Completion of Oocyte Meiosis

The oocyte released at ovulation has completed only meiosis I. At fertilisation it remains arrested partway through meiosis II, at metaphase II. Sperm entry is the trigger that finally releases it from this second arrest, allowing it to complete meiosis and become a mature, fully haploid ovum.

When a spermatozoon fuses with the oocyte membrane, the oocyte is not yet a fully mature, definitively haploid cell. It is a secondary oocyte, arrested at metaphase II of meiosis. Its chromosomes are aligned at the metaphase plate but not yet separated into the final haploid distribution that meiosis II achieves.

A specific cytoplasmic regulatory mechanism within the oocyte actively maintains this arrest. It persists indefinitely, until either fertilisation occurs or the unfertilised oocyte degenerates. Nothing internal to the oocyte is sufficient to release the arrest without an external trigger.

Sperm-oocyte fusion triggers calcium oscillations in the oocyte cytoplasm. These are the signal that releases the oocyte from metaphase II arrest. The cell cycle machinery responsible for the arrest is inactivated.

The oocyte then proceeds rapidly through the remainder of meiosis II. In anaphase II, sister chromatids finally separate from one another. In telophase II, the division is completed.

Meiosis II typically resumes and completes within a relatively short period after sperm entry. It is the final step in the oocyte's developmental journey. That journey began with entry into meiosis I during fetal life and, depending on the individual oocyte, may not reach this point until decades later.

Completion of meiosis II converts the arrested secondary oocyte into a mature ovum, now definitively haploid. It contains a single haploid maternal pronucleus, derived from the chromosomes the oocyte retains at the end of this final division.

The mature ovum retains almost the entire cytoplasmic volume of the original oocyte, with its organelles, nutrient reserves, and stored maternal RNA and protein. This reflects the same markedly unequal cytoplasmic division seen in meiosis I.

The resulting zygote depends on this substantial cytoplasmic contribution to support its earliest stages of development, before its own zygotic gene expression becomes fully active.

As with the first meiotic division, completion of meiosis II is markedly unequal in cytoplasmic distribution. The chromosomal material is divided correctly and equally between the two resulting cells. Almost all of the cytoplasm stays with the mature ovum. The second polar body, extruded at this point, receives an equal haploid chromosome complement but only a negligible, functionally insignificant quantity of cytoplasm.

The second polar body, like the first polar body produced at completion of meiosis I, has no further developmental capacity and does not participate in formation of the zygote. Both polar bodies are degraded and lost.

With extrusion of the second polar body, the oocyte's meiotic sequence is completely finished. It spanned fetal life through to this moment after fertilisation. The maternal haploid pronucleus is now ready to combine with the paternal pronucleus, already forming from the decondensing sperm nucleus within the same fertilised cell, to establish the diploid genome of the new zygote.