Oocyte: Function, Oogenesis, Structure, Fertility and Abnormalities

It is produced in the ovary by an ancestral cell called oogonium and gives rise to the ovule, which can be fertilized.

An oocyte is a female germ cell in the process of development.

An egg is the egg before it is released at ovulation.


An oocyte is the germ cell involved in the reproduction process of women.

It is one of the largest cells in the body (approximately 110 μm in diameter) and develops in the ovarian follicle, a specialized unit of the ovary, during the process of oogenesis / folliculogenesis in the cortex.

The oocyte must be supplied with numerous molecules that direct the growth of the embryo and control cellular activities.

Many of these molecules are delivered maternally to the oocyte.


  • Cytoplasm: Oocytes are rich in cytoplasm, which contains many types of molecules necessary to nourish the cell at the beginning of development.
  • Nucleus: During the primary stage of oocytes in oogenesis, the nucleus is called the germinal vesicle.
  • Mitochondria: The oocyte receives the mitochondria of the maternal cells, which will control the embryonic metabolism and the apoptotic events. The division of the mitochondria is carried out through a system of microtubules that will locate the mitochondria throughout the oocyte.
  • Nucléolo: It is a structure that is inside the nucleus, it is the place where the rRNA is transcribed, and it is assembled in the ribosomes. The nucleolus is dense and inactive in a mature oocyte, for the proper development of the embryo.
  • Ribosomes: Maternal cells also synthesize and provide a reserve of ribosomes that are required for the translation of proteins before the zygotic genome is activated.


The diploid germ cells that have the potential to become ovules are called oogonia.

In humans, all the oogony of a woman that she will create in her life is created when she is still a fetus and has not yet been born.

In fact, approximately one or two months before a girl is born, most of her approximately seven million oogonios die, and the remaining oogonia that remains enters meiosis I and becomes primary oocytes.

These primary oocytes press the pause button in their development in prophase I, after they have replicated their genomes, but before they have made the first meiotic division.

They remain under arrest at this stage of development for more than a decade until the girl begins her first menstrual cycle.

Then, during the next 30 to 45 years, monthly, the primary oocytes resume meiosis where they remained and complete the first meiotic division.

When the primary oocyte finally completes its first meiotic division, it divides the chromosomes into equal parts, as would be expected.

However, it does not divide your cytoplasm equally.

Almost all the cytoplasm remains in one of the two daughter cells, which becomes a secondary oocyte.

The other daughter cell, which gets half the chromosomes but very little cytoplasm, is called the polar body.

The polar body is not a functional oocyte, but degenerates and dies.

The formation of a polar body allows the primary oocyte to reduce its genome by half and retain most of its cytoplasm in the secondary oocyte.

The secondary oocyte still has two copies of each chromosome, so if it is to become a fully functioning ovum, it must undergo the second meiotic division.

This division is also uneven, like the first, with half of the chromosomes going to another very small degenerate polar body and half of the chromosomes are retained by the ovum along with almost the entire cytoplasm.

In this way, the ovum achieves its haploid state while retaining as much cytoplasm as possible.

There are two types of oocyte maturation that can be schematized in the following way:

Prenatal maturation:

The ovules are formed in the ovary from cells called oogonia that proliferate by mitotic division.

All oogonies are enlarged to form primary oocytes, of which around 2 million are present at the time of the female’s birth.

No more primary oocytes are formed after birth, in contrast to the continuous production of primary spermatocytes that form in the male after puberty.

Ovarian stromal cells surround the developing primary oocyte to form a single layer of flattened follicular cells.

The primary oocyte surrounded individually by its flat epithelial cells constitutes the primordial follicle.

The oocytes continue dividing by mitosis, increase their size and duplicate their DNA.

The number of oogonia increases rapidly until the fifth month of fetal development and it is there where the number of germ cells of the ovaries reaches its maximum number.

At this moment the stage of cellular degeneration begins and many oogonios and primary oocytes are destroyed spontaneously.

The primary oocytes that survive, initiate the first meiotic division before birth, but do not complete the prophase until after puberty, detained in the resting phase of the prophase, until before ovulation occurs.

Postnatal maturation:

In this phase the oocytes are divided into primary and secondary oocytes.

Primary oocytes:

These are arrested in prophase I and remain dormant in the ovaries until puberty and for 50 years.

The inhibitor of oocyte maturation is a factor created by follicular cells during the primary maturation of oocytes.

It is believed to be the reason why the oocyte remains so long in the immature state of meiosis.

During the first dictation in prophase I, the nucleus takes a special form of germinal vesicle.

The decomposition of the germinal vesicle , equivalent to the rupture of the nuclear envelope in somatic cells, indicates a resumption of meiosis and the extrusion of the first polar body, indicates the completion of the first meiotic division in oocytes.

Just before each ovulation, 5 to 15 primary oocytes complete the first meiotic division.

Unlike its male counterpart, the division of the cytoplasm is unequal, the secondary oocyte obtains almost all of the cytoplasm and the first polar body receives little cytoplasm, which is a small, non-functional cell.

At ovulation, the secondary oocyte nucleus begins the second meiotic division progressing only to the metaphase.

Secondary oocytes:

The primary oocytes were stopped in metaphase II until fertilization, at this time, the oocytes immediately enter the second meiotic division becoming a secondary oocyte.

The meiosis of a secondary oocyte is completed only if a sperm manages to penetrate its barriers.

Then meiosis II is resumed, producing a haploid ovule that, at the time of fertilization by a sperm (haploid), becomes the first diploid cell of the new offspring (a zygote).

Thus, the ovum can be thought of as a short, transitional, haploid stage between the diploid oocyte and the diploid zygote.

When fertilization occurs and the second meiotic division is completed, the mature oocyte retains most of the cytoplasm, while the second polar body degenerates.

The ovule, is the only fertilizable oocyte that measures around 120 to 150 mm and the polar bodies, which are not larger than 10 mm are not fertilizable.

About 2 million primary oocytes are found in the ovaries of a female newborn and between 30 and 40 thousand remain at puberty.

Only between 200 and 400 of these reach full maturity after puberty and are expelled during ovulation during the reproductive life of the female.

The secondary oocyte released at ovulation is a large cell and is surrounded by the zona pellucida and a layer of follicular cells, the corona radiated.


It has become a current social trend for women to delay motherhood.

However, the quality of the oocytes is compromised and the pregnancy rate is lower in older women but within the fertility range.

With the higher rate of delayed maternity, it is increasingly important to understand the mechanisms underlying the compromised quality of oocytes in older women, including mitochondrial dysfunctions, aneuploidy, and epigenetic changes.

The establishment of appropriate epigenetic modifications during oogenesis and early embryonic development is an important aspect in reproduction.

The reprogramming process may be influenced by external and internal factors that produce inappropriate epigenetic changes in the germ cells.

In addition, future generations could inherit the epigenetic changes in the germ cells.

Studies suggest that age-related effects, including epigenetic changes, in oocytes can be prevented with diets, medications or other methods.

However, until now, the harmful effects related to age in oocytes can not be effectively prevented.

The cause of female infertility is related to the problems of ovulation and damage to the fallopian tubes, which indicates the lack of health of the reproductive system in general.

These symptoms are mainly reflected in the quality and quantity of oocytes, which decrease in a deteriorated reproductive system.

The oocyte has the highest number of mitochondrial DNA copies of any cell.

The status and function of mitochondria are crucial for oocyte quality, oocyte fertilization and embryonic development.

The poor quality of the oocytes in one of the main causes of failed in vitro fertilization.

Other fertilization techniques such as intrauterine insemination also depend to a large extent on the ovulation cycles and the quality of the oocytes.

Disorders in the reproductive system are also characterized by additional complications, such as polycystic ovarian disease, accompanied by hyperandrogenism, insulin resistance, abnormal lipid profile and oxidative stress.

The health of oocytes affects reproductive and general health beyond fertility.


One of the disadvantages of the formation of the oocytes at a single time in the fetal stage of the woman, is that they age which results in abnormalities in the cells.

For example, meiotic non-disjunction results in aneuploidy, most are lethal embryonic and are not seen and this potential for genetic abnormalities increases with the age of the mother.

Among the abnormalities caused by aneuploidy we have the autosomal aneuploidy of the chromosome and the aneuploidy of the sexual chromosome, which will depend on the type of chromosomes that are affected.

The aneuploidies are caused by the addition or loss of chromosomes and depending on the number of chromosomes that are added or lost we have for example:

  • Trisomy 21: Down Syndrome.
  • Trisomy 18: Edwards Syndrome .
  • Trisomy 13: Patau Syndrome.
  • Monosomy X: Turner syndrome.
  • Trisomy X: Triple-X Syndrome.
  • 47 XXY: Klinefelter syndrome.