Embryo culture

Regardless of the technique, on the following morning embryologists observe the culture to establish how many oocytes have been fertilised. The egg and sperm chromosomes have meanwhile formed two pronuclei, which during the day converge to the centre of the oocyte and, finally, merge into a single and normal nucleus.

It is then that the first cell of the new body has been formed. We call this first cell "embryo". It is noted that this "embryo" still has the size of an oocyte (approx. one tenth of the millimetre). The presence of two pronuclei and two polar bodies indicate normal fertilisation.

The percentage of in vitro fertilised oocytes ranges from 50% to 100%. It depends on the oocyte quality, their maturity (immature and over mature oocytes are rarely fertilised), sperm fertilising capacity and culture condition.

In vitro fertilisation is a first, clear indication that the couple's oocytes and sperm can fuse.

In some cases of subfertility, it is discovered that a possible reason is that spermatozoa fail to fertilise the oocytes because either they do not adhere to oocytes or they lose their motility after a few hours in culture and cannot penetrate the zona pellucida.

The percentage of fertilised oocytes is not an indication of the quality of embryos. Embryos are in fact autonomous and often pregnancy is successfull with a single embryo. Therefore, a potentially small fertilisation percentage should not raise any concerns about the outcome.

On the second morning after egg retrieval, the fertilised eggs have already performed two cleavages. It is estimated that the percentage of embryos that perform cleavages exceed 70% (i.e. almost all fertilised eggs). Typically, the first cleavage occurs approx. 16 hours after fertilisation (the "embryo" consists of two cells) and then continue at an increasing rate (embryos with 4, 8, 16, 32 cells).