5 - 6 days after fertilization the embryo reaches the blastocyst stage -
a cyst (or cavity filled with fluid) develops and the cells begin to
differentiate into trophectoderm and inner cell mass. At this stage the the
blastocysts contains approximately 100 cells. The trophectoderm (tr) will form
the placenta a foetal membranes, and the inner cell mass (ICM) the fetus. The
blastocyst cavity increase in size, the pressure rises stretching the zona
pellucida.
The stages depicted above (From left to right). 1 Early blastocyst: 2
Late Blastocyst with obvious cavity: 3 Hatching from the zona pellucida: 4
Representation of Implanting in the endometrium (about day 10).
A greater understanding of the needs of the embryo in vitro (i.e. in a culture medium rather than in vivo - in the body) means that embryos can now be grown to the blastocyst stage before being transferred to the uterus. As the embryo develops in culture it needs different nutrients at different stages of development - the embryo also needs to 'get rid' of waste products which, if left in the culture medium surrounding it, will hinder its development. New culture media have been developed specifically to answer the needs of the embryo and this requires changing to different media at different stages during development.
In vivo, the human embryo enters the uterus at either the morula stage or the blastocyst stage on Day 4 to 5 post fertilization. Since 1978, IVF embryos have primarily been transferred into the uterus at the 4-8 cell stage on Day 2-3 post-fertilization. The reason for this has been due to limitations of the culture media and conditions to sustain normal growth of blastocysts at acceptable rates. Recently developed sequential culture media, known as sequential media has provided a successful alternative to early (Day 2-3) embryo transfers. The blastocyst development rate in this media is approximately 45%, thus providing adequate numbers of embryos that make Day 5 transfers a realistic option. Published studies using sequential media have reported implantation rates of greater than 50% using these media*.
Embryos are cultured in a similar fashion to the standard IVF protocol with the primary exception of the type of media and the duration of culture. From Day 1 post-fertilization the embryos are placed in Gl.2 ™ media, followed by G2.2 ™ for days 3-6. The two media differ in the concentration of non-essential and essential amino acids, glucose, lactate, pyruvate and presence of EDTA. These differences are designed to accommodate the metabolic requirements of the pre- and postgenomically activated embryo. The media are designed to be maintained at 37 'C for a maximum of 72 hours. Laboratory schedules are required to ensure that embryos are transferred between the media on the correct day. Embryo transfer occurs on Day 5 or 6 using the standard protocol.
*A prospective randomized trial of blastocyst culture and transfer in in-vitro fertilization. David K. Gardner et. al. Human Reproductionvol.13 no.12 pp.3434-3440,1998.
Information courtesy of Scandinavian Science.
There is growing body of opinion which suggests that the success of implantation after embryo transfer must be increased and that by growing to the blastocyst stage it is easier to judge the quality of the embryo - it should then be possible to restrict the number of embryos transferred and so reduce the multiple pregnancy rate often associated with IVF. However not all embryos will reach that stage and it is argued that those embryos would probably not implant even when transferred to the body's care. Other people might argue that it is better to get all the embryos back into nature's care as quickly as possible - nature knows best!
The improved implantation rate at the blastocyst stage also means that it should be possible to limit the number of embryos transferred to reduce multiple pregnancies and associated abnormalities which premature delivery might impose.