During her lifetime, a woman produces between 300 and 400 eggs, one per month. But even before this reproductive cell is emitted with a view to possible fertilization, it already has behind it, a long journey of several years … Back on the life cycle of this cell far from being like the others.

The oocyte is the female reproductive cell (the female gamete), like the sperm is the male reproductive cell. At 0,0001 mm, the oocyte is the largest cell in the human body. When she reaches childbearing age (puberty), a woman produces one (more rarely two) oocyte (s) per month, emitted alternately by each ovary. During the thirty years or so when she is fertile, between 300 and 400 matured oocytes are selected from the ovarian reserve, a kind of oocyte stock that a woman has at her birth.

If we often assimilate oocyte and ovum, there is still a subtlety. Indeed, according to the scientific acceptation, the term ovum describes the moment when the oocyte, having reached maturity, is penetrated by a spermatozoon. Very ephemeral, this stage evolves rapidly with the appearance of the egg, the fertilized oocyte, then the embryo, from the start of cell division.

The ovary plays an essential role in the production of the oocyte in two ways.

• on the endocrine level, the ovaries secrete progesterone and estrogen, two hormones essential for the good progress of the menstrual cycle and pregnancy. Among them, estrogen is particularly important in the life cycle of the oocyte. Indeed, it will not only participate in the thickening of the uterine lining (necessary to accommodate an embryo), but also allow the maturation of the oocyte in the ovary.
• The ovary also has an exocrine function decisive. Seat of the ovarian reserve, the ovary shelters, each month, several follicles reached different stages of maturation. At the end of the follicular phase of the menstrual cycle, only one of them reaches maturity before detaching from the ovary and beginning its migration. It’s ovulation.

The oocyte and its follicle begin their development well before the appearance of the female cycle, from uterine life. In fact, during the first weeks of gestation, the ovarian reserve is built up. It is oogenesis.

Its course? From 12 weeks onwards, oogonia, stem cells containing 23 pairs of chromosomes (called diploid), divide to form primary oocytes. This first phase of cell division, meiosis, is blocked at an early stage which does not resume until many years later, during ovulation. These primary oocytes, also diploid, are already surrounded by cells corresponding to a primordial follicle, on which the evolution of the said cell will depend. Around the 5th month of pregnancy, between 2 and 000 primordial oocytes are nested in the ovaries of the fetus. From then on, they begin to degenerate to be no more than 000 at birth.

Between birth and puberty, the oocytes remain dormant. Some of them degenerate over the years, so much so that at puberty, the ovarian reserve “only” counts about 200 oocytes. The other is the stock from which each month an oocyte is produced.

From then on, a menstrual cycle of 28 days (on average) begins. Divided into 3 phases, this cycle is regulated by the secretion of different hormones that allow each month an ovary to emit an oocyte with the prospect of pregnancy, then the body to evacuate the said cell if there is no has not had fertilization.

After menstruation and until the middle of the cycle (about 10 days) takes place the follicular phase of the cycle. In the ovary, several follicles and the oocyte they contain mature gradually over the course of two cycles. At the end of this first phase, one of them matures (De Graaf’s follicle) under the influence of the hormone FSH (follicular stimulation). It is then expelled from the ovary and begins its migration into the fallopian tube where it waits between 12 and 24 hours before fertilization. It is l’ovulation.

At the same time, the oocyte completes its first cell division to keep only 23 chromosomes (it is then haploid). It is only during fertilization that meiosis resumes with its second division, until the formation of 46 chromosomes which will constitute the genetic identity card of the embryo.

During the last phase of the female cycle, the luteal phase, the follicle breaks down to form the corpus luteum, which secretes progesterone. The objective: to participate in the thickening of the uterine lining in order to provide the ovum with a hospital environment and maintain a possible pregnancy.

 Two scenarios then arise each month:

• if the oocyte is fertilized, it gradually migrates from the tube to the uterine cavity and attaches itself to the mucous membrane around 7 days after fertilization. This is the start of The nidation. Under the influence of the hormone HCG, secreted by the outer layer of the fertilized oocyte, the corpus luteum continues to produce progesterone, beyond the luteal phase, for 3 months. Pregnancy sets in.
• if the oocyte is not fertilized, the corpus luteum secretes progesterone until the end of the cycle and the unfertilized oocyte is evacuated. In the absence of a fertilized oocyte, the corpus luteum is no longer stimulated and degenerates. Result: the progesterone level collapses and the rules appear. Another oocyte then resumes its maturation. The menstrual cycle begins again.

Egg donation at a glance

Governed in France by the law of July 29, 1994, egg donation allows a woman aged 18 to 37, in good health, to donate her gametes, anonymously and free of charge, to couples facing fertility or health problems related to women (early menopause, risk of transmission of a serious hereditary disease, following heavy treatment preventing ovulation, etc.). Note: since 2016, the donation is no longer conditional on prior maternity and donors are now authorized to request the conservation of part of the gametes removed in the event that they themselves are confronted with infertility.

This donation takes place in several stages within a CECOS (Center for the Study and Conservation of Eggs and Sperm), backed by a University Hospital.

• Information: a first medical consultation makes it possible to answer all the questions of the candidate for donation and to make her aware of the legal and medical questions related to the planned procedure.
• The consent : a pre-donation consent form is signed not only by the donor, but also by her spouse (whether or not they are related by marriage).
• A medical check-up then allows to take stock of the medical history of the donor, the possible contraindications to the donation and to carry out the first clinical and biological examinations necessary for the evaluation of the fertility of the candidate.
• An interview with the psychologist allows, before the start of medical treatment, the donor (and her spouse if she wishes) to verbalize her motivations, to move forward in her process.
• Ovarian stimulation promotes the maturation of several oocytes for 10 to 12 days. Similar in many ways to the medically assisted procreation treatments offered to couples in the context of IVF or ICSI IVF, this treatment is subject to close ultrasound and biological monitoring.
• Sampling : once the oocytes have reached maturity, the sample is taken under local or general anesthesia as part of a one-day hospital stay. The oocytes thus collected are then immediately allocated to a recipient or frozen for later use.