Embryonic Life-Week One

May 12, 2010Carole No Comments »

The first week of your new embryo’s life is a busy one.  By day two in the lab, your embryo has already established itself as a unique genetic organism and the original zygote has divided into at least two cells, starting down the road to the billions of cells your newborn will have. On day three in the lab, the embryo should be at least seven to eight cells in number. At this stage, the cells are only loosely associated with one another, and it is technically feasible to pull one cell away from the others for genetic analysis. Removing a single cell for testing is necessary for pre-implantation genetic diagnosis (PGD) so that embryos harboring genetic defects leading to disease can be identified and removed from  the group of embryos that will be transferred to you. If you are interested in PGD, Genesis Genetics has a brilliant website which discusses genetic testing of embryos in greater detail. I have no financial interest in Genesis Genetics but have known them to provide exceptional care and services to our patients.

Eight cell embryo on day 3. Note thick zona pellucida enclosing cells.Photo courtesy of Dr. Oleksil Barash, IVF clinic “Nadiya”

The first three days of embryonic life are fairly quiet, with the embryo quietly dividing and motoring on using maternal stores of fuel and other cytoplasmic goodies from the egg. Around day three and four, drastic changes occur in both the appearance and the metabolism of the embryo. Genome activation occurs which means that the embryo switches from passive reliance on gene transcripts from the egg and begins to use it’s own unique DNA to create gene transcripts and protein products that it needs to stay alive. Dr. David Gardner’s video podcast on embryo metabolism specifically addresses the changing metabolic and laboratory needs of the embryo’s first week of life. If you can get past the first chemical pathway slide, the rest is easy sailing.  The scientific work of David Gardner and other pioneers in both animal and human IVF culture systems is fundamental to how embryologists take care of embryos in the lab, particularly when IVF entered the uncharted waters of culture past day 3. Embryos used to stop dead after day 3 in the lab, because we didn’t understand that their nutrient needs changed drastically when they activated their own genome.

The ugly duckling morula.

By day four, the beautiful 8 cell embryo has changed into an ugly duckling- the morula. The morula is like a teenage embryo, all angst ridden, lumpy looking and totally disorganized. It’s trying to organize itself but it has so much new metabolism and cell division happening, it’s struggling to find it’s own way. The cells that were loosely associated in the 8 cell embryo are now tightly associated and cell-cell signaling is happening at a frantic pace. Signaling between cells is critical to orienting the embryo into distinct cell lines which will give rise to the basic body systems of the fetus.

On day five, Voila!!- your embryo has  transformed itself into an implantation-ready blastocyst.

Blastocyst embryo with well-formed inner cell mass. Photo courtesy of Dr. Oleksil Barash, IVF clinic “Nadiya”

Your blastocyst is an embryo with a vision- some of  its centrally located cells (appropriately called the inner cell mass) are destined to become the future cells of the baby. Other cells lining the zona pellucida create a structure called the trophectoderm which will burrow into the uterine lining during implantation. The zona pellucida is getting thinner, because the growing blastocyst, now around a hundred cells, is straining at the confines of the zona pellucida. In order to implant, the embryo must hatch from the zona pellucida. Hatching is possible because the blastocyst has been steadily pumping fluid into it’s central cavity  until the blastocysts is more than four times the size of the zona pellucida.  The next picture shows a hatching blastocyst. You can clearly see the mound of central cells which will form the cells of the baby.  Isn’t she cute?

Embryo hatching in action. Next....docking with the mother ship.

Along with the advent of new culture methods which made culture to the blastocyst stage possible, assisted hatching is not necessary because the zona thins naturally in newer culture medium. In the old days, embryologists would enzymatically dissolve a small window into the zona pellucida to “assist” the process of embryo hatching.  The technique of assisted hatching is not without technical risk. The enzyme used to dissolve the zona pellucida can just as easily dissolve the cells inside the zona.

Hatching is still performed in some labs if the embryo is going back on day 3 instead of day 5, or if the patient demands it. But if the lab is using a sequential medium system for continuous culture through day 5, assisted hatching isn’t necessary and may just add technical risk.

In the body, the egg is fertilized in the Fallopian tube  and these embryonic changes occur while the embryo is moving down the Fallopian tube, headed toward the uterus. No matter where the embryo spends it’s first five days, by the end of the first week, it is ready to burrow into the uterus and implant.

In the lab, we have been tracking your embryos progress and giving you progress reports, usually on day 3 and day 5, although programs vary in how often they check the embryos and update patients. Most embryologists like to minimize interventions and unnecessary time outside the incubator. In any case, by either day 3 or day 5, depending on the clinic, the embryos are returned to you with Best Wishes and high hopes for a successful pregnancy.

© 2010, Carole. All rights reserved.

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