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Embryo Culture
Very early stage embryos are placed in a medium containing nutrients and stimulating embryo growth reagents, and kept in special incubators that provide optimal conditions for development. The first cell division begins six to eight hours after fertilization assessment and usually is completed overnight.
We generally do not remove embryos from the incubator on the second day after retrieval. The second cell division from two to four cells follows immediately after the first, and on Day 2 embryos have usually developed to the 4-cell stage. It is possible that there are also some slower-dividing 2 and 3-cell embryos.
The third division takes place during the night of the second day. Early on Day 3 embryos have should have reached the 6 to 8-cell stage. On Day 3 some embryos start showing the signs of the next stage of development called compaction. Compaction leads to the morula stage which is typical for embryos on their fourth day of development.
Morula and compacting embryos, unlike their previous forms, are not beautiful and symmetrical to look at. During the compaction and morula stages of development the embryo compacts its cells to obtain the greatest amount of cohesion. Day 4 is a critical day for embryo development. The embryo undergoes several changes that require excessive energy. One of the largest changes is the activation of the embryonic genome. When the embryo activates it genome it becomes genetically unique as it now is a genetic blend of the parents. If the genetics do not blend correctly or the embryo does not have the needed energy then the embryo will arrest and fail to continue development.
By Day 5 the strongest embryos will have successfully activated their own genome and differentiated their cells to form a blastocyst with an inner cell mass and trophectoderm. The inner cell mass will contain the cells contributing to the fetus and the trophectoderm will contain the cells that will maintain the pregnancy such as the placenta. During blastocyst development the blastocysts expand and contract. The expansion occurs as the cells increase in number and also as the blastocyst stretches in efforts to hatch or break out of its shell.