Egg cells, also called ova or oocytes, are the reproductive cells produced by the ovaries. Unlike sperm, which are made continuously after puberty, egg cells are produced through a process that begins before birth and then continues in a limited, carefully regulated way throughout a person’s reproductive years.
The biology is unusual for another reason: an egg cell does not simply grow and divide like an ordinary body cell. Its development involves specialized cell divisions called meiosis, which reduce the chromosome number by half and create a cell capable of combining with sperm during fertilization.
Understanding how egg cells are made requires looking at what happens before birth, what changes at puberty, and what occurs during each menstrual cycle.
Egg cells begin developing before birth
The process starts during fetal development. In a developing fetus with ovaries, early reproductive cells called primordial germ cells migrate into the developing gonads. There, they become oogonia, the precursor cells of eggs.
Oogonia multiply by ordinary cell division, or mitosis, during fetal development. They then begin transforming into primary oocytes and enter meiosis.
Meiosis is a special type of cell division used to produce reproductive cells. A normal human body cell has 46 chromosomes arranged in 23 pairs. Meiosis ultimately produces cells with 23 chromosomes, so that when an egg and sperm unite, the resulting embryo can have the usual 46 chromosomes.
The primary oocytes do not complete meiosis before birth. Instead, they pause partway through the process. They remain in this arrested state, surrounded by supporting cells within structures called primordial follicles.
By the time of birth, the ovaries contain a large supply of immature oocytes. This reserve is finite; the body does not routinely create a new lifetime supply of oocytes after birth.
What happens to egg cells after birth?
After birth, most oocytes remain dormant inside their follicles. A follicle is more than a storage compartment: it is a small biological environment containing an oocyte and the surrounding cells that nourish it and help regulate its development.
Over time, many follicles naturally break down in a process called atresia. This happens throughout childhood and the reproductive years. Only a small fraction of the original pool will ever progress far enough to participate in ovulation.
At puberty, hormonal signals from the brain begin coordinating the ovaries with the menstrual cycle. Each cycle recruits a group of follicles to begin developing. Usually, one follicle becomes dominant and its oocyte proceeds toward ovulation.
How an egg develops during the menstrual cycle
The monthly development of an egg is controlled by a network involving the brain and ovaries.
The hypothalamus in the brain releases gonadotropin-releasing hormone (GnRH) in pulses. GnRH stimulates the pituitary gland to release two important hormones: follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
FSH helps stimulate the growth of ovarian follicles. As follicles develop, their surrounding cells produce hormones, particularly estrogen. One follicle generally becomes dominant as the cycle progresses.
Inside that developing follicle, the oocyte grows and undergoes important biochemical changes. The follicle provides nutrients and molecular signals that help prepare the oocyte for its next stage of development.
The egg is therefore not being manufactured from scratch during each cycle. Rather, an oocyte that has existed since fetal development is being recruited from the ovarian reserve and undergoing maturation.
Meiosis gives the egg the right chromosome number
One of the most important features of egg development is meiosis.
The primary oocyte began meiosis before birth but stopped during the first meiotic division. Years later, as the follicle matures, hormonal signals trigger the oocyte to resume meiosis.
Instead of producing two equal-sized cells, the oocyte divides very unevenly. Most of the cell’s cytoplasm—the material containing organelles, proteins, nutrients, and other cellular machinery—remains in one cell. The smaller cell is called a polar body.
This unequal division is important because the developing egg needs to retain substantial cellular resources that can support the earliest stages after fertilization.
The resulting cell is called a secondary oocyte. It has 23 chromosomes, although each chromosome still consists of two joined copies of its DNA.
Ovulation releases a secondary oocyte
Around the middle of a typical menstrual cycle, a rise in LH triggers ovulation. The mature follicle in the ovary ruptures and releases the secondary oocyte.
The released cell enters the nearby fallopian tube, where fertilization can occur if sperm are present.
An important detail is often lost when people casually refer to the released cell as an “egg.” At ovulation, it is technically a secondary oocyte, not a fully completed egg cell in the strict biological sense.
The secondary oocyte has paused again, this time during the second meiotic division. It generally completes that division only if fertilization occurs.
Fertilization completes the egg’s development
If a sperm successfully enters the secondary oocyte, the oocyte resumes and completes meiosis II. Another small polar body is produced, while the larger cell retains most of the cytoplasm.
The genetic material from the egg and sperm then organizes into two pronuclei. These eventually combine their chromosomes, restoring the embryo’s diploid chromosome number of 46.
If fertilization does not occur, the secondary oocyte does not complete meiosis II. It eventually degenerates.
This timing is one of the distinctive features of human egg development: the final stage of meiosis is normally completed only in response to fertilization.
Egg cells and sperm are made very differently
Egg and sperm production both use meiosis, but the processes are not mirror images.
| Egg development | Sperm development |
|---|---|
| Begins before birth | Begins around puberty |
| Starts with a finite ovarian reserve | Stem cells continually replenish sperm production |
| Usually produces one large functional cell from each meiotic sequence | Each meiotic sequence produces four functional sperm cells |
| Cell divisions are highly unequal | Cell divisions produce cells of roughly equal size |
| The developing egg retains most of the cytoplasm | Sperm cells discard most cytoplasm as they specialize for movement |
| The final meiotic division is completed when fertilization occurs | Meiosis is completed before mature sperm are released |
The difference reflects their different roles. An egg must supply the early embryo with a large amount of cellular machinery and cytoplasm. A sperm is much smaller and specialized primarily to deliver its genetic material.
What determines how many eggs are available?
The number of remaining oocytes generally decreases over a person’s lifetime because follicles are continually lost through atresia, with a much smaller number progressing toward ovulation.
This is why ovarian reserve is limited. A menstrual cycle does not normally replace the oocyte that was ovulated with a newly manufactured one. Instead, another oocyte from the existing pool is recruited.
With increasing age, both the quantity and the average quality of the remaining oocytes tend to change. One important biological reason is that oocytes can remain arrested in meiosis for many years. The cellular machinery responsible for accurately separating chromosomes becomes more vulnerable to errors over time. Such errors can produce eggs with abnormal chromosome numbers, a condition known as aneuploidy.
Not every age-related change follows the same pattern in every individual, but the decline in ovarian reproductive potential is a fundamental feature of human reproduction.
Does the body make new eggs after birth?
For humans, the established biological model is that the ovaries contain a finite population of oocytes established before birth, followed by continuous loss of that population.
There has been scientific debate over whether rare cells in adult ovaries might have the ability to generate new oocytes under particular experimental conditions. However, this has not overturned the established understanding of human ovarian biology: there is no demonstrated normal physiological process that replenishes the ovarian egg reserve after birth.
For practical purposes, egg development after birth means the maturation and selection of oocytes that were already present in the ovaries, not the routine production of new ones.
The process in one sequence
Egg development can be summarized as a long process with several distinct stages:
Fetal development: germ cells become oogonia, multiply, and enter meiosis.
Before birth: oogonia become primary oocytes, which begin meiosis I and then pause.
Childhood: most primary oocytes remain dormant in primordial follicles, while many follicles undergo atresia.
After puberty: hormones recruit follicles during menstrual cycles, allowing selected oocytes to resume development.
Before ovulation: one dominant follicle usually reaches maturity, and its primary oocyte completes meiosis I, producing a secondary oocyte and a polar body.
Ovulation: the secondary oocyte leaves the ovary and enters the fallopian tube.
If fertilization occurs: the secondary oocyte completes meiosis II, producing another polar body and a mature female gamete.
If fertilization does not occur: the secondary oocyte degenerates without completing meiosis II.
The key idea is that an egg cell is not made anew each month. Its story begins before birth, pauses for years, resumes during the reproductive years, and reaches its final meiotic stage only if fertilization takes place.
