Oogenesis

Oogenesis is a cycle of female gametes formation. Before conception, this cycle starts inside the Embryo. Before birth, the steps in Oogenesis up to primary oocyte production occur. The primary oocytes are not further divided. They either become or degenerate into secondary oocytes.

Oogenesis takes place in the outermost ovary layers. Oogenesis starts with an oogonium-like germ Cell and undergoes mitosis to increase in number. The Oogenesis process is carried out in the following three stages:

• Preborn

• Check-in

• Pre-ovulatory therapies

In the human female reproductive system, Oogenesis is the process of growth in which the primary Egg Cell (or ovum) becomes a mature ovum. In any human generation, the development of the Egg begins before the female carrying it is even born; 8 to 20 weeks after the fetus has begun to grow, Cells that are to become mature ova have multiplied, and by the time the female is born, all the Egg Cells that the ovaries will release during the female's active reproductive years are already present in the ovaries. Known as the primary ova, these Cells number around 400,000. The primary ova remains dormant until when an Egg is released from the ovary, just before Ovulation. For 40 years, some Egg Cells do not mature; others will degenerate, and never mature. The Egg Cell remains as the primary ovum until the time has come for its release from the ovary. The Egg then breaks into a Cell. The nucleus divides in such a way that half of the chromosomes go into one Cell and a half into another. Typically one of these two new Cells is larger than the other and is known as the secondary ovum; the smaller Cell is known as a polar organism. In the ovary, the secondary ovum grows until it reaches maturation; it breaks loose and is transported into the fallopian tubes. The secondary Egg Cell, once in the Fallopian tubes, is suitable for fertilization by male Sperm Cells. 

Process of Oogenesis

The process of Oogenesis is completed in the following three stages:

Pre-Natal Stage: The primary oocyte grows while at meiosis-I being arrested. They proliferate and form a stratified cuboidal epithelium. Such Cells are referred to as granulosa. Those Cells secrete glycoproteins around the primary oocyte to form zone pellucid.

Antral Stage: The fluid-filled spaces between granulosa Cells merge together to form a central fluid-filled space called the antrum. These are referred to as secondary follicles. Those secondary follicles develop during each monthly cycle under the influence of follicle-stimulating hormone and luteinizing hormone.

Pre-Ovulatory Stage: LH surge induces this stage and meiosis-I complete here. Inside the follicle are formed two haploid Cells of unequal sizes. A polar body forms one of the daughter Cells which receives less cytoplasm. This Cell is not involved in the development of an ovum. The secondary oocyte is known as the other daughter Cell. Meiosis-II occurs in the two daughter cells. The polar body replicates to form two polar bodies whereas the secondary oocyte arrests in the meiosis-II metaphase stage.

Ovulation

Oocyte formation occurs in the ovaries. To form a follicle each oocyte is neighbored by follicle Cells.

As the menstrual cycle begins, primary oocytes begin to grow larger, and the number of follicle cells increases, causing the follicle to grow larger too.

Some nursing oocytes usually degenerate and leave one follicle only to mature. Fraternal twins, which are genetically distinct, can be born here.

The primary oocyte begins its primary meiotic division when a follicle reaches maturity and becomes a secondary oocyte. Shortly after, in the Fallopian tube, the follicle splits and secondary oocytes are released even though the second meiotic division has not occurred. That release from ovaries of a secondary oocyte is known as Ovulation.

Fertilization

On fertilization Meiosis-II is complete. This produces a third polar structure. When there is no fertilization, the oocyte degenerates 24 hours after Ovulation while remaining arrested in Cell division meiosis-II.

The diploid germ Cells which could develop into ova are called oogonia. In humans, all the oogonia of a female she will make in her lifetime are created when she is still a fetus and wasn't even born yet. Yes, about one or two months before a baby girl is born, most of her about seven million oogonia die, and the remaining oogonia that survive joins meiosis I and becomes primary oocytes. These primary oocytes, after having replicated their genomes, press the pause button on their development in prophase I but before they have made the first meiotic division. They remain detained for more than a decade at this stage of development before the girl starts 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.

Once the primary oocyte eventually completes its first meiotic division, as you would assume, it equally separates the chromosomes. It's not dividing its cytoplasm equally though. In one of the two daughter cells remains nearly all of the cytoplasm, which becomes a secondary oocyte. The other daughter cell is called a polar body, which gets half the chromosomes but very little cytoplasm. The polar body is not a functional oocyte but degenerates and dies instead. The creation of a polar body allows the primary oocyte to halve its genome and retain the remainder of its cytoplasm in the secondary oocyte. The secondary oocyte still has two copies of each chromosome, so it must undergo the second meiotic division if it is to become a fully-functional ovum. Similar to the first, this division is also unequal with half of the chromosomes heading to another very small degenerate polar body and half of the chromosomes being retained by the ovum along with nearly all the cytoplasm. The ovum retains its haploid state in this way while maintaining as much cytoplasm as possible.

Benefits of Referring to Vedantu for Studying Oogenesis

The topic of Oogenesis has been categorized among the various important concepts of class 12 Biology. It is counted in one of the initial chapters of the textbook. 

In particular, Oogenesis is from chapter-3 'Human reproduction' of the Class 12 NCERT textbook. 

• On the website of Vedantu, you can download the full Pdf for the Class 12 Biology textbook for referring to chapter 3- Human reproduction. This chapter gives a detailed account of Oogenesis along with the important diagrams. 

• Vedantu covers a detailed definition of Oogenesis. Students are advised to make sure and check the weightage of their answers before attempting the questions. In this way, they will avoid vague points and overdoing them. 

• The definition related to the process of Oogenesis is then followed by another main component of the topic- three stages of Oogenesis. 

• Experts at Vedantu have explained the concept in short and clear paragraphs for each stage. These stages are very important from the exam point of view. 

• Instead of making your own points, you can check the website and refer to the points already made by the experts. The points are kept quite comprehensible and easy to understand. 

• Vedantu makes sure that the explanations involved with each chapter or any particular topic are matched with the level of every student who is joining hands in the learning process with Vedantu. 

• Experts make sure that the points are- ‘easy to remember’. In addition, they break down the complex words into simple language so that every student is able to understand them easily. 

• Followed by these topics, Vedantu covers all the details about the Ovulation process. While learning the process of Oogenesis, Ovulation and fertilization come as a compulsory package that you should make sure to understand well. Experts at Vedantu explain both topics in detail.  

• The concept of Oogenesis has a good number of diagrams. Students are advised to refer to practice all of them if they are planning to score well on it. Vedantu gives a diagram that subject experts have considered to be important. 

• In diagrammatic representations, it's important that students practice the diagram related to the process of Oogenesis first. Followed by it, they are advised to practice the diagrams indicating the difference between the two processes. 

• For Instance- Students are asked to explain the difference between - Spermatogenesis and Oogenesis. While giving a theoretical explanation for the answer, you can support your answer by adding simple diagrams of both processes. It will help you in scoring the best on it.