What is oogenesis?

• Definition: The process of maturation and differentiation of PGC to oogonia, primary oocytes, secondary oocytes and to mature ova in the female genital tract.

 • Location: Ovarian cortex.

 • Peculiarities of oogenesis:
 – Starts before birth (10th week)
 – Stops in the middle (birth to puberty)
– Restarts at puberty (11–13 years)
 – Continues up to menopause (45–55 years)

 • Processes: The various processes in oogenesis are:
 – Mitosis
– Meiosis
 – Growth of follicles
– Differentiation of follicles
• The cortex contains many large round cells called “oogonia”. All the oogonia to be utilized throughout the life of a woman are produced at a very early stage (possibly before birth) and do not multiply thereafter.

 • On arrival in the gonad the primordial germ cells differentiate into oogonia. The oogonia pass through the stages of primary and secondary oocyte and ovum.

• Oogenesis at different phases of life  can be described as:
 – Before birth:
 - Before 3rd month: The PGCs undergo mitosis to form oogonia. This occurs in the absence of testicular differentiation factor (TDF).
 - Before 7th month: The oogonia continue to divide mitotically. The oogonia are surrounded by a layer of flat epithelial cells. Some of the oogonia enlarge to form primary oocytes.
 - 7th month to birth: Formation of primordial follicles (primary oocyte with its surrounding flat epithelial cells) and multiplication of primary oocytes to produce millions of germ cells occurs. Primary oocyte enters prophase I of meiosis I at that phase the meiosis is arrested by oocyte maturation inhibitor (OMI) factor.
- The oogonia are diploid (2n) in chromosome content. Many of these oogonia and primary oocytes degenerate before birth.
 – Birth to puberty:
- There will be both maturation and degeneration of primordial follicle resulting in reduction in the number of primary oocytes.
 - At the time of birth all primary oocytes are in the prophase of first meiotic division. At birth approximately two lakh primary oocytes in primordial follicles are present in each ovary.
- Instead of entering metaphase the primary oocytes enter prolonged resting or diplotene stage.
– After puberty—cyclical preparation for fertilization is known as ovarian cycle.

- From the time of birth to puberty there is degeneration of number of primary oocytes. Rest of the primary oocytes remain in prophase and do not complete their first meiotic division until they begin to mature and are ready to ovulate.
  - The first meiotic division of a primary oocyte produces two unequal daughter cells. Each daughter cell has the haploid number of chromosomes (23). The large cell, which receives most of the cytoplasm, is called the secondary oocyte, and the smaller cell is known as “the first polar body”. The secondary oocyte immediately enters the second meiotic cell division.
 - Ovulation takes place while the oocyte is in metaphase. The secondary oocyte remains arrested in metaphase till fertilization occurs. The second meiotic division is completed only if fertilization occurs. This division results in two
unequal daughter cells. The larger cell is called ovum. The smaller daughter cell is called the second polar body. The first polar body may also divide during the second meiotic division making a total of three polar bodies.
 - If fertilization does not occur, the secondary oocyte fails to complete the second meiotic division and degenerates about 24 hours after ovulation.

Reproductive Period

 In an individual, the formation of gametes takes place only during the reproductive period which begins at the age of puberty (10–14 years). In women, it ends between the ages of 45 years and 50 years, but in men it may continue till the age of 60 years or more.

The period of a woman’s life in which she can bear children is during the reproductive period. The most obvious feature of this period is a monthly flow of blood from the uterus that is referred to as menstruation (or menses). The onset of menstruation (menarche) takes place at about 12 years of age. Menstruation ceases to occur at about 45 years of age, and this is referred to as menopause. The monthly menstruation is the external manifestation of a series of cyclic changes taking place in the uterus. These changes constitute the menstrual cycle.
Simultaneously, cyclic changes also take place in the ovaries, and these constitute the ovarian cycle. The most important event in the ovarian cycle is ovulation. During the reproductive life of a female in each month/menstrual cycle several primary and secondary follicles start developing but only one reaches maturity for release of ovum in that cycle.
  The process of fusion of sperm and ovum is known as fertilization. If fertilization does not take place the  secondary oocyte degenerates. The secondary oocyte at fertilization completes its meiosis II.  In each menstrual cycle, 5–30 primary oocytes in primordial follicles start maturing, but only one of them reaches maturity and is ovulated and the remaining degenerate. During the entire reproductive life of a female, only around 400 ova are discharged (out of 40,000 primary oocytes available). 
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