Oogenesis: Definition, Process, Development, Maturation & Fertilization

Edited By Irshad Anwar | Updated on Jul 02, 2025 07:15 PM IST

What Is Oogenesis?

Oogenesis refers to the process of formation of egg cells in females. This process initiates before birth, continues in monthly cycles during the reproductive years of a woman's life, and concludes at menopause. Sexual reproduction requires this process; it consists of the growth and maturation of oocytes within the ovaries.

This Story also Contains

What Is Oogenesis?
Oogenesis Process
The Ovarian Cycle
Stages Of Oogenesis
Hormonal Control Of Oogenesis
Folliculogenesis
Recommended Video On 'Oogenesis'

Oogenesis Process

This is an extremely intricate process, comprising successive stages of development. All these stages are under the control of hormones and ultimately lead to the formation of mature oocytes.

Definition and Process

Initiates during foetal development.
Involves the development of primary oocytes.
Primary oocytes go into arrested development until puberty.
At each menstrual cycle, some oocytes re-enter meiosis.
Usually, only one oocyte completes the meiosis and is released during ovulation.

Stages in Oogenesis

Primordial germ cells
Development of primary oocytes
Growth and maturation of the oocytes
Ovulation - release of a mature oocyte
Maybe fertilised and developed as an embryo

The Ovarian Cycle

The ovarian cycle refers to the monthly series of events associated with the maturation of an egg and its preparation for possible fertilization.

Overview Of The Ovarian Cycle

Controlled by hormonal changes
Required the development of ovarian follicles
Prepares the endometrium for a potential pregnancy

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Phases Of The Ovarian Cycle

The phases of the ovarian cylcle are:

Follicular Phase

FSH stimulates follicle growth
Dominant follicle secretes estrogen
Ends with ovulation

Ovulation

LH surge triggers the release of the mature egg
Occurs around day 14 of the cycle

Luteal Phase

Corpus luteum formation
Progesterone and estrogen secretion
Prepares the endometrium for implantation

Stages Of Oogenesis

Oogenesis consists of successive stages of development from the time primordial germ cells form until a mature ovum matures and is released.

Prenatal Phase

Formation of Primordial Germ Cells:

Derivate from the yolk sac
Migrate to the developing ovaries

Development into Primary Oocytes:

Enter meiosis I and then arrest in prophase I
Get surrounded by a layer of granulosa cells forming the primordial follicles.

Postnatal Phase

It includes:

Follicular Development

Activation of primordial follicles
Growth to become primary, secondary, and Graafian follicles

Growth And Maturation Of Oocytes

Oocytes increase in size
Granulosa cells grow and multiply into several layers

Puberty To Menopause

The details are given below:

Role Of Hormones

FSH and LH promote the growth and development of the follicle.
Estrogen and progesterone control the menstrual cycle.

Mechanism Of Ovulation

The mature oocyte re-enters meiosis
Ovulation liberates the oocyte from the ovary
Elaborate drawing of stages of oogenesis

Hormonal Control Of Oogenesis

Hormones play a significant role in regulating oogenesis and the ovarian cycle.

Role of Gonadotropin-Releasing Hormone GnRH

Produced by the hypothalamus
Stimulates the release of FSH and LH from the pituitary gland

Follicle-stimulating hormone, FSH

Stimulates follicular growth
Promotes estrogen production

Luteinizing Hormone, LH

Triggers ovulation
Supports corpus luteum formation

Estrogen

Produced by growing follicles
Prepares the endometrium

Progesterone

Secreted by the corpus luteum
Maintains endometrial lining

Hormonal Feedback Mechanisms

Negative and positive feedback loops
Regulation of GnRH, FSH, and LH levels

Folliculogenesis

Folliculogenesis is the process through which ovarian follicles mature and is important for successful oogenesis.
Formation and development of follicles are necessary for the production of a viable oocyte.

Stages of Folliculogenesis

Primordial Follicles: Resting stage
Primary Follicles: Activation and growth begins
Secondary Follicles: Further growth with the formation of the antrum
Graafian Follicles: Mature follicle ready for ovulation
Follicular Atresia: Degeneration of non-dominant follicles. The natural selection process for the healthiest oocyte.

Recommended Video On 'Oogenesis'

Frequently Asked Questions (FAQs)

1. How long does oogenesis take?

Oogenesis is a rather prolonged process that starts during the fetal stage of life, and continues until menopause. Each cycle of maturation takes approximately 28 days.

2. What are the major stages of oogenesis?

The major stages of oogenesis include the prenatal phase of the formation of primordial germ cells and primary oocytes, the postnatal phase of follicular development and maturation, and the phase from puberty to menopause of ovulation.

3. How is oogenesis hormonally regulated?

Oogenesis is controlled by the intricate play of hormones: GnRH, FSH, LH, estrogen, and progesterone. All these hormones orchestrate the ovarian and menstrual cycles.

4. What are common disorders of oogenesis?

The common disorders are Polycystic Ovary Syndrome, in which ovulation does not occur regularly, Premature Ovarian Failure, and endometriosis— all these can affect the normal process of oogenesis.

5. What is the effect of age on oogenesis?

With the advancing age of women, there is a decline in both the quality and quantity of oocytes which leads to a decrease in fertility potential and an increased risk of genetic abnormalities in the offspring.

6. How does the menstrual cycle relate to oogenesis?

The menstrual cycle is closely tied to oogenesis. During each cycle, follicle-stimulating hormone (FSH) stimulates the growth of several follicles, each containing an oocyte. Usually, only one follicle fully matures and releases its egg (ovulation) in response to the luteinizing hormone (LH) surge. If fertilization doesn't occur, the unfertilized egg is shed along with the uterine lining during menstruation, and a new cycle of oogenesis begins.

7. What is the role of cumulus cells in oocyte maturation and fertilization?

Cumulus cells surround the oocyte and play crucial roles in its maturation and fertilization. They provide nutrients and regulatory signals to the developing oocyte, help maintain meiotic arrest, and assist in the final stages of oocyte maturation. During fertilization, cumulus cells help guide and activate sperm, facilitating the fertilization process.

8. How does maternal age affect oocyte quality?

Maternal age significantly affects oocyte quality. As women age, the quality of their eggs tends to decline due to several factors: accumulation of genetic mutations, decreased mitochondrial function, and reduced ability to properly segregate chromosomes during meiosis. This decline in oocyte quality can lead to increased risk of chromosomal abnormalities, miscarriages, and fertility issues in older women.

9. What is the function of the zona pellucida in a mature egg cell?

The zona pellucida is a glycoprotein layer surrounding the egg cell. It serves several crucial functions: it protects the egg during its journey through the fallopian tube, allows only species-specific sperm to bind during fertilization, and prevents multiple sperm from fertilizing the egg (polyspermy). After fertilization, it also helps protect the developing embryo.

10. How does the cortical reaction prevent polyspermy?

The cortical reaction prevents polyspermy (fertilization by multiple sperm) by triggering changes in the egg's surface immediately after the first sperm enters. When a sperm fuses with the egg, it causes cortical granules just beneath the egg's surface to release their contents. This alters the zona pellucida, making it impenetrable to other sperm and preventing multiple fertilizations.

11. How does follicle stimulating hormone (FSH) influence oogenesis?

Follicle stimulating hormone (FSH) plays a vital role in oogenesis by stimulating the growth and development of ovarian follicles. It promotes the maturation of primary oocytes, encourages the production of estrogen by follicular cells, and helps prepare the egg for potential fertilization. FSH is essential for the cyclical nature of oogenesis in mature females.

12. How does the luteinizing hormone (LH) surge trigger ovulation?

The luteinizing hormone (LH) surge triggers ovulation by stimulating the resumption of meiosis in the primary oocyte. It causes the follicle to rupture, releasing the mature egg. LH also initiates the transformation of the remaining follicular cells into the corpus luteum, which produces progesterone to support potential pregnancy.

13. How does the corpus luteum form, and what is its function?

The corpus luteum forms from the remnants of the ovarian follicle after ovulation. The granulosa and theca cells of the ruptured follicle transform into luteal cells, which produce progesterone and some estrogen. The main function of the corpus luteum is to secrete these hormones to prepare the uterus for potential implantation and to support early pregnancy. If fertilization doesn't occur, the corpus luteum degenerates, leading to a drop in hormone levels and the onset of menstruation.

14. What is the role of estrogen in oocyte development?

Estrogen plays several important roles in oocyte development. It stimulates the growth and maturation of ovarian follicles, promotes the proliferation of granulosa cells surrounding the oocyte, and helps in the formation of the antrum (fluid-filled cavity) within the follicle. Estrogen also provides feedback to the hypothalamus and pituitary gland, regulating the release of gonadotropins (FSH and LH) that control the oogenesis process.

15. What is the significance of the germinal vesicle in oocyte maturation?

The germinal vesicle is the nucleus of the immature oocyte. Its breakdown, known as germinal vesicle breakdown (GVBD), is a crucial step in oocyte maturation. GVBD marks the resumption of meiosis I, which had been arrested since fetal development. This process is triggered by the LH surge and is essential for the oocyte to progress to the metaphase II stage, making it ready for potential fertilization.

16. What is the significance of meiotic spindle formation in oocyte maturation?

Meiotic spindle formation is a critical step in oocyte maturation. The spindle is responsible for properly aligning and segregating chromosomes during meiosis. In human oocytes, the meiotic spindle forms after germinal vesicle breakdown and is crucial for reducing the chromosome number to haploid. Proper spindle formation and function are essential for preventing chromosomal abnormalities that could lead to infertility or genetic disorders in offspring.

17. What is the significance of cytoplasmic maturation in oocytes?

Cytoplasmic maturation is a crucial aspect of oocyte development that occurs alongside nuclear maturation. It involves the accumulation of mRNAs, proteins, and organelles necessary for fertilization and early embryonic development. Proper cytoplasmic maturation ensures that the egg has the resources needed to support fertilization, zygote formation, and initial cell divisions before the embryonic genome is activated.

18. What is the significance of the first polar body in genetic testing of oocytes?

The first polar body, formed during the first meiotic division of the oocyte, is significant in genetic testing because it contains a complementary set of chromosomes to those retained in the oocyte. By analyzing the DNA of the first polar body, geneticists can infer the genetic makeup of the oocyte without disturbing the egg itself. This technique, known as polar body biopsy, is used in preimplantation genetic testing to screen for chromosomal abnormalities or specific genetic disorders before in vitro fertilization.

19. How does the zona reaction differ from the cortical reaction?

While both the zona reaction and cortical reaction are mechanisms to prevent polyspermy, they differ in their timing and specific effects. The cortical reaction occurs immediately after sperm entry and involves the release of cortical granules, which modify the egg's surface. The zona reaction, which follows the cortical reaction, specifically alters the zona pellucida, making it impenetrable to additional sperm. The zona reaction is a more prolonged process that ensures long-term protection against polyspermy.

20. How does the oocyte regulate its own maturation through oocyte-secreted factors?

Oocytes play an active role in regulating their own maturation through the secretion of various factors, collectively known as oocyte-secreted factors (OSFs). These include proteins like growth differentiation factor 9 (GDF9) and bone morphogenetic protein 15 (BMP15). OSFs influence the surrounding granulosa cells, regulating their proliferation, differentiation, and function. This bidirectional communication between the oocyte and its surrounding cells is crucial for coordinating oocyte growth, maintaining meiotic arrest, and preparing for ovulation.

21. What is the significance of the first polar body in oogenesis?

The first polar body is formed during the first meiotic division of the primary oocyte. Its significance lies in its role in reducing the chromosome number to haploid while allowing the oocyte to retain most of its cytoplasm and nutrients. The polar body typically degenerates and does not participate in fertilization, ensuring that only one functional egg is produced per ovulation cycle.

22. What is the difference between primary and secondary oocytes?

Primary oocytes are immature egg cells that have not yet completed their first meiotic division. They are formed before birth and remain arrested in prophase I of meiosis until puberty. Secondary oocytes, on the other hand, have completed the first meiotic division and are released during ovulation. They contain half the number of chromosomes of primary oocytes but are still diploid until fertilization triggers the completion of the second meiotic division.

23. What is the significance of meiotic arrest in oogenesis?

Meiotic arrest in oogenesis is crucial because it allows eggs to remain dormant for years or even decades. Primary oocytes arrest in prophase I of meiosis before birth and resume only at puberty. This pause helps conserve cellular resources and ensures that eggs are not fully mature until they are needed for potential fertilization.

24. How does the process of polar body formation contribute to genetic diversity?

Polar body formation during oogenesis contributes to genetic diversity by ensuring that each egg receives a unique combination of chromosomes. When the oocyte undergoes meiosis, it divides unequally, with most of the cytoplasm remaining in the egg and a small portion forming the polar body. This process allows for genetic recombination and random assortment of chromosomes, increasing genetic variability in potential offspring.

25. Why do human females are born with all their potential eggs?

Human females are born with all their potential eggs because oogenesis begins during fetal development. This ensures a large pool of eggs is available for future reproduction. However, this also means that eggs age along with the woman, which can impact fertility and increase the risk of genetic abnormalities in offspring as a woman gets older.

26. How does the process of follicle selection determine which oocyte will be ovulated?

Follicle selection is a complex process that determines which oocyte will be ovulated in each menstrual cycle. Initially, several follicles begin to grow in response to FSH. As they develop, they produce estrogen, which provides negative feedback to reduce FSH levels. The follicle with the most FSH receptors can continue to grow even as FSH levels decline, becoming the dominant follicle. This follicle then produces more estrogen and inhibin, further suppressing other follicles and ensuring its own continued development until ovulation.

27. What is the role of gap junctions in oocyte-granulosa cell communication?

Gap junctions play a crucial role in communication between the oocyte and surrounding granulosa cells. These intercellular channels allow for the direct exchange of small molecules, ions, and metabolites between the cells. This bidirectional communication is essential for coordinating oocyte growth, maintaining meiotic arrest, and regulating metabolic processes. Gap junctions also help transmit hormonal signals that control oocyte maturation and prepare it for ovulation.

28. How does the process of follicular atresia contribute to reproductive efficiency?

Follicular atresia is the degeneration and resorption of ovarian follicles that do not reach ovulation. While it may seem counterintuitive, this process contributes to reproductive efficiency by ensuring that only the healthiest and most viable oocytes are selected for ovulation. Atresia helps conserve energy and resources by eliminating less viable follicles and allows the body to focus on developing the most promising oocytes, thereby increasing the chances of successful reproduction.

29. What is the role of progesterone in oocyte maturation and early embryo development?

Progesterone plays several important roles in oocyte maturation and early embryo development. During oocyte maturation, progesterone helps maintain meiotic arrest and supports cytoplasmic maturation. After ovulation, progesterone produced by the corpus luteum prepares the uterine lining for potential implantation. If fertilization occurs, progesterone continues to support early embryo development by maintaining the uterine lining and suppressing further ovulation.

30. How does the oocyte-cumulus complex facilitate fertilization?

The oocyte-cumulus complex, consisting of the oocyte surrounded by cumulus cells, plays a crucial role in facilitating fertilization. The cumulus cells produce hyaluronic acid, creating a matrix that helps capture and guide sperm towards the oocyte. They also secrete progesterone and other factors that activate sperm and promote capacitation. Additionally, the cumulus cells provide metabolic support to the oocyte during the fertilization process.

31. What is oogenesis and how does it differ from spermatogenesis?

Oogenesis is the process of female gamete (egg cell) formation in the ovaries. It differs from spermatogenesis (male gamete formation) in several ways: oogenesis produces fewer, larger gametes; it begins before birth and pauses until puberty; and it results in only one functional gamete per cycle, while the other potential eggs become polar bodies.

32. What is the role of mitochondria in oocyte quality and fertility?

Mitochondria play a crucial role in oocyte quality and fertility. They are the primary source of energy (ATP) for the egg and early embryo. The number and function of mitochondria in an oocyte can significantly impact its ability to mature, be fertilized, and support early embryonic development. As women age, mitochondrial function in oocytes tends to decline, which is one factor contributing to reduced fertility and increased risk of chromosomal abnormalities in older mothers.

33. What is the significance of the germinal vesicle stage in in vitro maturation of oocytes?

The germinal vesicle (GV) stage is significant in in vitro maturation (IVM) of oocytes because it represents the starting point for artificial oocyte maturation. At this stage, the oocyte is still immature and arrested in prophase I of meiosis. IVM techniques aim to mimic the natural maturation process, guiding the oocyte from the GV stage through germinal vesicle breakdown, completion of meiosis I, and arrest at metaphase II. Successfully maturing oocytes from the GV stage in vitro can provide more options for fertility preservation and treatment.

34. What is the function of the antral cavity in a developing follicle?

The antral cavity, also known as the antrum, is a fluid-filled space that forms within the developing follicle. It serves several important functions: it provides nutrients and hormones to the developing oocyte, allows for expansion of the follicle, and helps in the selection of the dominant follicle. The antral fluid also aids in the expulsion of the oocyte during ovulation and contributes to sperm capacitation during fertilization.

35. How do primordial follicles differ from primary follicles?

Primordial follicles are the most immature stage of ovarian follicles, consisting of a primary oocyte surrounded by a single layer of flattened granulosa cells. Primary follicles, the next stage of development, have a larger oocyte surrounded by a single layer of cuboidal granulosa cells. The transition from primordial to primary follicle marks the beginning of follicular growth and is characterized by the activation of previously dormant follicles.

36. What is the role of cumulus expansion in oocyte maturation and ovulation?

Cumulus expansion is a critical process that occurs just before ovulation. The cumulus cells surrounding the oocyte produce a hyaluronic acid-rich matrix, causing them to spread apart. This expansion serves several purposes: it provides a larger target for sperm, helps guide sperm to the oocyte, protects the oocyte during its journey through the fallopian tube, and may play a role in the mechanical process of ovulation by helping to rupture the follicle.

37. How does the zona pellucida change after fertilization to prevent polyspermy?

After fertilization, the zona pellucida undergoes a process called the zona reaction to prevent polyspermy. This involves the release of enzymes from cortical granules within the egg, which modify the structure of the zona pellucida. These changes alter the zona's protein composition, making it impenetrable to additional sperm. The hardened zona also protects the developing embryo as it travels through the fallopian tube to the uterus.

38. How does the process of follicular waves contribute to oocyte development?

Follicular waves are the cyclical pattern of follicle development in the ovaries. Each wave involves the recruitment, selection, and growth of a cohort of follicles, with usually only one becoming dominant and reaching ovulation. This process ensures a continuous supply of developing oocytes, increasing the chances of having a mature, high-quality egg ready for ovulation each cycle. Understanding follicular waves has important implications for fertility treatments and contraception methods.

39. How does the oocyte's cytoskeleton reorganize during maturation?

The oocyte's cytoskeleton undergoes significant reorganization during maturation. This includes the breakdown of the germinal vesicle, formation of the meiotic spindle, and migration of cortical granules to the oocyte surface. Microtubules play a crucial role in chromosome alignment and segregation, while actin filaments are involved in polar body extrusion an