Selection based on morphological assessment of oocytes and embryos at different stages of preimplantation development: a review

In contrast to IVF, in ICSI the surrounding cumulus and corona cells must be removed completely, as only denuded oocytes can be successfully manipulated by the holding pipette. This ancillary effect of ICSI allows us to focus on the morphology of preimplantation development from the earliest stages. Early prognosis regarding the developmental fate of oocytes would help to limit a negative impact of culture conditions. However, little evidence is available that non-invasive selection at the oocyte stage (first polar body, granular cytoplasm) may be of prognostic value. Recently, certain patterns of pronuclei (number and the distribution of nucleoli) at the zygote stage were found to correlate with treatment outcome in IVF and ICSI cycles, offering an additional prognostic tool prior to cleavage. As there is evidence that embryo selection on day 2 or 3 based on morphological criteria (fragmentation, number of blastomeres, multinucleation, uneven cleavage) may be imprecise, patients might benefit from extended embryo culture to day 5. However, not all major chromosomal aberrations are incompatible with blastocyst formation, and prolonged culture in vitro does not exclusively select embryos with a normal chromosomal complement. Consequently, special care should be taken to minimize the presence of aneuploid concepti in culture. In addition, multiple selection at different stages of development will be required to filter out the correct 'candidate' embryo which will result in a healthy newborn.