Culturing the embryos in the laboratory
After collection, the eggs and sperm are brought to the laboratory where the eggs will be fertilised and the embryos cultured for 5 days. In conventional IVF, about 50,000 to 100,000 washed sperm are left in a small plastic dish with the eggs. The sperm spend the next few hours getting through the layers of cumulus cells, and hopefully one sperm will successfully fertilise the egg.
By the next day - some 15 hours after introducing the sperm to the eggs - the scientists will check to see if the eggs have fertilised by looking for the presence of pronuclei. In normal fertilisation there should be 2 pronuclei - one from the sperm and one from the egg.
Not all eggs will fertilise normally, which is common. We consider it a good result if 80% of the eggs collected have two pronuclei on Day 1.
Intracytoplasmic sperm injection (ICSI)
Intracytoplasmic sperm injection (ICSI) can be used to fertilise eggs when your doctor thinks there is a decreased chance of fertilisation occurring with conventional IVF, either because of problems with low sperm numbers or low sperm motility, or because of other barriers to the fertilisation process such as sperm antibodies or previous failure to fertilise through IVF.
A single sperm is injected into each egg. The sperm is selected mainly on the basis of its normal shape and size.
Once fertilisation has occurred, the embryo will divide and rapidly increase in cell number over the next few days. By Day 4, the cells have divided rapidly but the embryo has not yet increased in size. It is now compacting (you cannot distinguish the cells) and is called a morula.
If the embryo survives to Day 5 - the blastocyst stage - it will contain between 75 and 100 cells. It is a 3-dimensional ball of outer cells (the trophectoderm) surrounding a fluid-filled cyst in which an inner group of cells, the inner cell mass can be seen. The trophectoderm will go on to form the placenta, membranes and umbilical cord, while the inner cell mass will become the baby.
We cannot tell the difference between a good embryo and a bad embryo just by looking at them. Embryos at the best of times are busy transforming and repairing themselves, so can develop fragmentation (which is when small bits of cells are pinched off during division) or vacuoles, which are small spaces within the substance of the cells. The significance of these changes is not known and many fragmented and vacuolated embryos can go on to form perfectly healthy pregnancies.
While many embryos can survive 2 or 3 days to reach the 4-6 cell stage, only the strongest will have the ability to keep developing into a blastocyst and then a baby. One way of identifying the better embryos, therefore, is to let them grow a little longer in the laboratory and to transfer them at the blastocyst stage. It is a good way of determining which embryos have the most developmental potential. Our world-leading success with blastocyst culture and implantation means that blastocyst transfer is standard at Superior A.R.T. centre.
An embryo needs two things to reach Day 5: enough energy, and normal chromosomes.
An embryos energy supply comes from tiny structures inside its cells called mitochondria. The embryo needs to survive on the energy produced by the mitochondria it inherits from the egg until it has implanted and formed a placenta. Because all the mitochondria in an embryo come from the egg they are inherited from the mother. And because women are born with all their eggs for their lifetime already formed, the mitochondria in your eggs are as old as the eggs themselves.
Embryos must also have the right genetic makeup to develop normally. In humans, genes are contained in 23 pairs of chromosomes. An incorrect number of chromosomes leads to failure of an embryo to implant or to progress to a normal birth. Pregnancy is a great filter of abnormal embryos. When chromosome analysis is performed on cells from Day 3 embryos, studies have shown that only one third will have the normal number.
If an embryo progresses to Day 5 and becomes a blastocyst, it has a two-thirds chance of being chromosomally normal. 90% of chromosomally abnormal pregnancies will miscarry in the first trimester. 93% of chromosomally normal pregnancies will continue to term.