The process of fertilization is defined as the female and male gametes fusion to produce a zygote, the first phase of embryo development.
The fertilisation is a physiologically complex process that lasts approximately 18 hours. Naturally, takes place in the Fallopian tubes once the ovulation has occurred and that the spermatozoids have reached to this region of the male reproductive system. It has been calculated that in between 500 and 1000 spermatozoids are needed to achieve the oocyte fertilisation.
The basis of in vitro fertilisation is to emulate in the laboratory the process that happens in a natural way in the Fallopian Tubes. The oocytes, or to be exact the cumulus oophorus, are placed with a proper concentration of sperm in culture plates with fertilisation medium. The spermatozoids eliminate the cumulus oophorus and do the different phases of fertilization in the culture plate. After 18-22 hours, the oocytes are transferred to a new culture medium (without sperm) and under the microscope the presence of pronuclei is checked.
The oocytes that have not developed pronuclei or that had polyspermy (more than two pronuclei) are removed as are non-viable.
When the number of spermatozoids obtained with the sperm capacitation (REM) is low, the IVF process described previously cannot be achieved and instead is used another technique in which a spermatozoid in microinjected in an oocyte. In this case, the oocytes must be denudated with an enzymatic method with hialuronidasa and placed in microdrops. An unique spermatozoid is completely introduced in the oocyte with inverted microscopy and holding and ICSI needles, going through the pellucid zones (a cover that oocytes have that is similar to the plasma membrane). Although this technique seems to be more agressive nevertheless it does not interfere with the normal develop of the embryo.
Once inseminated the oocytes, these are returned to the plate and on the following day the presence os pronuclei is checked in a similar way as described for IVF. The fertilization percentages described for ICSI are similar to those for IVF and are between 70 and 80 %.
Alter fertilisation and the formation of the zygote the embryos are cultured within the incubator until the moment for the embyo transfer (ET). The day for the ET is in between day 2 and day 6 after the Ovarian Puncture (OP). The development between the formation of the zygote an day 5 consists basically in embryo cleavage to form a mass of cells (2-4-8-16-32). This ball of cells is called morula because is similar to one of those, undergoes a process of cavitation to generate the blastula that consists in a layer of peripheric cells called trophoectoderm (the-placenta-to-be) and a inner mass cell (the strictly speaking embryo)
The embryo development in the lab emulates the situation that occurs in vivo and nowadays are routinally done embryo cultures to blastocysts routinely in the IVF lab, mainly due to the improvement of sequential embryo culture media suitable to each stage of development.