What’s Blastocyst and Blastocyst Quality Grading

April 18, 2013

In our previous IVF or FET cycles, we always used day 3 embryos. We used a different clinic this time and they suggested to transfer day 5 blastocyst to increase the chance of getting pregnant. We did a little bit more internet research about blastocyst and would like to share with people who might be interested.

What is a blastocyst?

A blastocyst describes an embryo stage reached usually after about five days of development post-fertilization. It has about 50-150 cells and has started to develop specific regions with different cellular destinies. The blastocyst is working hard; pumping fluids towards its center, creating a fluid-filled center and expanding like a water filled balloon. The inner cell mass is a clump of cells  protruding into  the middle of the fluid-filled cavity. This inner cell mass will continue to grow and ultimately will be the source for all the cells of the future baby.

The trophectoderm cells line the inner surface of the zona pellucida (the glycoprotein shell around the embryo) and play a supporting role, supplying cells to form the fetal part of the future placenta. So the grading system takes into account how much progress the embryo makes in hatching from the “shell” and also how richly endowed the inner cell mass and trophectoderm are in terms of cell number and quality. More expansion is better than less and more cells are better than fewer cells.

5-day-blastocyst-embryo
Above is a picture of a high quality expanded human blastocyst embryo 5 days after fertilization.
The clump of cells in the 10 to 12 o’clock area is the inner cell mass (ICM) which become the fetus
The trophectoderm cells (TE) that will form the placenta surround the fluid cavity.
The fluid-filled blastocoel cavity is in the center

A healthy blastocyst will implant within one to four days following a day 5 transfer, with invasion of the cells into the uterine lining occurring soon after blastocyst hatching.

Blastocyst Development and Blastocyst Quality Grading (or scoring)

  • There are 3 distinct parts to a blastocyst – the two cell types and the fluid cavity (see picture above)
  • As development of blastocysts progresses, cells in the 2 components divide and the fluid cavity enlarges
  • With continued blastocyst development the embryo expands, the shell thins, and it hatches from its shell
  • Blastocyst implantation into the lining of the uterus can then begin

Many IVF clinics that do blastocyst culture and transfer use a blastocyst scoring system originally developed and published by Dr. David Gardner in the late 1990’s. Dr. Gardner is well known for being first to develop a commercially available embryo culture media system that supported high rates of viable blastocyst development. There are other grading systems for blastocysts , but this one is most widely used.

The Gardner blastocyst grading system assigns 3 separate quality scores to each blastocyst embryo:

  1. Blastocyst development stage – expansion and hatching status
  2. Inner cell mass (ICM) score, or quality
  3. Trophectoderm (TE) score, or quality
Expansion
grade
Blastocyst development and stage status
1

Blastocoel cavity less than half the volume of the embryo

2

Blastocoel cavity more than half the volume of the embryo

3

Full blastocyst, cavity completely filling the embryo

4

Expanded blastocyst, cavity larger than the embryo, with thinning of the shell

5 Hatching out of the shell
6 Hatched out of the shell

 

ICM grade

Inner cell mass quality

A

Many cells, tightly packed

B

Several cells, loosely grouped

C

Very few cells

 

TE grade

Trophectoderm quality

A

Many cells, forming a cohesive layer

B

Few cells, forming a loose epithelium

C Very few large cells

Blastocysts are given a quality grade for each of the 3 components and the score is expressed with the expansion grade listed first, the inner cell mass grade listed second and the trophectoderm grade third.

For example, a blastocyst quality grade of 4AB means that the blastocyst is expanded (grade 4), has many tightly packed cells in the inner cell mass (grade A), and has a trophectoderm with few cells forming a loose epithelium (grade B). See the examples of blastocyst grading pictures below.

hatchingblastocyst

The IVF blastocyst picture above shows hatching out from the shell – at lower right of photo
This blastocyst is grade 5AA.
After hatching, the embryo can invade and begin implantation in the uterine lining
This picture was taken a few minutes prior to embryo transfer.

** Please note, most of above information regarding blastocyst are from Richard Sherbahn MD from Advanced Fertility Center of Chicago. It’s very informative and easy to understand.

Before the transfer, our embryologist showed us the picture of our lovely blastocyst. It looks very similar to the the above blastocyst graded 5AA. 🙂

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