Gametogenesis in females happens in ovaries and the formation of gametes in the ovaries is termed as oogenesis.

Oogenesis in females begins before they are even born. It starts at the 6-8th week of the fetus development. At this stage, the primordial (primitive) germ cells starts differentiating into oogonium within the ovaries. They start dividing vigoursly, at around 20th week of fetus development, the amount of oogonium present will be around 6-7 million in number. After 20th week, the oogonium starts degenerating and by the time of birth around 2 millions of oogonium will be found in the fetal ovaries. [1]

Oogonium, are diploid (2n) stem cells that divide mitotically to produce millions of germ cells. Even before birth most of the cells degenerate in process known as atresia. The oogonium that is survived starts differentiating, where only few of them forms primary oocytes. So at the time of puberty only around 60,0000 to 80,000 primary oocytes are formed and will be in arrested phase of meiosis I prophase (diplotene stage). During this arrested phase each primary oocyte is surrounded by a single layer of flat follicular cells, and the entire structure is called a primordial follicle.

Oogenesis Process

Out of 60,000 to 80,000 primary oocytes at puberty only around 400 will mature and ovulate during the women’s reproductive lifetime. Each month after puberty until menopause, gonadotropins (FSH & LH) are secreted by the anterior pituitary gland which further stimulates the development of several primordial follicles. Out of which only one will typically reach the maturity needed for ovulation and develops into primary follicles.

The primary follicle which was arrested at meiosis I prophase (diplotene stage) starts dividing and forms secondary follicle. The secondary follicle eventually becomes larger turning into mature (graafian) follicle.[2]

Just before ovulation the diploid primary oocyte completes meiosis I producing 2 haploid (ncells of unequal size each with 23 chromosomes. The smaller cell produced by meiosis I is called the first polar body, is essentially a packet of discarded nuclear material. The largest cell known as secondary oocyte receives most of the cytoplasm. After the formation of secondary oocyte it begins meiosis II, but will be arrested at the metaphase II stage.

The mature (graafian) follicle soon ruptures and releases its secondary oocyte, a process known as ovulation. At ovulation the secondary oocytes are swept into the uterine tube, if fertilization doesn’t occur the cell degenerates. If the sperm is present in the uterine tube and one penetrates the secondary oocyte, the arrested meiosis II resumes. The secondary oocyte splits into two haploid cells again of unequal size. The larger cell is the ovum or mature egg. The smaller one is the secondary polar body. The nuclei of the sperm cell and the ovum then unites forming a diploid zygote. [2]

References :

1. Principles of Anatomy And Physiology 12th edition By Gerard J. Tortora and Bryan Derrickson

2. Langman’s Medical Embryology 12th edition by T.W Sadler

The Ovulation Cycle

During the time of puberty in females, out of 60,000 to 80,000 available primary oocytes, only 400 of them gets a chance to form a mature oocyte. Only one of the mature oocyte out of the 400 will be released every month which can get fertilized when it comes in contact with the sperm. [2]

How does all this happen?

The hypothalamus produces a hormone called the gonadotropin-releasing hormone or GnRH. GnRH stimulates the anterior lobe of the pituitary to secrete follicle-stimulating hormone (FSH).FSH travels to the ovaries and stimulates a group of follicles to grow, were one of them will  survive and become a mature follicle, the rest die.

FSH triggers the stimulation of the follicles

FSH stimulates estrogen production. The increasing level of estrogen acts on the hypothalamus and the anterior pituitary to increase the level of GnRH and induce the production of another hormone, luteinizing hormone (LH). [3]

FSH triggers the production of estrogen

A surge in LH secretion triggers ovulation – the release of the egg from the follicle and the ovary.

Release of mature oocyte

Fertilization by a spermatozoon, when it occurs, usually takes place in the ampulla, the widest section of the Fallopian tube. The fertilized egg immediately begins the process of development while travelling toward the uterus.





  1. Textbook of medical physiology, 11th edition , Arthur C. Guyton and John E. Hall.
  2. www.youtube.com/watch?v=RFDatCchpus
  3. www.youtube.com/watch?v=pNe43KGZTl8&t=101s

Stimulated Ovulation Cycle

The main steps involved in this cycle are:

  • Super-Ovulation:
    Naturally, in each ovarian cycle one follicle becomes mature and gets ready for fertilization. Now women who have infertility problem and is going to undergo IVF, FSH are given as subcutaneous (under the skin) injection that will regulate ovulation, the growth and development of eggs in the ovaries.[4]
Administration of FSH for Ovarian follicle stimulation
  • Trans-vaginal Ultrasound: 
    After FSH treatment, ultrasound scans are needed to monitor the response of the follicles growth in the ovaries. The growth of the follicles is assessed by observing their increase in size using a trans-vaginal ultrasound.[4]
Trans-vaginal Ultrasound

If the follicles seen on the scan is in the range of 16 mm to 20 mm in size, then the trigger shot is given as mentioned in the next step. This hormone initiates the final maturation and release of the eggs. This mimics the LH surge that stimulates ovulation during normal cycle.

  • Taking a hCG shot:
    The next step in IVF treatment is triggering the oocyte for the last stage of maturation, before retrieval. This last growth is triggered with an injection. This is also called the “hCG (human Chorionic Gonadotrophin) Trigger shot”.[4]
hCG trigger shot


The injection is given when the follicles have grown in range of 16 to 20 mm in size. This shot is typically a one-time injection.


  • Going for the Gold- Retrieval of Eggs:
    About 34 to 36 hours after the “trigger shot” is received, the egg retrieval or ovum pick up will take place.
Retrieval of the egg




  1. cimarindia.org/specialities/fertility/ivf-treatment/
  2. www.lifefertility.com.au/ivf-step-by-step/
  3. http://www.fertilitytoday.org/frequent_questions.html
  4. http://www.thebridgeclinic.com/Doctors-on-fertility/entryid/68/steps-to-ivf-treatment-administration-of-drugs