When you think about your fertility clinic, you probably picture only the part you see during your appointments and meetings with your doctor, but there’s a lot happening behind the scenes in the embryology lab that's critical to the process.
We teamed up with embryologist Jessica Manns to learn some of the basics and some of the uniquely technical things going on in the embryology lab.
What is an embryologist?
Embryologists are medical professionals (but not medical doctors) who manage eggs, sperm, and embryos outside the body. Some of their responsibilities include freezing and thawing sperm, eggs, and embryos, assisting in fertilization of eggs during ICSI, and overseeing and grading the development of embryos. They also maintain the laboratory and equipment to ensure a safe and optimal environment for handling and storing human tissue.
What is an embryologist’s job during the egg retrieval process?
During an egg retrieval procedure, a physician inserts a needle into the Follicles inside of the ovaries and draws up the contents of the follicles into a sterile tube. The contents typically consist of follicular fluid, eggs, and some blood. One sterile tube may contain the contents of multiple follicles.
These tubes are then taken to the IVF lab, where an embryologist empties the contents of the tubes and examines them under a microscope. If eggs are found, they are transferred into a separate dish. In most clinics, the remaining contents are thrown away.
Retrieved eggs are typically still surrounded by the cumulus cells that surrounded them while they were inside of the follicles. Most of the cumulus cells can be carefully trimmed off using a needle, and the remaining cumulus cells can be removed by passing them through an enzyme known as hyaluronidase. Once the eggs are cleaned, the embryologist can assess the maturity of each egg.
Note: Eggs are cleaned prior to ICSI or egg freezing, but not conventional insemination. Insemination or egg freezing typically occurs a few hours after the eggs are retrieved.
Metaphase II (also known as “M2”) is the stage of maturity in which an egg is capable of being fertilized. Once the cumulus cells are removed, only the M2 eggs are fertilized in IVF cycles using ICSI or frozen in egg freezing cycles.
In IVF cycles using conventional insemination, the embryologist combines the sperm with the retrieved eggs and allows the sperm to fertilize the eggs on their own.
How do you know if the eggs fertilized successfully when you look at them the day after the retrieval?
Typically on the day after your egg retrieval you will get an update from the lab about how many of the eggs that were retrieved are showing signs of normal fertilization. There are two indicators of successful fertilization on day 1 (the day after your egg retrieval) in the lab:
The presence of two pronuclei in the embryo. Pronuclei contain the genetic information from the egg and sperm. Eventually, they will fuse together to create the nucleus of the embryo, which will contain two sets of DNA.
The presence of two polar bodies. Eggs go through developmental stages that begin during fetal development. However, eggs must reach a certain level of development in order to be fertilized by a sperm. Eggs that have one polar body are often called M2 (mature) oocytes and are capable of being fertilized. If fertilization occurs, the egg will complete its development and generate a second polar body. Thus, only fertilized eggs should have two polar bodies.
Can you explain briefly how embryos are graded and what patients should keep in mind when they receive their embryo grading report?
The most common system for grading embryos is known as the Gardner scale. This grading system begins once the embryo reaches the blastocyst stage of development (normally around day 4 of development). At this point, a cavity (blastocoel) and two distinct cell lines develop within the embryo. The Gardner scale records an embryo’s appearance based on 3 criteria:
Its stage of development, or level of expansion. This is often recorded as a number from 1-6:
1 = early blastocyst (the cavity takes up less than 50% of the embryo’s volume)
2 = blastocyst (the cavity takes up more than 50% of the embryo’s volume)
3 = full blastocyst (the cavity takes up 100% of the embryo’s volume, but the embryo has not increased in size and the shell (zona pellucida) around it is still thick)
4 = expanded blastocyst (the embryo increases in size as the cavity expands and the shell around the embryo begins to thin out)
5 = hatching blastocyst (the embryo begins to hatch from its shell)
6 = hatched blastocyst (the embryo is fully hatched from its shell)
The appearance of its inner cell mass (ICM). These are the cells that ultimately develop into the fetus. They are usually graded on an A, B, C (sometimes D) scale, with A being the best:
A = compact and round, above average
B = fairly compact, average
C = not compact, below average
D = non-existent or degenerate
The appearance of its trophectoderm (TE). These cells line the shell of the embryo and develop into the placenta and non-fetal tissues. They are usually graded on an A, B, C (sometimes D) scale, with A being the best:
A = many uniform cells, no gaps, above average
B = mostly uniform cells, some gaps, average
C = few to no uniform cells, many gaps, below average
D = non-existent or degenerate
Some labs use different grading systems or variations of the Gardner scale, such as including +/- grades. ask your lab about its grading system to best understand your embryo’s grade.
When receiving the grades for each of your embryos, keep in mind that an embryo’s appearance is only one factor for determining its chance of success. For example, an embryo’s grade cannot determine if the embryo has the correct number of chromosomes (preimplantation genetic testing for aneuploidy, or PGT-A, is required to determine this).
In general, the higher an embryo’s quality, the higher its chance of success. However, lower graded embryos are capable of resulting in healthy pregnancies. Also, embryo grading is subjective, meaning that it can vary slightly between embryologists. In other words, one embryologist may look at your embryo and assign it a certain grade, but another embryologist may look at it and assign it another grade. Your physician can walk you through more specifics about your embryos and chances of success based on their grades.
How are embryos stored when frozen? Is there a time limit for how long they can be safely kept frozen?
When embryos are frozen, they are moved through fluid media that does two important things to help the embryos avoid the formation of ice crystals that can damage them during the freezing process:
Removes water from the cells of the embryo
Coats the cells with a cryoprotectant agent
Once frozen, the embryos are placed in a specialized cryo device (usually a labeled, thin plastic device) and plunged into liquid nitrogen, which is roughly -196℃.
Cryo Devices
These devices are secured inside a plastic cup (goblet) that is attached to a metal cane. One cane can hold multiple devices. The embryos are stored in liquid nitrogen inside specialized containers known as cryotanks or Dewars until they are thawed or discarded. During this time, no cellular activity occurs, meaning that the embryos do not develop. Embryos can remain safely stored in liquid nitrogen indefinitely. While they are stored, they should always be submerged in liquid nitrogen. Embryologists measure the cryotanks/Dewars at least three times a week and fill the tanks at least once a week to ensure that the stored embryos are always submerged in liquid nitrogen. Cellular activity resumes as soon as embryos are removed from liquid nitrogen.
Cryotank
Embryologists have one of the most important jobs in the fertility clinic, but so much of it goes on behind the scenes. To learn more about embryology and all things IVF, find Jessica on Instagram, @explainingivf.