The first sign of fertilisation, the formation of 2 pronuclei (arrowed) within a fertlilised human egg. This process occurs around 16-20 hours post insemination.
8 Cell Embryo
Cell division continues; the embryo reaches the 8 cell stage at around 65-70 hours post insemination
The embryo comes out of compaction and begins to form a blastocyst with the formation of a blastocoele, a fluid filled cavity in the centre of the embryo which continues to swell causing the blastocyst to expand and ultimately hatch. The blastocyst will begin to form around 112 hours post insemination.
2 Cell Embryo
The pronuclei disappear and the zygote divides into two cells to form the two cell embryo. This usually occurs 25 to 30 hours post insemination
The embryo continues to grow in cell number until it becomes impossible to accurately count the number of cells. This is referred to as a morula and forms around 80-85 hours post insemination
The blastocyst continues to increase in size and form two clear cohorts of cells, the inner cell mass which forms the foetus itself and the trophectoderm cells around the outside of the sphere which go on to form the blastocyst. Expansion occurs rapidly, usually between 118 and 123 hours post insemination.
4 Cell Embryo
The cells continue to divide within the embryo until it reaches the 4 cell stage, usually at 40-45 hours post insemination
The embryo enters a stage of reorganisation where it begins its transition to a blastocyst. The cells compact upon each other making the embryo look ‘scruffy’ and assessing the cell number impossible. This is a key stage of normal embryo development and occurs approximately 90 hours post insemination.
The blastocyst continues to expand and put pressure on the zona pellucida (egg shell) until it splits and the blastocyst can escape in the hope that it can implant into the lining of the uterus and form a pregnancy. This stage in the development of the embryo is critical as the blastocyst can never implant if it cannot escape from the zona pellucida. Hatching may start to occur from around 125 hours post insemination.