An Embryo’s Journey

An Embryo’s Journey


Many times, the days an embryo spends growing in the lab are just a waiting game for patients.  While there is a vague idea of what happens, it seems like a lot of acronyms are thrown around and then 5 days later, there is the call with numbers.  In this post, we will break down what happens with a patient’s embryos throughout their stay in the lab.

On the day of retrieval, oocytes (eggs) are collected from the patient and then undergo a process called “trimming and stripping”.  During this procedure, the cumulus cells are removed from the eggs.  In the follicle, cumulus cells surround the egg and assist in the egg’s growth and maturation.  Once the egg is retrieved, these cells are no longer required.  After the eggs are trimmed and stripped of their cumulus cells, they are placed into the incubator to await Intracytoplasmic Sperm Injection (ICSI) that afternoon.  When it is time for ICSI, maturity of the eggs is assessed to determine how many are ready for sperm injection.  To assess maturity, a high-powered microscope is used to locate the Polar Body.  This extrusion indicates that the egg has undergone the first division of meiosis, allowing it to accept the other half of the DNA from the sperm.

With the mature eggs identified, ICSI begins.  The sperm are carefully selected based on morphology (shape) and movement.  This allows the best sperm from the sample to be used for insemination of each mature egg.  One sperm is then injected into each mature egg.  After ICSI is complete and all mature eggs have been injected with sperm, the eggs are placed back into the incubator.  The next morning, known as Day 1 in the lab, eggs are assessed for fertilization.  This assessment is done by visualization of two pronuclei (2PN).  These appear to be two circles within the egg, indicating that the egg has fertilized and is now a zygote.  All fertilized zygotes are placed back into the incubator to remain there for the next two days.  On Day 2, the embryos remain in culture within the incubator and start to divide.  They are typically a 2-4 cell embryo at this stage.

By Day 3, the embryos have likely reached the 8 cell stage.  They are removed from the incubator and cell division is assessed.  Not only are the number of cells within each embryo counted, but quality and symmetry of the cells is also noted.  Day 3 is also the time a small hole is created within the shell of the embryo- known as Assisted Hatching.  This is done through the use of a small laser attached to one of the microscope objectives.  It safely allows the hole to be created which, in turn, allows the embryo to easily expand out of the shell on Day 5.  The embryos are placed back in the incubator to continue growth in culture.

Day 4 is spent in the incubator.  This is a very stressful time for the embryos as they grow- dividing from an 8-celled embryo to a morula.  The morula stage embryo is undergoing a very big change.  Cellular connections are being made between cells, compaction of these cells occurs, and the metabolic driving forces of the embryo change.  There is a switch from the maternal genome driving cell divisions to the embryonic genome driving these divisions.

On Day 5, the embryos are assessed for blastocyst development.  The blastocyst stage is the first time we see any cellular differentiation.  Two distinct cell types are now visible: the Inner Cell Mass (ICM) and the Trophectoderm.  The ICM will go on to develop into cell lines forming the baby, while the trophectoderm goes on to form part of the placenta.  If patients are having a fresh embryo transfer, it is done on Day 5.  Alternatively, patients may elect to freeze blastocyst stage embryos.  This is also the first of two days when biopsies are performed for genetic testing.  In order to be biopsied, an embryo must reach the hatching blastocyst stage with good cell quality.  Using the same high-powered microscope as earlier in the week, a few trophectoderm cells are separated from the rest of the embryo using the laser microscope objective.  These few cells are then sent to a genetics lab for chromosome testing.  After the embryos are biopsied, they are frozen using a technique known as vitrification.  Vitrification is a form of rapid freezing that allows for significantly higher embryo survival post-thaw.  After the embryos are frozen, they are stored for later use during a frozen embryo transfer (FET).

Although this process is generally seen as a long waiting game, these embryos have been busy growing, expanding and reaching their full potential.  Their time in the lab not only allows for a successful pregnancy, but it also gives us a much deeper understanding of fertility issues that may be occurring.

                                      ~Conceptions Embryology Department: 1/4/17


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