Kinetochore appearance during meiosis, fertilization and mitosis in mouse oocytes and zygotes

The events of mammalian fertilization overlap with the completion of meiosis and first mitosis; the pro-nuclei never fuse, instead the parental genomes first intermix at the mitotic spindle equator at metaphase. Since kinetochores are essential for the attachment of chromosomes to spindle microtubules, this study explores their appearance and behavior in mouse oocytes, zygotes and embryos undergoing the completion of meiosis, fertilization and mitoses. Kinetochores are traced with immunofluorescence microscopy using autoimmune sera from patients with CREST (CREST = calcinosis, Raynaud's phenomenon, esophageal dysmotility, sclerodactyly, telangiectasia) scleroderma. These sera cross-react with the 17 kDa centromere protein (CENP-A) and the 80 kDa centromere protein (CENP-B) found at the kinetochores in human cell cultures. The unfertilized oocyte is ovulated arrested at second meiotic metaphase and kinetochores are detectable as paired structures aligned at the spindle equator. At meiotic anaphase, the kinetochores separate and remain aligned at the distal sides of the chromosomes until telophase, when their alignment perpendicular to the spindle axis is lost. The female pronucleus and the second polar body nucleus each receive a detectable complement of kinetochores. Mature sperm have neither detectable centrosomes nor detectable kinetochores, and shortly after sperm incorporation kinetochores become detectable in the decondensing male pronucleus. In pronuclei, the kinetochores are initially distributed randomly and later found in apposition with nucleoli. At mitosis, the kinetochores behave in a pattern similar to that observed at meiosis or mitosis in somatic cells: irregular distribution at prophase, alignment at metaphase, separation at anaphase and redistribution at telophase. They are also detectable in later stage embryos. Colcemid treatment disrupts the meiotic spindle and results in the dispersion of the meiotic chromosomes along the oocyte cortex; the chromosomes remain condensed with detectable kinetochores. Fertilization of Colcemid-treated oocytes results in the incorporation of a sperm which is unable to decondense into a male pronucleus. Remarkably kinetochores become detectable at 5 h post-insemination, suggesting that the emergence of the paternal kinetochores is not strictly dependent on male pronuclear decondensation.