What is Oogenesis?

Oogenesis is the process that describes how a mature egg in females develops. Oogenesis is part of a very important process called gametogenesis. Oogenesis helps to ensure that the human species keeps on continuing on the face of the Earth. Oogenesis describes the remarkable journey an oocyte undergoes after it originates from the primordial germ cell. The whole intricate process of oogenesis takes place in the ovarian follicles present in the ovaries. Understanding the process of oogenesis helps us to comprehend female fertility, the menstrual cycle, and female reproductive health.

What is Gametogenesis?

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 whereas the process of formation of the sperm is known as spermatogenesis. In this article, we will look into the oogenesis.

What is Oogenesis?

Oogenesis is the complex process of egg cell development in females. It begins before birth, with oogonia, which differentiates into primary oocytes. These primary oocytes get arrested in the prophase I of meiosis. During a woman's menstrual cycle, some primary oocytes get activated and continue meiosis. Ovulation leads to the release of the secondary oocyte from the Graadian follicle. If fertilization occurs, that would lead to the completion of meiosis II, forming a mature ovum and a second polar body. If fertilization does not occur meiosis II will not take place and the follicle will denature and get ejected out of the body with menstrual blood.

Process of Oogenesis

The process of oogenesis takes place in three steps:

Pre-natal stage

In the process of fetal development, germinal epithelium in the ovary grows much larger than others and divides to form egg mother cells or oogonia. After birth, oogonia continues to divide and develop into primary oocytes. These are covered by granulosa cells to create primary follicles. Before puberty, many follicles degenerate, leaving only around 60,000-80,000 primary follicles in each ovary.

Antral stage

The antral stage of oogenesis involves the formation of the antrum, a fluid-filled cavity, within the primary follicle. It marks the transition from a primary oocyte to a secondary oocyte, which is arrested in metaphase II. This stage leads to the eventual release of a mature ovum during ovulation, ready for fertilization.

Pre-ovulatory stage

Two maturation divisions occur in each primary oocyte; the first and second meiotic division. In the first meiotic division, the primary oocyte divides into two haploid daughter cells, one of which is a larger secondary oocyte and the other a smaller first polar body. In the second meiotic division, the secondary oocyte further divides, forming two more haploid daughter cells, one of which is a larger ootid and the other a smaller second polar body. Eventually, the ootid develops into a haploid ovum, while the three polar bodies degenerate. The ovum is the main female gamete involved in reproduction.

Phases of Oogenesis 

In oogenesis, the process of egg cell development in females, there are three main phases;

• Multiplication phase: In this phase, oogonia multiply through cell division during fetal development, creating a pool of potential eggs. Some of these oogonia become primary oocytes arrested in prophase I, which remain inactive until later in life.
• Growth phase: This phase starts with around 60,000 to 80,000 primary follicles in each ovary. In this phase, selected primary oocytes resume development during the menstrual cycle. They grow in size and are surrounded by granulosa cells within a mature Graafian follicle, and one primary oocyte proceeds to meiosis I. 
• Maturation phase: In this phase, ovulation occurs, releasing the secondary oocyte into the fallopian tube. If fertilization happens, the secondary oocyte completes meiosis II, forming a mature egg ready for fertilization.

Ovulation 

Ovulation is an important event in the menstrual cycle where a mature Graafian follicle, in response to hormonal signals i.e. a surge in Luteinizing hormone (LH), ruptures. This releases the secondary oocyte from the ovary into the fallopian tube, making it available for potential fertilization. Ovulation marks the midpoint of the menstrual cycle and is a key process in human reproduction.

Oogenesis in Non-Human Mammals 

In non-human mammals, oogenesis begins in the germinal epithelium of the ovary, which creates ovarian follicles. Oogenesis involves three stages: oocytogenesis, ootidogenesis, and maturation to form the ovum. Folliculogenesis is a separate sub-process that accompanies and supports all three oogenetic sub-processes.

Formation of Oogonia 

Oogenesis starts with folliculogenesis, oocytogenesis, and ootidogenesis. Oogonia become oocytes through meiosis during embryonic development. Meiosis involves DNA replication and crossing over and ends in early prophase.

Preservation of Meiotic Arrest 

1. Mammalian oocytes are continued in meiotic prophase arrest: Mammalian oocytes are arrested in meiotic prophase due to a lack of cell cycle proteins initially, but later cyclic AMP plays a crucial role. Follicles surrounding the oocyte help in preserving meiotic arrest. Granulosa cells, which make up the follicles are connected by gap junctions. Cyclic GMP, produced by granulosa cells, enters the oocyte and prevents meiotic arrest breakdown.
2. The reinitiation of meiosis and stimulation of ovulation by the luteinizing hormone: As follicles grow, they acquire receptors for the luteinizing hormone, which restarts meiosis in the oocyte, leading to ovulation of a fertilizable egg. The luteinizing hormone acts on receptors in the granulosa cells, causing a decline in cyclic GMP and restarting meiosis in the oocyte. Meiosis stops again at the second metaphase until fertilization. The 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 stimulating 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 the 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 remaining residue of the Graafian follicle is excited by LH to develop into corpus luteum. The ascending level of progesterone checks the ejection of GnRH and, in succession, checks the production of FSH, LH & progesterone.

Significance of Oogenesis

Following are the 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

Also Read:

• Human Reproduction
• Reproduction
• Menstrual Cycle