Here’s what will be happening with our little cells while we wait:
*Day 2 in the IVF Lab – Embryo Divides
On Day 2 the embryos start to divide and should have 2-4 cells. The embryo itself does not grow bigger; the single cell that was the fertilized egg divides to become 2 cells and then 4 cells with each cell being half the size of its predecessor.
They can start to see slight variations between the embryos in the group and they could be graded at this stage. Most clinics do not look at the embryos on this day but it is possible to do an embryo transfer or freeze the embryos if that is the clinic’s policy. It is usually better to wait as long as possible for transfer and freezing as this allows more diversity within the group and enables the embryologist to more easily select the best embryos. The splitting of cells to create identical twins happens on day 2 or 4.
Day 3 in the IVF Lab – Embryo Grading and Potential Transfer
On Day 3 the embryos should be around 8 cells. This is the day when some clinics move them into a new Petri dish with different media (solution to support growth) that is similar to uterine fluid for their next stage of growth. They grade the embryos but still do not know how many will continue to grow to the blastocyst stage (Day 5 and 6).
Some clinics perform embryo transfers and freezing on Day 3 and some clinics wait until Day 5. This depends on the number and quality of embryos that you have available and also the clinic policy. From what I understand, Shady Grove does not touch the cells until day 5 to let them grow in a healthy, untouched environment.
On Day 3 we know:
- How many cells the embryos have
- If the embryos are of good appearance up to this point
- An average of 95% of fertilized eggs will grow to the Day 3 stage
- The appearance on Day 3 does not tell us how many embryos will continue to grow
Day 4 in the IVF Lab – From Cleavage Stage to Blastocyst
Day 4 is a transformation day where the embryos are in between the cleavage stage and blastocyst stage.
What is a blastocyst?
- A blastocyst is a fertilized egg that has developed for five to six days and contains 3 distinct features. These features include a fluid-filled cavity and two distinct types of cells:
- Trophectoderm (T) cells – T cells consist of a single layer of cells around the circumference of the embryo that become the placenta and embryonic sac.
- Inner cell mass (ICM) – The ICM is a distinct clump of cells that form the actual baby
The clinic usually does not look at the embryos on this day, but if they did they would see that some of them would be making the transition from a multi-celled embryo with clear cell outlines to what is called a morula which is the stage before an embryo becomes a blastocyst. Sometimes embryos do not reach the morula stage until Day 5.
*Information from Fertility Smarts
2 thoughts on “Bio Lesson: Cells the Grow into Human”
I love all your biology lessons. Thanks for letting us “ride along” one day at a time!
You are so welcome, this has been so fascinating for me to learn!