Do you know what the process called for the formation of gametes in females is? This process of formation of gametes is called oogenesis. Why is oogenesis important? How is the female gamete formed? Do you know some processes necessary for the formation of female gametes that take place before the birth of the female child? Did you know the female body produces all the lifetime egg cells before birth itself? If yes, then how much? Do all the primitive germ cells that multiply eventually end up with the gamete formation? Or do some of them get degenerated? Read on to explore the answers to all these questions and to study the process of oogenesis in detail.

Oogenesis Definition

Gametogenesis refers to the processes of the formation of gametes, which in turn takes place by oogenesis and spermatogenesis. Oogenesis refers to the formation of mature female gametes (ova). This process occurs in the ovaries.  The ovary is one of the primary female reproductive organs.

Spermatogenesis refers to the formation of sperms that occurs in testis.

Oogenesis Process

Prenatal Development

1. Primordial germ cells (PGCs) of both testes and ovaries are extragonadal in origin. It originates from the endoderm of the yolk sac and then proliferates and migrates to the gonads of the developing embryo.
2. Primordial germ cells are the most primitive germ cells. These first appear in the wall of the yolk sac around the fourth week of development. These germ cells proliferate by the process of mitosis and migrate to the gonads of the developing female embryo.
3. In the first month of the gestation period, there are about 1700 primordial germ cells in the ovary.
4. In the ovary, primordial germ cells further multiply to form oogonia.
5. After a significant number (7 million) of oogonia has been achieved, some oogonia enlarge, and others degenerate. At this point, mitotic division stops.
6. These enlarged oogonia are surrounded by flat epithelial cells and are called primary oocytes.
7. Primary oocytes with a sheath of flat epithelial cells are called primordial follicles.
8. As of now, mitosis goes, its chromosome number remains the same as that of primordial germ cells.
9. Now, after the oogonium has changed to the primary oocyte, the mitosis process stops, and meiosis begins.
10. When primary oocytes reach the diplotene stage of the prophase of Meiosis-I division, they arrest their further progression. Till this stage, the progression of oogenesis takes place before birth.
11. At birth, there are 2 million primary oocytes.
12. Now, we can say that all the primary oogonia have been converted into primary oocytes.
13. At birth, there are no oogonia.

Postnatal Development

1. Most of the primary oocytes degenerate from birth till puberty, and their number from 2 million becomes 40,000.
2. These all 40,000 primordial follicles having primary oocytes have been arrested at the diplotene stage of Meiotic-I.
3. During the monthly cycle, 15-20 primordial follicles start growing, and many of them change to primary follicles.
4. Primary follicles have a primary oocyte surrounded by multilayers of follicular cells, which in turn are surrounded by some connective tissue.
5. Surrounding the primary oocyte, there is a glycoprotein layer called zona pellucida, which is being secreted by primary oocyte and follicular cells.
6. The primary follicle then forms a secondary follicle.
7. The secondary follicle has a fluid-filled space called the antrum. Some follicular cells that remain around the ovum and zona pellucida are called cumulus oophorus.
8. Surrounding the secondary follicle is a layer of connective tissue called theca folliculi. Its cells which are more internally placed, are called theca interna, while those that are externally placed are called theca externa.

Fig: Secondary Follicle

9. In the end, usually, only one secondary follicle develops, and others get degenerated. The mature secondary follicle is also called the Graffian follicle.

Fig: Graafian Follicle

10. The primary oocyte in the secondary follicle completes its Meiotic-I division and forms a first polar body and a secondary oocyte, which has more cytoplasm. It has 23 chromosomes.
11. After completion of meiosis-I, the secondary oocyte enters its meiosis-II but gets arrested at the metaphase stage. During this time, it gets ovulated with the polar body and some oophorus cells from the Graffian follicle.
12. This secondary oocyte travels to the fallopian tube.
13. The cells of the oophorus rearrange themselves around secondary oocytes to form corona radiata.
14. This secondary oocyte waits for sperm for 24-48 hrs. If it does not come, then the secondary oocyte will degenerate, while if it does, then the secondary oocyte rapidly completes its second meiotic division and forms a mature ovum and a second polar body. This mature ovum has 23 chromosome numbers.
15. When sperm induces its nucleus into the ovum, then both male and female pronuclei fuse to form the zygote.

Fig: Ovarian Follicular Development

The process of Oogenesis can also be divided into three phases:

1. Multiplication Phase: It refers to the phase where primordial germ cells continue to multiply mitotically. Primordial germ cells form oogonia. Oogonia changes to primary oocytes. Once the primary oocyte is formed, the mitosis process stops, and meiosis starts. This process multiplication phase starts from the fetal stage itself.

2. Growth Phase: The growth phase of primary oocytes is very long, extending over many years. The oogonium grows into a large primary oocyte. Each primary oocyte gets surrounded by many follicular cells to form a primordial follicle. Which, after attaining puberty, grows further into the primary follicle. Primary oocyte in primary follicle secretes glycoprotein to form zona pellucida around it.  This primary follicle then changes into the secondary follicle. A mature secondary follicle is the Graafian follicle. During this phase, yolk accumulation occurs in an egg called vitellogenesis. The size of an egg increases 200 times in humans. Size of the nucleus, number of mitochondria, reserve food, etc., increases.

3. Maturation Phase: This phase is characterized by meiosis. In this, the diploid primary oocyte undergoes meiosis-I (reductional division) to form two unequal haploid cells, i.e. first polar body, which has very little cytoplasm and a larger secondary oocyte. This secondary oocyte undergoes meiosis II but gets arrested again in metaphase II. Ovulation occurs at this time, i.e., when the oocyte has completed meiosis-I, but meiosis II is stopped at metaphase-II, which is completed only after sperm entry, i.e. when sperm touches the secondary oocyte. After meiosis II (equational division), there is the formation of a second polar body and a mature ovum.

During ovulation, the secondary oocyte, polar body, and some oophorus cells also come out of the ovary. Oophorus cells form corona radiata around the oocyte.
The first polar body may divide to form two polar bodies. All three polar bodies (see in the figure given below) get degenerated. So, only one ovum is formed from one primary oocyte.
When the nucleus of sperm, the male pronucleus, enters the ovum, fuses with the female pronucleus, the zygote is formed.

Fig: Oogenesis

How Does Oogenesis Differ from Spermatogenesis?

The differences between oogenesis and spermatogenesis are tabulated below:

Oogenesis	Spermatogenesis
It is the process of formation of ova in females.	It is the process of the formation of sperms in males.
It occurs in the ovary.	It occurs in testis.
It is a discontinuous process.	It is a continuous process.
It starts before birth.	It starts after puberty.
An oogonium produces one functional ovum and three nonfunctional polar bodies.	Spermatogonium produces four functional spermatozoa.
Ovum is much larger than sperm.	The sperm is much smaller than the ovum.
Vitellogenesis occurs.	No yolk formation occurs here.

Significance of Oogenesis

1. Haploid ovum with 3 polar bodies is produced by this process.
2. A sufficient amount of cytoplasm is retained in the ovum to support the life of the early embryo.
3. Crossing over during Meiosis-I may bring variation.
4. It is an important and complex process in mammals facilitating female gamete development, which begins inside the fetus before birth.

Summary

Oogenesis is the process by which mature haploid ovum is formed in females. In this article, we came to know about its complete formation process, which starts before the birth of a female child.  Primordial germ cells multiply to form oogonia. Some of these oogonia get enlarged to form primary oocytes, which, when surrounded by flat epithelial cells, form primordial follicles. At birth, there are about 2 million primordial follicles. Once oogonia is changed to the primary oocyte, mitosis stops, and meiosis begins. Before birth, meiosis-I halts at the diplotene stage.

After puberty, a few primordial follicles grow and change to primary follicles that still contain primary oocytes during each monthly cycle. One of the primary follicles grows further to form a secondary follicle. When matured is called Graafian follicle, it contains a secondary oocyte having completed its meiosis-I and started meiosis-II but gets arrested at metaphase stage. Ovulation of the secondary oocyte takes place at this stage. Completion of meiosis-II, triggered by the arrival of sperm, forms a mature ovum.

FAQs

Q.1. What are the three different phases of oogenesis?
Ans: Three different phases of oogenesis are:-
(a) Multiplication phase
(b) Growth phase
(c) Maturation phase

Q.2. Does oogenesis begin at puberty?
Ans: No, oogenesis does not begin at puberty, but it continues at puberty that has already started before birth.

Q.3. How many eggs does oogenesis produce?
Ans: One egg is produced.

Q.4. Does oogenesis occur in the ovary?
Ans: Yes, oogenesis occurs in the ovary.

Q.5. Does oogenesis produce polar bodies?
Ans: Yes, oogenesis produces polar bodies.

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