The first and second order oocytes: characteristics and role in IVF

What is an oocyte?

Oocytes are female sex cells that contain half of the genetic material necessary for the formation of an embryo. They mature and are then fertilized by sperm, giving rise to new life. The number of oocytes is strictly limited throughout a woman's life, unlike sperm which are constantly developing.

Difference between primary and secondary oocytes

Primary oocytes

At the end of the third month of intrauterine development, cells that have accumulated enough nutrients and completed the mitosis phase form in the fetus's ovaries. These cells are called primary oocytes. At birth, their number reaches two million, but only 300 to 500 of them go through all the developmental stages and transform into oocytes ready to be fertilized.

Secondary oocytes

After puberty, the oocytes in the follicles enter the first prophase of meiosis and become secondary oocytes. During each menstrual cycle, 15,000 to 20,000 reproductive cells simultaneously enter this phase, but only one (rarely two) reaches maturity and transforms into a complete oocyte equipped with a set of haploid chromosomes (n chromosomes).

Oocytes in an IVF program

During a woman's natural menstrual cycle, only one secondary oocyte typically matures. However, as part of an in vitro fertilization (IVF) program, controlled hormonal stimulation can produce multiple mature oocytes, thus increasing the chances of successful conception.

Influence of the number of oocytes on the effectiveness of IVF

The effectiveness of IVF is directly linked to the number of oocytes obtained. To increase the chances of success, each patient is selected according to an individual scheme of hormonal stimulation of the ovaries, allowing for an average of 15 to 20 secondary oocytes to be obtained.

During follicle puncture, embryologists extract the oocytes, denude them, and prepare them for ICSI fertilization (intracytoplasmic sperm injection). This method is used in cases of reduced sperm motility or low sperm count, and in situations where precise sperm selection is necessary to increase the chances of successful fertilization.

Reduced ovarian reserve and possible solutions

Women with reduced ovarian reserve often obtain only 5 to 7 oocytes after stimulation. A low ovarian reserve is more common in women aged 35 to 40 and can be detected by an anti-Müllerian hormone (AMH) test, which assesses the functional reserve of oocytes in the ovaries. However, depletion of the ovarian follicular reserve is also possible at an earlier age.

Before the IVF program, the patient's ovarian reserve is assessed. If the quantity and quality of the patient's oocytes are insufficient for successful conception, other methods may be proposed:

• IVF with egg donation - a method that uses oocytes from a donor if the patient has a low ovarian reserve or genetic abnormalities.
• IVF with double donation - a variant that uses both an egg and a sperm from donors, increasing the chances of successful conception.

Where can I get advice on oocytes and IVF?

To learn more about primary and secondary oocytes and IVF programs, make an appointment with a specialist at the Natuvitro clinic. The specialists at the clinic will help you assess the status of your ovarian reserve and choose the optimal method for infertility treatment.