Meiosis is the specialized cell division that reduces the number of chromosomes by half. This reduction ensures that when two reproductive cells, or gametes, combine, the resulting offspring has the correct, full set of chromosomes. The formation of the female gamete, or egg cell, is called oogenesis, and it is a process markedly different from sperm production in males. Oogenesis involves long periods of cellular inactivity, where the developing egg cell pauses its division timeline for years or even decades. The final completion of this cell division occurs only under a single, specific biological condition.
The Initial Stages and First Arrest
The process of egg formation begins during the fetal development of the female. Precursor cells called oogonia begin the first stage of reduction division, Meiosis I, transforming into primary oocytes. This division quickly enters a prolonged state of cellular arrest, pausing at a stage called Prophase I.
The primary oocyte remains dormant in this arrested state until the female reaches puberty, a pause that can last over a decade. With the onset of the reproductive years, monthly hormonal signals, primarily a surge in luteinizing hormone, trigger the resumption of Meiosis I in select oocytes.
The completion of Meiosis I is an asymmetrical division, resulting in two cells of vastly different sizes. The larger cell, which retains nearly all the original cytoplasm and nutrients, is the secondary oocyte. The much smaller cell, consisting mainly of extruded chromosomes, is called the first polar body, and it generally degenerates.
The State of the Ovulated Egg
After Meiosis I finishes, the secondary oocyte begins Meiosis II. This division is quickly halted, entering a second state of arrest at the Metaphase II stage. The secondary oocyte is released from the ovary during ovulation in this arrested state.
The ovulated egg travels into the fallopian tube, awaiting potential fertilization while paused at Metaphase II. This second arrest is maintained by internal cellular factors, including the activity of Maturation Promoting Factor (MPF). The egg remains viable in this state for only about 12 to 24 hours.
If fertilization does not occur, the secondary oocyte will degrade and be expelled from the body without completing Meiosis II. The egg encountered by sperm is therefore not a fully mature ovum but a secondary oocyte suspended mid-division.
The Final Trigger: Completion upon Fertilization
An ovum finishes meiosis at the moment of successful fertilization. The sperm’s penetration of the secondary oocyte’s membrane provides the biological stimulus required to break the Metaphase II arrest. This trigger initiates a rapid cascade of internal signaling events within the egg.
This activation is characterized by a series of calcium ion oscillations that sweep through the egg’s cytoplasm. These calcium waves act as the internal signal that deactivates inhibitory complexes, such as MPF. The inactivation of these complexes allows the secondary oocyte to progress past Metaphase II and complete meiosis.
The completion of Meiosis II occurs after the sperm enters but before the genetic material from the sperm and egg fuse. This timing ensures the egg’s nucleus is properly haploid before combining with the sperm’s nucleus to form the diploid cell of the new embryo.
The Resulting Mature Cell
The completion of Meiosis II results in the formation of the large, mature ovum and the small, second polar body. Similar to the first division, this second division is highly asymmetrical to conserve the maximum amount of cytoplasm and stored nutrients within the single mature ovum. The mature ovum contains a single, haploid nucleus ready to merge with the sperm’s haploid nucleus.
The second polar body, extruded to the periphery of the cell, contains the remaining half of the chromosomes from the secondary oocyte. The extrusion of the polar bodies is the mechanism by which the egg sheds its excess genetic material while retaining the large volume of cytoplasm needed for early embryonic development. The now-fertilized egg, containing both sets of haploid chromosomes, is officially termed a zygote.