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Oogenesis Medically Reviewed

Last Updated: April 15, 2020

Definition - What does Oogenesis mean?

Oogenesis is the process of development of female gametes (also called ova or eggs), that takes place in ovaries. The process of oogenesis begins before birth and then development is suspended until puberty. Development resumes with ovulation and the final step (meiosis II) is completed only if fertilization occurs.


FertilitySmarts explains Oogenesis

Oogonium

The process of oogenesis begins before birth with the formation of germ cells called oogonia that have the ability to develop into mature eggs. These oogonia are formed before birth. However, the majority of these oogonia degenerate prior to birth and the remaining enter first meiotic division as primary oocytes (also called immature eggs).

Primary Oocyte

After duplicating their genetic material post-birth, primary oocytes are arrested in prophase I and remain arrested at this stage of development for years until the sexual maturity of the individual when ovulation starts. No new primary oocyte will be produced after birth.

Meiosis I

The immature ova then resume the cell division where they left off until they finish the meiosis I (cell division). This yields two daughter cells, each receiving an equal number of chromosomes. This is not the case with the cytoplasm of the primary oocytes, as the entire cytoplasm remains with one of the daughter cells, which is then designated as the secondary oocyte. The other daughter cell that lacks cytoplasm develops into a first polar body. The polar body is nonfunctional, and therefore, degenerates or gives rise to 2 more polar bodies.

Secondary Oocyte

The secondary oocyte is released from the ovary during ovulation. It is destined to become a mature egg but still contains two copies of each chromosome. A mature ovum should have a single set of chromosomes and not two. So, the oocyte has to undergo a second meiotic division. The secondary oocyte is arrested in metaphase II until fertilization. If the secondary oocyte is not fertilized by a sperm within a day, it degenerates.

Meiosis II

The penetration of the sperm into the secondary oocyte induces the second meiotic division. The second meiotic division finally yields a mature egg and since this division is also unequal, half of the chromosomes are transferred to the second polar body and the other half is conserved by the ovum along with all of the cytoplasm. If fertilization is completed, the zygote undergoes further mitotic changes to become an embryo.



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