Trying For A Baby / 30 October, 2023 / Ellie Thompson

What Happens Behind The Embryology Laboratory Door? Top Embryologist Reveals All

What happens in an embryology laboratory? After weeks of fertility investigations, blood tests, scans and injections, the day of egg collection is finally upon you. Then, just like that, you walk out of the clinic and must now entrust the embryology team with your precious eggs and sperm, along with your hopes and dreams of having a baby. But have you ever wondered what happens behind that laboratory door and what goes into creating your embryos? Victoria Wigley, a Senior Clinical Embryologist of 14 years, and Founder of All About Embryology is on hand with an overview of all aspects of embryology, explaining how the laboratory team work so meticulously for every one of their patients on each day of their IVF/ICSI treatment cycle.

Day -1 (the day before egg collection)

Our work in the laboratory starts even before the patient’s egg collection has begun. It is crucial that the day before the egg collection, the laboratory team prepare all the necessary dishes and tubes of culture media that are required to collect and nourish the eggs and sperm. It is necessary we do this a day in advance to allow all the culture media to warm and equilibrate to the correct temperature and pH to mimic the natural environment. We also ensure all the plasticware is correctly labelled with the patient’s name, date of birth and unique identification number. In some cases, they will also be marked with an electronic tagging system to enhance the sample security. All our incubators and storage systems are also installed with external alarms so the Embryologist will be alerted if anything goes amiss in the laboratory (even at 3 am!).


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Day 0 (the day of egg collection)

On this morning, the Embryologists are all set and ready for their patient. Along with the medical team, we first perform an identification check on the patient prior to the sedation to ensure the correct pre-labelled dishes and tubes are used.

During the egg collection procedure, the doctor will aspirate each follicle on the ovaries and draw the fluid out into a tube. This fluid will then be meticulously examined by the embryologist under a highly magnified microscope to ensure all eggs are retrieved and collected into the culture dishes. Some eggs are easier to spot than others, so the same fluid will be checked numerous times if the egg isn’t instantly found. Once the egg collection is complete, the dishes containing the retrieved eggs are placed into the incubator to ensure the eggs are kept in optimal conditions. It is vital that the embryologist works with speed and diligence when the eggs are outside of the incubator to maintain these conditions.

At the same time as the egg collection, the male sperm sample is prepared ready for insemination. Using centrifuges and a suspension solution, we can isolate the highest quality sperm from the ejaculate.

In the afternoon, we perform the insemination. If the sperm quality is within normal parameters, we can do conventional IVF. With this method of insemination, we mix a small volume of the prepared sperm sample, equivalent to approximately 100,000 motile sperm, with the culturing eggs. We then leave these to attempt to fertilise overnight. I have been known on many occasions to sing to the eggs and sperm when I perform the insemination to try and set a more romantic scene! If the sperm sample is poor, or there has been previous poor/failed fertilisation with conventional IVF, then we need to use ICSI as the method of insemination. ICSI is a technique whereby the individually selected sperm is injected directly in the centre of the egg using a microscope set up that somewhat resembles a PlayStation! It requires intensive concentration, excellent hand-eye coordination, and precision – and often results in very achy shoulders after a long case.

Day 1

This day is usually a very early start for the embryologists (often as early at 06:30/07:00!). This early start is to ensure that we can assess the eggs at the point in time where they show their fertilisation status. This transient period is when the embryologist can visualise the male and female pronuclei in the centre of the egg. These pronuclei contain the genetic material from the mother and the father, and visualising two pronuclei confirms normal fertilisation. Eggs can fertilise abnormally or may not fertilise at all. On average, with both conventional IVF and ICSI, we’d expect approximately 60-70% fertilisation. This percentage will vary between patients and can often be indicative of the cause of infertility if it is low. The embryologist will call the patient once they have assessed the fertilisation – this can often be a very difficult phone call for us to make if it is bad news for the patient, but we ensure we are as supportive and considerate to the patients as possible.

Day 2

On day 2 of development, the second stage of development occurs; cell division. The fertilised egg divides into two cells and each of these cells divide again, and so on. The patient now has developing embryos, and by checking the embryos on the morning of day 2, the embryologist has the job to start assessing each embryo for its cell number and quality. To grade the quality, we primarily look at the level of cell fragmentation within the embryo, the evenness of the cells and the granularity within the cells. We often need to roll the embryo around in the dish using a fine pipette to be able to count the cells and gauge the quality (you must remember the embryo is 3-dimensional and not a flat object as it appears in images).

Day 3

By day 3 of development, it is expected that the embryo contains at least 6-8 cells, and each embryo can again be assessed according to cell number and quality in the morning. Today is often the first day that a patient may be called in for embryo transfer. If a patient only has one or two embryos growing, or embryos are not developing as expected, then it may be advised to transfer on day 3.

At embryo transfer, the embryologist and doctor will discuss with the patient the number of embryos to transfer. The doctor will then position the catheter in the middle of the uterine cavity and the embryologist will load the embryo(s) into the catheter to be released. I used to always have a quiet word with the embryos at this point to politely ask them to please implant!

Day 4

If the patient has embryos remaining after a day 3 transfer or they are having a day 5 transfer, the embryos will continue to be cultured in the incubator. Day 4 is a transition day for the embryos; they are undergoing huge changes and the cells start to compact together to form a ball of cells called a morula. We often won’t assess the embryos on this day and leave them to progress in the incubator until the following day.

Day 5

This day marks the final stage of embryo development that occurs around day 5-6, and this is blastulation (blastocyst formation). At this stage, the cells within the embryo start differentiating into two distinct cell lines: the inner cell mass and the trophectoderm. The inner cell mass cells are destined to form the baby itself, whilst the trophectoderm cells form all the extra-embryonic material such as the placenta and the amniotic sac. A new grading system is applied on this day and the embryologist must now look at the expansion of the blastocyst, the quality of the inner cell mass and the quality of the trophectoderm cells. Approximately only 50% of cleavage stage embryos will make the transition to the blastocyst stage, which can feel disheartening for the patients, but this helps the embryologist identify the strongest embryos from the cohort, and those that are most likely to achieve the patient a pregnancy.

Embryo transfer most commonly occurs on day 5, with the selection of the strongest blastocyst(s) for transfer. Any remaining embryos that have formed good quality blastocysts can be vitrified (‘frozen’). This is a very rapid process where all the fluid is first removed from the blastocyst to ensure no ice crystals form, and then the blastocyst is exposed to a solution of cryoprotectant to protect it in the freezing process. It is then loaded onto a straw-like device and plunged into liquid nitrogen at -196°C. This sounds very daunting for the blastocysts, but this new rapid freezing technique has greatly improved their survival, and most clinics can boast a 90-95% survival rate. In fact, there is a higher chance that the Embryologist’s finger cells will be hurt with a cold burn during the process than the blastocyst’s cells be damaged, which is certainly a good thing (for the embryos anyway!).

Day 6

The remaining embryos that have not been transferred or vitrified are cultured until day 6 with the hope for further vitrification. If they have not formed blastocyst by this day, then they are deemed non-viable and are discarded. Today marks the final day of the embryo’s development in the laboratory. For that patient’s cycle, the embryologists job is now complete, other than to wish our patients all the very best and know we have done everything we could for them that cycle.

Article by Victoria Wigley, All About Embryology. For independent embryology advice and support through your ertility journey, please contact Victoria via email or online form.

Follow Victoria and All About Embryology on Facebook and Instagram


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