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Oogenesis: How the Female Reproductive System Produces Eggs

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  • 0:05 The Ovum
  • 0:55 Meiosis
  • 1:28 Oogenesis
  • 3:47 Ovary Function and Ovulation
  • 4:54 Fertilization
  • 5:44 Lesson Summary
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Lesson Transcript
Instructor: Joshua Anderson
Did you know that a human female already has all of the oocytes that she will make in her entire life before she is even born? In this lesson, you'll learn how the human body produces an ovum through the process of oogenesis. You'll also learn how polar bodies help to ensure that female gametes are of high quality even though they are not produced in high quantities.

The Ovum

Oogenesis is the process by which the female gametes, or ova, are created. The female gamete is called an ovum. Sometimes people will refer to female gametes as eggs, but the term egg can include more than one stage of development, and the definition of an egg also changes depending on the type of organism.

Definition of oogenesis
Oogenesis Definition

For example, the entire prenatal development of birds occurs inside an egg, but in placental mammals, after the egg is fertilized and starts dividing, nobody calls it an egg anymore. So, we're going to use the more precise term for a mature female gamete which is ovum (or ova for the plural form). Now you may recall, each ovum must be haploid and contain only one copy of each chromosome.

Meiosis

You may also remember that in order to create haploid gametes, a cell must go through the process of meiosis, which involves replicating its genome and then dividing, not once, but twice. In theory, this would create four haploid gametes from a single diploid cell, like it does during spermatogenesis.

Illustration of meiosis in action
Meiosis Illustration

However, this isn't the case for human female gametes. Let's go through the process of oogenesis to see how this works and how many haploid gametes are made from a diploid cell.

Oogenesis

The diploid germ cells that have the potential to develop into ova are called oogonia. In humans, all of a female's oogonia that she will make in her lifetime are created when she's still a fetus and hasn't even been born yet. In fact, about one or two months before a baby girl is born, most of her approximately seven million oogonia die, and the remaining surviving oogonia enter meiosis I and become primary oocytes. These primary oocytes press the pause button on their development in prophase I, after they've replicated their genomes, but before they've made the first meiotic division. They stay arrested at this stage of development for over a decade until the girl begins her first menstrual cycle. Then, for about the next 30 to 45 years, on a monthly basis, primary oocytes resume meiosis where they left off and complete the first meiotic division.

When the primary oocyte does finally complete its first meiotic division, it divides the chromosomes evenly, just as you would expect. However, it does not divide its cytoplasm equally. Almost all of the cytoplasm remains in one of the two daughter cells, which becomes a secondary oocyte. The other daughter cell, which gets half of the chromosomes but very little cytoplasm, is called a polar body. The polar body is not a functional oocyte, instead it degenerates and dies. The formation of a polar body allows the primary oocyte to reduce its genome by half and conserve most of its cytoplasm in the secondary oocyte.

The secondary oocyte still has two copies of each chromosome, so if it's going to become a fully-functional ovum, it must undergo the second meiotic division. This division is also uneven, like the first one, with half of the chromosomes going to another very small degenerate polar body and half of the chromosomes being retained by the ovum along with almost all of the cytoplasm. In this way, the ovum achieves its haploid state while conserving as much cytoplasm as possible.

Illustration of oogenesis in action
Oogenesis Illustration

Ovary Function and Ovulation

In humans, oocyte development occurs in the ovaries. Each oocyte is surrounded by a number of follicle cells to create a follicle. When the menstrual cycle begins one, or maybe even a few, primary oocytes begin to grow larger and the follicle cells increase in number and cause the follicle to grow larger too. Usually, some of the developing oocytes will degenerate and only leave one follicle that will mature, but every once in a while two, or even more, follicles will mature. In these cases, fraternal twins or triplets that are genetically distinct may be born. In any case, as a follicle reaches maturity, the primary oocyte completes its first meiotic division and becomes a secondary oocyte. Very soon after that, the follicle ruptures, and the secondary oocyte is released into the fallopian or uterine tube, even though the second meiotic division hasn't occurred yet. The release of a secondary oocyte from the ovaries is called ovulation.

Definition of ovulation
Ovulation Definition

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