of genetically unique offspring when fertilization occurs.

During telophase II, sister chromatids are delivered to opposite cell edges and untangle into haploid unreplicated chromosome sets. New nuclear envelopes form around each set, and the cell divides via cytokinesis, producing two distinct haploid cells with unique genetic combinations ready for gamete development.

During intracytoplasmic sperm injection (ICSI), a needle inserts sperm directly into an egg's cytoplasm. Damaging the meiotic spindle apparatus could harm the microtubule framework and cause an abnormal number of chromosomes in the resulting embryo. Embryologists predict spindle location using the polar body position or polarized light microscopy to ensure safe injection.