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Primary Oocytes: Definition & Concept

Instructor: Christine Morgan

Christine has taught college Biology and Anatomy, and has a Master's degree in Anatomy.

Primary oocytes arrive by the millions, but each one is unique. Learn more about the precursors of life and what it takes for these cells to survive to maturity.

Genetics - It's A Numbers Game

It may seem hard to believe, but the story of the oocyte, or egg, actually starts just weeks after a female has been conceived - long before she is ready to have her own children. By the fourteenth week of fetal growth, many of her seven million or so oocytes have even begun a process of dying off that will continue throughout her life. Roughly a quarter million are left by the time she reaches puberty, or reproductive maturity -- and each one is unique thanks to the special process that starts not long after her own conception.

In the bundle of embryonic cells that will become a new individual, you can see a special layer that gives rise to gametes, or reproductive cells. In females, these are undifferentiated oogonia, not yet specialized for their purpose of joining with a sperm to give rise to a new life.

Mature Sperm and Egg at Conception
Photo of sperm and egg at conception

Oogonia have the same amount of genetic material, or DNA, as all the other cells in the body. Just think what would be created from a match of sperm with those cells! The individual would have way too much DNA and could not survive.

Oocyte DNA: Less Is More

It probably makes sense that all regular body cells, including oogonia, must grow and divide by a process called mitosis, where the cellular and genetic material, or DNA, is equally shared between the two new cells after division. These are all called diploid cells because they have the usual two copies of DNA (di means two - one copy from mom and one from dad).

Of course, mature gametes, eggs and sperm, need to have half of the original DNA. This is called a haploid genetic arrangement -- the egg has mom's DNA copy and the sperm has dad's. Because of this, the oogonia have to begin a different route of division called meiosis, which ensures they end up with the right amount of DNA so that conception will result in a healthy individual.

Chart of Mitosis (diploid) vs. Meiosis (haploid) Development
Chart of Mitosis vs. Meiosis development

As this process of meiosis starts, the cells are now called primary oocytes and begin to grow and change. The DNA of these cells has condensed, or thickened, and been duplicated. If you've ever wondered how inherited information becomes so mixed up and random throughout the generations, here is where it happens. First, the copied and condensed DNA strands, called chromosomes, become closely paired. Then, there is literally an exchange of DNA material from one to another in a process called crossing over -- this ensures no two oocytes are ever exactly the same.

Depiction of Chromosomes Crossing Over
Drawing of Crossing Over

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