2023 Author: Anita Thornton | [email protected]. Last modified: 2023-05-22 03:30
In vitro fertilization (IVF) is now considered one of the greatest successes of modern medicine. It is the most effective treatment in the fertility clinic.
It consists of several steps, including ovarian stimulation, egg retrieval, laboratory fertilization and embryo transfer into the uterine cavity.
After the egg retrieval by the doctor, the entire process of fertilization and embryo monitoring takes place in the confined world of the embryology laboratory before the embryo transfer.
During the puncture, the embryologist identifies the cumulo-oocyte complexes (CCOs), these are the eggs surrounded by a layer of cells (the cumulus). About ten CCOs are recovered on average, but this number varies from one cycle to another, depending on the response to ovarian stimulation. The CCOs are then placed in an incubator at 37°C for approximately 2 hours.
Simultaneously with the preparation of the eggs, the sperm are prepared. They are centrifuged on a density gradient and washed in order to select the most mobile and the most beautiful morphologically.
There are twoways to achieve fertilization: conventional IVF and IVF assisted by intracytoplasmic sperm injection (ICSI). For conventional IVF, the CCOs are cultured with the spermatozoa in a petri dish placed in an incubator at 37°C overnight. It is up to the sperm to make their way to meet the egg.
For ICSI, it is an embryologist who organizes this meeting. For this, it is necessary to prepare the eggs: this is decoronization. It is achieved through mechanical and enzymatic action. Thus the cells of the cumulus will detach from the ovum. Before micro-injecting, it is necessary to check the maturity of the eggs thanks to the presence of the polar body. Indeed, an immature egg cannot be fertilized.
The sperm are then observed under a microscope (sometimes at high magnification: IMSI), in order to select the best candidates. This can also sometimes be done by PICSI. More conventionally, the choice of sperm to be injected is based on several criteria. Mobility and movement are the first selection criteria because it is the assurance of a living sperm. Then, the size of the flagellum, the neck and the head of the spermatozoa are carefully evaluated. After selection, the embryologist immobilizes the spermatozoon by a mechanical action on the flagellum and aspirates it using the micro-injection pipette. The mature egg is then held in place by the holding pipette and the spermatozoon is injectedin this last. This technical gesture must be perfectly mastered so as not to lyse the egg. The eggs are then placed in a 37°C incubator overnight.
Revelation and Division
The signs of fertilization are observed the next day:
The expulsion of the second polar body by the egg indicates that it has completed its meiosis thanks to the presence of the sperm. The egg is then haploid, it has 23 chromosomes.
The presence of the nucleus of each of the two gametes is also a sign of fertilization. This is because the genetic material from the sperm and the egg is still distinct at this stage and is contained in two structures called the pronuclei (figure). The two pronuclei will migrate towards each other and the male and female chromosomes will pair up to generate a new original set of chromosomes. The cell, called a zygote, is then diploid, it has 23 pairs of chromosomes and fertilization is complete.
The first cell division, or mitosis, occurs 25 to 30 hours after fertilization. It makes it possible to generate another cell identical to the first, the zygote then passes to the embryo stage. The embryo then undergoes a series of cell divisions. On the second day, the embryo is made up of 2 to 6 cells and 6 to 14 cells on day 3. On the fourth day, the embryo, made up of about thirty cells, compacts to form a morula. Between days 4 and 5, male DNA is expressed and influences the structure of the embryo. The cells differentiate, rearrange and a cavity forms: this is the blastocyst stage. The blastocyst is composed of the trophectoderm which will form the future placenta and the embryonic button which will form the future fetus.
Embryo transfer is usually performed 3 or 5 days after fertilization. The choice of the day of the transfer depends on the evolution and the embryonic quality. Indeed, a selection is made naturally on the embryos, because they are not all viable. If one or two embryos stand out on day 3, then the transfer is performed on that day. Otherwise, the transfer takes place on day 5.
The chosen embryo is mounted in a catheter and then given to the gynecologist. The latter, under ultrasound guidance, will place the embryo in the uterine cavity. Once deposited, the embryo is juxtaposed with the uterine microvilli. The implantation takes place a few days after the transfer.
It is important to remember that Quebec law prohibits the transfer of two embryos, for ladies under the age of 37. Surplus embryos of good quality are cryopreserved.
The cryopreservation of embryos has been done for several years using the vitrification technique. The survival rate has increased considerably thanks to this method which consists of emptying the embryonic cells of their water and filling them with cryoprotectant, before immersing them in liquid nitrogen (-196°). Thus, the formation of crystals in the cells is avoided. The embryos arethus frozen in time and preserved in nitrogen until they warm up.
Cécile Adam, PhD, embryologist at Fertilys
Marjorie Disdier, M. Sc, embryologist at Fertilys
Véronique Blais, M. Sc, embryologist at Fertilys