Meiosis in females begins surprisingly early, during fetal development, but it does not finish until decades later. A human female’s germ cells enter meiosis while she is still in the womb, typically around the third to fifth month of gestation. Those cells then pause partway through the process and remain frozen in that state for years, sometimes for over forty years, until a hormonal signal at ovulation nudges them forward. Even then, the process only finishes if the egg is fertilized. This drawn-out timeline, stretching from before birth to the moment of conception, makes female meiosis one of the longest cell-division events in the human body.
It Starts Before Birth
Female germ cells begin meiosis during embryonic development. In the developing fetal ovary, precursor cells called oogonia stop dividing by mitosis and transition into meiosis, entering the early stages of prophase I. In humans, this shift starts around weeks 10 to 12 of gestation and ramps up through mid-pregnancy. A gene called STRA8 acts as a gatekeeper for this transition and is expressed in the embryonic ovary just before meiotic initiation.1PubMed Central. Retinoic acid regulates sex-specific timing of meiotic initiation in mice
For years, researchers believed that retinoic acid, a derivative of vitamin A, was the master signal that told germ cells to begin meiosis in the fetal ovary. That story has gotten more complicated. Mouse studies have shown that fetal ovaries lacking the key enzymes for producing retinoic acid still express STRA8 normally and produce functional oocytes.2Nature Communications. Sex-specific timing of meiotic initiation is regulated by Cyp26b1 independent of retinoic acid signalling Separately, female mouse fetuses engineered to lack all retinoic acid nuclear receptors still initiated meiosis, produced meiotic gene expression, and even yielded viable offspring when their ovarian tissue was transplanted.3PubMed Central. Meiosis occurs normally in the fetal ovary of mice lacking all retinoic acid receptors The search for the actual meiosis-triggering substance in the ovary remains open. Whatever the signal turns out to be, the outcome is consistent: by the time a human female is born, her germ cells have already entered meiosis and progressed into prophase I.
The Long Pause at Prophase I
After entering meiosis and moving through the early substages of prophase I, where chromosomes pair up and exchange genetic material through recombination, fetal oocytes hit the brakes. They arrest at a late phase of prophase I called the diplotene stage, sometimes referred to as the dictyate stage. This is not a brief timeout. From birth until the onset of puberty, and then continuing for each individual oocyte until it is selected for ovulation, the cell sits in this paused state. An oocyte ovulated at age 35 has been arrested for roughly 35 years.
The arrest is actively maintained by chemical signals passing between the oocyte and the granulosa cells that surround it within the ovarian follicle. Granulosa cells produce a molecule called C-type natriuretic peptide (CNP), which triggers the neighboring cumulus cells to make cyclic GMP. That cyclic GMP flows into the oocyte through tiny gap-junction channels and prevents the breakdown of cyclic AMP inside the egg.4PubMed Central. Signaling mechanisms and their regulation during in vivo or in vitro maturation of mammalian oocytes High cyclic AMP levels keep a key enzyme, maturation-promoting factor, inactive, and as long as that enzyme stays quiet, the oocyte cannot resume dividing.5PubMed Central. The art of oocyte meiotic arrest regulation In other words, the egg is not simply dormant; it is being actively held in place by ongoing molecular communication with its surrounding cells.
Quality Control During Fetal Meiosis
Not every oocyte that enters meiosis survives to birth. During prophase I, when chromosomes are pairing and recombining, internal checkpoints scan for errors. Oocytes that accumulate too much DNA damage or fail to properly pair their chromosomes are culled. Research in mice has shown that a DNA-damage checkpoint driven by the kinases CHK2 and ATM eliminates oocytes with severe synapsis failures, triggered when they accumulate a threshold level of unrepaired DNA breaks in late prophase I.6Molecular Cell. The DNA Damage Checkpoint Eliminates Mouse Oocytes with Chromosome Synapsis Failure Structural proteins at the nuclear envelope also play a role: when a protein called NEMP1 is missing, meiotic progression is delayed, DNA damage increases, and the ATM-CHK2 checkpoint kicks in to destroy the defective oocytes.7Nature Communications. Loss of Nemp1 disrupts female meiosis and activates a conserved ATM-CHK2 checkpoint
The result of all this quality control is dramatic attrition. A human fetus may produce several million oogonia by mid-gestation, but by birth the number of surviving oocytes has dropped to roughly one to two million. By puberty, only a few hundred thousand remain. Of those, only about 400 to 500 will ever be ovulated over a lifetime. The rest gradually die off in a process called atresia.
Waking Up at Ovulation
Each menstrual cycle, a surge of luteinizing hormone (LH) from the pituitary gland signals the dominant follicle to resume meiosis. The LH surge dismantles the chemical brake system that was keeping the oocyte paused. It reduces CNP levels in the follicle and disrupts the gap-junction connections between the cumulus cells and the egg, which causes cyclic GMP levels inside the oocyte to drop. With less cyclic GMP, cyclic AMP gets broken down, and maturation-promoting factor finally activates.8PubMed Central. Luteinizing Hormone Action in Human Oocyte Maturation and Quality: Signaling Pathways, Regulation, and Clinical Impact Retinoic acid signaling and epidermal growth factor signaling within the follicle cooperate to rapidly suppress CNP production after the LH surge, speeding the process along.9PubMed Central. Intrafollicular Retinoic Acid Signaling Is Important for Luteinizing Hormone-Induced Oocyte Meiotic Resumption
Within hours, the oocyte finishes meiosis I. This involves an extremely lopsided cell division: instead of splitting into two equal-sized daughter cells, the oocyte pushes out a tiny package of chromosomes and cytoplasm called the first polar body, keeping almost all of its cellular resources for itself.10PubMed Central. Symmetry breaking and polarity establishment during mouse oocyte maturation This asymmetric division is critical. The egg needs to retain its large store of proteins, organelles, and nutrients to support an embryo if fertilization occurs, so it sacrifices a minimal cell (the polar body) while keeping everything else.
The Second Arrest and Fertilization
After completing meiosis I, the oocyte does not barrel straight through meiosis II. It pauses again, this time at metaphase of the second meiotic division. The egg that sits in the fallopian tube awaiting a sperm is frozen at metaphase II, held there by a molecular brake called cytostatic factor. This factor works by stabilizing maturation-promoting factor and preventing the cell’s protein-destruction machinery from dismantling the components needed for division.11PubMed Central. How eggs arrest at metaphase II: MPF stabilisation plus APC/C inhibition equals Cytostatic Factor
A protein called Emi2 is central to this second arrest. It inhibits the cell’s protein-recycling complex by blocking a key interaction that would otherwise allow the complex to become active and degrade cyclin B, the partner of maturation-promoting factor.12Nature Communications. Emi2 mediates meiotic MII arrest by competitively inhibiting the binding of Ube2S to the APC/C When a sperm penetrates the egg, a flood of calcium ions triggers the destruction of Emi2, which releases the brake and allows the oocyte to finish meiosis II.13PubMed Central. Mammalian Emi2 mediates cytostatic arrest and transduces the signal for meiotic exit via Cdc20 The second polar body is extruded, leaving behind a cell with a single set of chromosomes ready to merge with the sperm’s set. Second polar body extrusion is visible under a microscope and serves as the hallmark sign that the oocyte has completed its final meiotic division.
If the egg is never fertilized, it simply degenerates without ever completing meiosis II. Most eggs meet this fate.
Why the Decades-Long Wait Matters for Fertility
The fact that oocytes sit arrested for so long has real consequences. During those years of prophase I arrest, the protein complexes that hold paired chromosomes together, known as cohesins, gradually deteriorate. Research on human oocytes has shown that the distance between sister chromatids at the point where they are connected increases significantly with maternal age, indicating weakened cohesion, and that unpaired sister chromatids appear in oocytes from older women.14PubMed Central. Chromosome cohesion decreases in human eggs with advanced maternal age A protective protein called SGO2, which helps maintain cohesion, is also lost in oocytes from women of advanced age.15Current Biology. Age-dependent loss of cohesion protection in human oocytes
When cohesion weakens, chromosomes are more likely to separate incorrectly during the two meiotic divisions, leading to eggs with the wrong number of chromosomes, a condition called aneuploidy. This is the primary reason fertility declines and the risk of conditions like Down syndrome rises with maternal age. The crossovers established back in fetal development are supposed to physically link paired chromosomes and hold them stable through the long arrest, but cohesin deterioration and suboptimal crossover placement can undermine that stability.16Nature. Common variation in meiosis genes shapes human recombination and aneuploidy Genetic variation in meiosis-related genes also influences how many crossovers form and where they land, meaning some individuals carry inherently higher aneuploidy risk than others.
How Female and Male Meiosis Differ
Male meiosis is a fundamentally different affair in terms of timing. Males do not begin meiosis until puberty, and from that point they continuously produce new waves of sperm throughout life. Each round of male meiosis takes about 74 days from start to finish. By contrast, a single round of female meiosis can span decades. The checkpoint systems also differ between the sexes: studies of mouse mutants lacking various meiotic regulators have revealed that females tolerate certain meiotic defects that would be eliminated in males, and vice versa.17PubMed Central. Cytological studies of human meiosis: sex-specific differences in recombination originate at, or prior to, establishment of double-strand breaks
Recombination levels also differ between the sexes. Genome-wide, females undergo more recombination events than males, and the placement of those crossovers differs, with sex-specific patterns emerging as early as the formation of the initial DNA double-strand breaks that start the recombination process.17PubMed Central. Cytological studies of human meiosis: sex-specific differences in recombination originate at, or prior to, establishment of double-strand breaks The relaxed stringency of some female meiotic checkpoints may partly explain why aneuploidy is far more common in eggs than in sperm. Male meiosis culls defective cells more aggressively, while the female system is more permissive, allowing some chromosomally abnormal oocytes to survive and potentially be ovulated.18PubMed Central. Not all germ cells are created equal: Aspects of sexual dimorphism in mammalian meiosis
Can New Eggs Form After Birth?
The traditional view, taught in textbooks for most of the twentieth century, holds that females are born with their entire lifetime supply of oocytes and never make new ones. Over the past two decades, that orthodoxy has been challenged. Several research groups have reported isolating cells from adult mammalian ovaries that display molecular markers of both germ cells and stem cells. When transplanted into mouse ovaries, some of these cells formed follicle-like structures and, in rodents, even produced healthy offspring.19PubMed Central. The existence and potential of germline stem cells in the adult mammalian ovary
The controversy remains unresolved. Skeptics point out that there is no evidence these cells contribute to the natural primordial follicle pool or participate in normal ovarian function in a living animal. In humans, there is only limited evidence that such cells can form oocyte-like structures when placed in a supportive environment, and none showing they produce functional eggs under natural conditions.20Maturitas. The controversial existence and functional potential of oogonial stem cells For practical purposes, the fixed-pool model still governs clinical thinking: the eggs you are born with are, functionally, the eggs you have for life.
Environmental Threats to Fetal Meiosis
Because female meiosis begins in the womb, the developing fetal ovary is vulnerable to chemical exposures during pregnancy. Bisphenol A (BPA), a synthetic compound found in certain plastics, has received particular attention. In animal models, fetal exposure to BPA disrupts meiotic prophase I, interfering with the DNA double-strand break repair machinery that is essential for proper chromosome recombination.21PubMed Central. Bisphenol A impairs the double-strand break repair machinery in the germline and causes chromosome abnormalities Related compounds, including BADGE and BPAF, have also been shown to impair meiosis through oxidative DNA damage in mouse fetal ovaries.22Environmental Pollution. Foetal exposure to the bisphenols BADGE and BPAF impairs meiosis through DNA oxidation in mouse ovaries
More recent mouse work has identified the onset of meiotic prophase as the specific vulnerable window. Brief maternal exposure to BPA during this window induced complex molecular differences in the fetal germline, affecting chromatin structure and several interrelated meiotic pathways.23PubMed Central. Effect of Brief Maternal Exposure to Bisphenol A on the Fetal Female Germline in a Mouse Model The concern is that damage inflicted during fetal meiosis could persist through the decades-long arrest and manifest as chromosome errors when those oocytes are eventually ovulated in adulthood. Translating these findings from animal models to human risk assessment remains difficult, but the fact that the critical meiotic events happen only once, before birth, and cannot be redone makes the fetal ovary uniquely susceptible.
Why Arrest at All?
The extended meiotic arrest in female mammals is puzzling from an evolutionary standpoint. Why not just finish meiosis and store completed eggs? One evolutionary hypothesis proposes three possible advantages. First, producing all eggs before birth may reduce variation in the amount of genetic damage they accumulate, lowering the overall risk of severely damaged eggs. Second, keeping oocytes at a stage where they still have four copies of each DNA sequence (the sister chromatids from both homologs) may allow the cell to use those extra copies as templates for DNA repair during the long waiting period. Third, arresting before the final divisions may prevent competition between egg cells for resources, a form of intracellular conflict that could select for “selfish” genetic elements at the expense of the organism.24Journal of Theoretical Biology. Why is Meiosis Arrested? These ideas remain theoretical, but they highlight that the long arrest is probably not just an accident of development. It may be a solution to deep biological problems.
Clinical Relevance in Fertility Treatment
Understanding meiotic arrest and resumption has direct applications in assisted reproduction. In vitro maturation (IVM) is a technique where immature oocytes are retrieved from the ovary before they have resumed meiosis and are then coaxed to mature in the lab. The challenge is replicating the natural signaling that occurs within the follicle. Pre-IVM systems that use chemical agents to prevent spontaneous meiotic resumption and instead allow a more controlled maturation have shown promise for improving egg quality, though results remain inconsistent.25Molecular Human Reproduction. A pre-in vitro maturation medium containing cumulus oocyte complex ligand-receptor signaling molecules maintains meiotic arrest, supports the cumulus oocyte complex and improves oocyte developmental competence Mouse studies have found that oocytes cultured in follicle-growth systems gain meiotic competence over time, reaching higher rates of polar body extrusion with longer culture, though fertilization rates can paradoxically drop.26PubMed. Acquisition and loss of oocyte meiotic and developmental competence during in vitro antral follicle growth in mouse
The broader point is that each step in the meiotic timeline, from fetal initiation to fertilization-triggered completion, represents a potential intervention point or a potential vulnerability. Knowing where an oocyte is in its meiotic journey matters for egg freezing, embryo creation, and diagnosing certain kinds of infertility. An egg retrieved too early may lack the competence to complete maturation. An egg from an older patient may carry cohesin damage accumulated over decades. The biology of female meiosis is not just an academic curiosity; it sits at the center of reproductive medicine.