Laser Assisted Hatching is used on Day 3 embryos to help the embryo break out of the zona pellucida.
As an egg matures within the follicle of an ovary, it develops a shell around it called a zona pellucida. This shell protects the egg and later, protects the embryo. The zona also provides an enclosure which keeps the cells of the dividing embryo in close proximity to each other and leads to compaction (a kind of cohesion) of those cells that is necessary further along in the development of the embryo.
While the embryo traverses the Fallopian tube and grows from a single cell to 8 cells over the course of three days, the relative thickness of the zona is not important. However, it becomes important after the embryo enters the uterus.
As an embryo grows in the mother’s uterus it continues to divide and, by the fifth day after ovulation, develops into a stage of growth known as a blastocyst. The embryo contains between 80 and 100 cells by this time and has an internal fluid-filled cavity known as a blastocoel. While the blastocyst expands due to an increasing number of cells and an increasing volume of fluid within its blastocoel, the zona begins to stretch and thin. By day 6 or 7, the blastocyst has made enough cells and has expanded to the point that it breaks the zona open and the blastocyst hatches from its shell. The embryo is now ready for implantation onto the uterine lining. If the embryo does not hatch from its shell, it cannot implant and it will stop growing.
When embryos grow in the IVF laboratory, noticeable differences can be observed in the thickness of the zona pellucida from one embryo to the next. This variation in thickness does not have any direct relationship to the apparent health of the embryo. The thickness, however, can have an effect on the potential of an embryo to implant. If the zona is too thick, the expansion of the blastocyst cannot occur and the embryo will not break through its shell.
Ovation Fertility Austin’s current protocol is that the embryos in the IVF lab are observed on day 3 for the number of cells produced and the overall quality of the embryos. This information is communicated to the patient’s physician and the decision is made to transfer on either day 3 or on day 5. If the transfer is to occur on day 3, the physician will notify the embryologist as to the number of embryos that should be transferred. The embryologist will select the embryos for transfer based on which ones appear to be the healthiest. The embryos for transfer are separated and placed on the microscope for assisted hatching. A laser will then be used to perforate the zona. This perforation can be done safely by a trained embryologist on day 3 because there are always gaps between the cells of the embryo that allow for the exact placement of a channel through the shell without touching any of the cells. This method of making a perforation in the zona pellucida is very safe for the embryos and allows the embryologist to return them to the incubator quickly.
Embryos that are transferred on day 5 do not have assisted hatching performed on them.
There are two reasons: The expansion of the embryos at this time (now they are blastocysts) is so great that they have completely filled their zona and there are no gaps to safely place a channel through the shell. Also, on day 5 the embryologist has more information about the embryos and can more easily choose embryos for transfer that are expanding and already thinning their zona on their own.