Figure 5:Appearance of normal and abnormal pre-embryo (a) The assessment of normal fertilization on day 1 of embryonic development (day 1 zygote) requires the visualization of two pronuclei (PN), 2PN (top panel). Abnormally fertilized day 1 embryos (bottom) containing 0PN, 1PN or 3PN are considered to be of poor quality and are not used for clinical purposes. White arrows show the PN. (b) A comparison of cleavage-stage embryos (day 2, two-cell stage; day 2, four-cell stage and day 3, eight-cell stage) with optimal preimplantation development and embryos exhibiting poor-quality traits that are commonly encountered during in vitro culture, including uneven cleavage, multinucleated blastomeres and several degrees of fragmentation. (c) An optimal morula-stage embryo (day 4) and a fragmented morula. (d) Human blastocysts (day 6) can be assessed for optimal or suboptimal blastocoel expansion, TE and ICM quality. The embryos used here were donated with informed consent under the guidelines approved by the Ghent University Hospital Ethical Committee (project no. 2009/281) and by the Belgian Federal Ethical Committee on Embryo Research (project no. ADV 022). Scale bars, 100µ. (Adapted from [58]).