What is Oogenesis?

Gametogenesis is the formation of both male and female gametes, known as gametogenesis. 

It is of two types, i.e., spermatogenesis and oogenesis. The process of formation of the ovum is known as oogenesis In other words, the female gamete is an egg and this includes various stages of development. Differentiation of the ovum is oogenesis. Gametogenesis in females is known as oogenesis, and it forms the ovum. The process begins in the fetus of a female. Some steps take place in oogenesis until the production of primary oocyte occurs before birth. Primary oocytes do not divide further. They may change into secondary oocytes or degenerates. Oogenesis happens in the outer layer of the ovaries. Germ cell starts the process of oogenesis.

Process of Oogenesis

The process of oogenesis takes place in three steps:

• Pre-natal stage: In fetal development, in the germinal epithelium of the ovary fetus definite cells are of huge size than others. By mitosis, these cells divide and give rise to egg mother cells or oogonia. After birth, the formation of oogonia ends. By mitotic division, oogonia multiply to create primary oocytes. Granulosa cells covered the primary oocyte to create the primary follicle. During the time from birth to puberty, follicles in a large amount degenerate. So, at the time of puberty, only 60,000–80,000 primary follicles are left in each ovary. The antrum is the fluid-filled cavity of the follicle. 
• Antral stage: It is a very long phase of the primary oocyte. Inside a large primary oocyte, the oogonium grows by taking nutrition from the surrounding follicle cells after puberty.
• Pre-ovulatory stage: Two maturation divisions take place in each primary oocyte, i.e., first mitotic and second meiotic. In the first meiotic division, two unequal haploid daughter cells are formed by the division of the primary oocyte. A large haploid daughter cell is a secondary oocyte, and a very small haploid daughter cell is the first polar body. In the second meiotic division, the first polar body divides into the second polar body. The secondary oocyte splits and forms two unequal daughter cells, a large daughter cell is an ootid and a very small daughter cell is the second polar body. The ootid develops into the haploid ovum. Hence, from oogenesis, one ovum and three polar bodies are formed. The ovum is the main female gamete. Only the ovum takes place in reproduction, and three polar bodies degenerate.

In human beings, the ovary eject ovum in the secondary oocyte stage. In the fallopian tube, the maturation of secondary oocytes takes place after the entry of sperm for fertilization. 

In humans, the first polar body does not go through meiosis II, although the secondary oocyte takes about the metaphase stage of meiosis II. But, it afterward ends proceeding further; it waits for the entry of sperm for the accomplishment of meiosis II. The cell cycle breaking down MPF (M—phase promoting factor) and turning on APC (Anaphase of promoting complex) restarts, after the entry of sperm. Fulfillment of meiosis II changes the secondary oocyte into a fertilized ovum or zygote. 

Phases of Oogenesis 

• Multiplication phase: In the germinal epithelium of the female ovary, definite cells present are bigger than compared to others during fetal development. In consequence, these cells break by mitosis, generating a few million oogonia in every ovary present in the fetus. There are no oogonia left, which are formed after birth. 
• Growth phase: This specific course of action of the primary oocyte tends to be elongated. In this, the oogonium brings up larger oocytes. Later, each primary oocyte acquire adjoined by a granulosa cells layer to generate a primary follicle. Afterward, many follicles acquire immoral while birth to puberty. Consequently, at puberty 60,000 to 80,000 primary follicles may be found in each ovary. 
• Maturation phase: Alike to a primary spermatocyte, each primary oocyte experiences two maturation dividing lower oogenesis are very different from those which happen in spermatogenesis. Thinking the first meiotic division, the primary oocyte separates into two irregular haploid daughter cells. These are called large secondary oocytes and small polocytes. 

Ovulation 

Inside the ovaries development of oocytes takes place. Follicle cells are attached to oocytes to form follicles. As the menstrual cycle begins, primary oocytes begin to grow large, and follicle cells increases and bring about to grow larger follicle. Usually, a few bring-up oocytes degenerate and only one oocyte remains here to get mature. Now, identical twins are created which are genetically distinct. 

When a follicle achieves full growth, the primary oocyte completes its primary meiotic division and becomes a secondary oocyte. Immediately, the follicle breaks and a secondary oocyte gets free in the fallopian tube, even second meiotic division does not occur yet. This liberation of a secondary oocyte from the ovaries is known as ovulation.

Oogenesis in Non-Human Mammals 

In mammals, in the germinal epithelium oogenesis begins, which produces the development of ovarian follicles, of the ovary. Oogenesis comprises some sub-processes: oocytogenesis, ootidogenesis, and maturation to create the ovum. Folliculogenesis is an unconnected sub-process that goes with and holds up all three oogenetic sub-processes. 

Formation of Oogonia 

The formation of oogonia commonly does not belong to oogenesis actual, nevertheless rather, it is to the usual procedure of gametogenesis, which in human females, starts with the procedure of folliculogenesis, oocytogenesis, and ootidogenesis. Oogonia burst into meiosis through embryonic development, becoming oocytes. Meiosis starts with DNA replication and meiotic crossing over. It then ends in the early prophase.

Preservation of Meiotic Arrest 

Mammalian oocytes are continued in meiotic prophase arrest for interminability – months in mice, and years in humans. At first, the arrest was because of a lack of enough cell cycle proteins to allow meiotic development. But, as the oocyte increases in size, these proteins are systemized, and meiotic arrest becomes based on cyclic AMP. The cyclic AMP is caused by the oocyte by adenylyl cyclase in the oocyte membrane. The adenylyl cyclase is remain energetic by an essentially energetic G – protein – combined receptor known as GPR3 and a G—protein, Gs, also existing in the oocyte membrane.

Preservation of meiotic arrest is also based on the existence of a composite compound of cells, known as follicles, that nearby the oocyte. Taking away the oocyte from the follicle brings about meiosis to development in the oocyte. The cells that consist of follicles, known as granulosa cells, are attached to one and all by proteins as gap junctions, that permit tiny molecules to move between the cells. The granulosa cells give rise to tiny molecules, cyclic GMP, that spreads into the oocyte through the gap junctions. In oocytes, cyclic GMP stops the breakdown of cyclic AMP by the phosphodiesterase PDE3, and thus, continues to maintain meiotic arrest. The cyclic GMP is caused by the guanylyl cyclase NPR2.

The reinitiation of meiosis and stimulation of ovulation by the luteinizing hormone 

As follicles get bigger, they obtain receptors for luteinizing hormone, a pituitary hormone that reinitiates meiosis in the oocyte and is the cause of the ovulation of an egg that is eligible for fertilization. Luteinizing hormone function on receptors in the external layer of granulosa cells of the follicle, and be a cause of the decline in cyclic GMP in the granulosa cells. Since the granulosa cells and oocytes are linked by gap junctions, cyclic GMP also declines the oocyte, causing meiosis to restart. Meiosis then continues to the second metaphase, where it stops again until fertilization. Luteinizing hormone also restores gene expression leading to ovulation.

Ovarian Aging

Menopause is the last step in the process mentioned as ovarian aging. The age-connected decline in follicle numbers gives orders to begin the cycle irregularly. The equivalent decomposition in oocyte quality contributes to a drop in fertility and the final presence of natural sterility. Endocrine swap mostly reduces ovarian factors at the hypothalamic-pituitary unit.

Hormonal Control of Oogenesis

Hypothalamus produces GnRH which excites the anterior lobe of the pituitary gland to produce LH and FSH. Graafian follicles and the maturation of oocyte inside the follicle are excited by the FSH hormone to accomplish meiosis I to form a secondary oocyte. The emergence of estrogen is also excited by FSH. Whereas, breaking of the mature Graafian follicle and release of secondary oocyte prompt by LH. The rest part of the Graafian follicle is excited by LH to develop into corpus luteum. The ascending level of progesterone holds back the ejection of GnRH, in succession, holding back the production of FSH, LH & progesterone.

Significance of Oogenesis

• One oogonium gives rise to one ovum and three polar bodies.
• The emergence of polar bodies continues half the number of chromosomes in the ovum.
• In the course of meiosis first, crossing over takes place which brings variation.

Oogenesis Timeline and Stages

Cell Type	Chromosomes	Chromatids	Process	Time of completion
Oogonium	diploid/46(2N)	2C	Oocytogenesis(mitosis)	Third trimester
primary oocyte	diploid/46(2N)	4C	Ootidogenesis (meiosis I) (Folliculogenesis)	Dictyate in prophase I for up to 50 years
secondary oocyte	haploid/23(1N)	2C	Ootidogenesis (meiosis II)	Halted in metaphase II until fertilization
Ootid	haploid/23(1N)	1C	Ootidogenesis (meiosis II)	Minutes after fertilization
Ovum	haploid/23(1N)	1C

FAQs on Oogenesis

Question 1: What is oogenesis? 

Answer: 

The development of the female gamete ( ovum ) is called oogenesis. It takes place in the ovaries. 

Question 2: What is spermatogenesis?

Answer:

It is a process of the formation of sperm (male gametes) known as spermatogenesis.

Question 3: What is a secondary oocyte? 

Answer:  

In the first meiotic division, two unequal haploid daughter cells are formed by the division of the primary oocyte. A large haploid daughter cell is a secondary oocyte.

Question 4: Name the hormones that control oogenesis. 

Answer:

Hypothalamus produces GnRH which excites the anterior lobe of the pituitary gland to produce LH and FSH. Thus, LH and FSH control oogenesis.

Question 5: What is the significance of oogenesis? 

Answer:

• One oogonium gives rise to one ovum and three polar bodies.
• The emergence of polar bodies continues half the number of chromosomes in the ovum.
• In the course of meiosis first, crossing over takes place which brings variation.