Morphological assessment of preimplantation embryo quality in cattle.

The extensive use of embryo technologies has emphasized the need for assessing embryo quality by morphological techniques, such as transmission electron microscopy, immunocytochemistry for confocal laser scanning microscopy and fluorescence in situ hybridization. By a combination of these techniques, it has been possible to demonstrate: (i) that rRNA gene activation, as monitored by embryonic nucleolar development, is comparable in bovine embryos developed in vivo and produced in vitro, whereas reconstructed nuclear transfer embryos may be deviant, (ii) that generating embryos by both in vitro production and reconstruction by nuclear transfer is associated with increased occurrence of apoptosis, in particular in the inner cell mass of blastocysts, and (iii) that these two embryo production techniques are associated with increased occurrence of mixoploidy that is, embryos presenting a large population of normal diploid cells and a small population of abnormal haploid or polyploid cells. It is clear that blastocysts that appear healthy at stereomicroscopy may have subcellular defects. Therefore, the possibility of long-term evaluation in vitro of embryos after hatching has been examined. However, whereas embryos developing in vivo after hatching present a number of well defined developmental milestones, such as elongation of the trophoblast, formation of hypoblast and epiblast followed by differentiation of endoderm, mesoderm and ectoderm, in vitro culture systems for development beyond the blastocyst stage currently allow the embryo to complete only a single milestone, namely hypoblast formation.