Oogenesis: Definition, Stages and Sample Questions

Oogenesis is the gametogenesis process through which mature female gametes are formed. It is a vital part of the human reproductive process. Oogenesis occurs in the female reproductive system. It is a growth process through which a mature ovum is formed from a primary egg cell (or ovum) in the oogonia. This process of development starts in the female ovary even before conception. In this article, we will have a look at the process of oogenesis, its various stages, ovulation and fertilization.

Keyterms: Oogenesis, gametogenesis, gametes, human reproductive process, reproductive system, primary egg cell, ovum, ovary, ovulation, fertilization, spermatogenesis, sperms

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Gametogenesis

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Gametogenesis is defined as the process through which gametes are formed in reproducing organisms. In mammals, the gametogenesis process in males is called spermatogenesis (formation of sperms) and in females, it is called oogenesis (formation of ovum).

The video below explains this:

Oogenesis Detailed Video Explanation:

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Oogenesis

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Oogenesis begins in the female when the female fetus is of 8-20 weeks. In the female, each ovary has about one million oogonia or gamete mother cells at this stage. No other oogonia are produced or added after the birth of the female. These cells divide mitotically to form the primary oocyte. An oocyte is a cell in the ovary which might undergo division to form the ovum. These are around 400,000 in number and are known as the primary ovum. The production of all primary oocytes occurs before birth after which they do not divide further until the female reaches puberty. Those primary oocytes either develop into secondary oocytes or degenerate. 

When these egg cells are released from the ovary, it breaks down into a cell and the nucleus divides in a way that half chromosomes go into one cell and a half into another. A secondary ovum is formed which is the largest of the two cells that are formed, the smaller one is referred to as the polar organism. The secondary ovum is transported to a fallopian tube after it breaks down on maturation. It is then suitable for fertilization by the sperm cells.

Oogenesis takes place in the outermost layers of the ovaries. It occurs in the germinal epithelium cells inside the ovary of a female fetus. The epithelial cells undergo mitosis to form an oogonium which further divides mitotically to form several oogonia. Both these cells are diploid.

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Stages Of Oogenesis

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The process of oogenesis is completed in three stages.

• Pre-Natal Stage 

The oogonia divide rapidly and one cell enlarges to form the primary oocyte. The other cells form the layer of follicular epithelium around the oocyte. This whole structure is known as the primary follicle. The nucleus of the primary oocyte undergoes division but gets arrested in Prophase-1 of the meiotic division. No further division occurs in the primary oocyte before puberty.

In the period from birth to puberty, a huge number of primary follicles are degenerated Therefore, at the age of puberty, each female ovary has just around 60,000-80,000 primary follicles.

The follicular cells form a stratified columnar epithelium called the Granulosa cells. These cells secrete glycoproteins which later form the thick transparent membrane called zona pellucida around the oocyte. When the primary follicle gets surrounded by several layers of granulosa cells and a new theca (or membrane), it is called the secondary follicle.

• Antral Stage

This stage is characterized by the presence of antrum which is formed as a result of the combination of a fluid-filled area in between the granulosa cells. The secondary follicles thus become the tertiary follicles. The theca layer is now differentiated into inner theca interna and outer theca externa.

• Pre-Ovulatory Stage 

The primary oocyte in this stage grows in the follicle and completes the first meiosis division, induced by the LH surge. As a result, two unequal cells are formed - a large haploid secondary oocyte and a tiny first polar body. The secondary oocyte retains almost the whole nutrient-rich cytoplasm of the primary oocyte. But, the fate of the tiny first polar body remains unknown for now. The polar body does not participate in the formation of the ovum any further.

The tertiary follicle now grows further to form the mature Graafian follicle. The secondary oocyte also forms the new membrane, Zona Pellucida, around it.

Read about: Internal Formation of Zygote

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Ovulation

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The Graafian follicle rises to the surface of the ovary, under the influence of the FSH and estrogen hormone. At the surface of the ovary, the Graafian follicle ruptures and releases the ovum around the 14th day of the menstrual cycle. This process is called Ovulation. At the time of ovulation, the egg is in the secondary oocyte stage.

When the menstrual cycle begins, follicles rise in number and the primary oocytes start to grow bigger. As a result, the follicle also becomes larger. Usually, some oocytes degenerate and leave behind one follicle to mature, forming fraternal twins which are genetically distinct. On maturity, a secondary oocyte is formed as a result of primary meiotic division. Ovulation refers to the release of this secondary oocyte from the ovaries when the follicle breaks even without the second meiotic division. After ovulation, the secondary oocyte undergoes meiosis – II. But it gets arrested at the metaphase stage of Meiosis-II.

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Fertilization

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Meiosis-II of the secondary oocyte is completed during fertilization. When the sperm enters the zona pellucida layer during fertilization, the secondary oocyte completes its meiosis-II. This division is also unequal and gives rise to a second polar body and a haploid ovum (egg cell). Then, the haploid sperm nucleus and the haploid ovum fuse to form the diploid zygote. This process is called Fertilization.

If the fertilization does not occur, the secondary oocyte will degenerate, 24 hours after ovulation, while still being arrested in the meiosis-II stage. Thus, the menstrual cycle continues.

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Things To Remember

• Oogenesis is the process of formation of mature female gametes, starting before the birth of the female which continues till fertilization.
• The primary oocyte gets surrounded by a layer of follicular cells to form the primary follicle.
• These primary follicle keeps growing and matures into the secondary follicle, the tertiary follicle (characterized by fluid-filled space called antrum) and then finally the mature Graafian follicle.
• The Graafian follicle ruptures in the middle of the menstrual cycle under the influence of FSH and estrogen hormones. The egg in the Graafian follicle is in the secondary oocyte stage.
• The secondary oocyte divides to form ovum only in the case fertilization is to happen. Otherwise, it degenerates and continues the menstrual cycle in females.
• The primary oocyte is a diploid cell whereas both secondary oocyte and ovum are haploid cells.
• The first polar body also replicates to form two polar bodies. Thus, a total of three polar bodies can be formed in the process of oogenesis.
• The difference between oogenesis and spermatogenesis is that the process of oogenesis starts before birth in females whereas spermatogenesis begins at puberty in males.