Assessment of mammalian embryo quality: what can we learn from embryo morphology?

Embryo morphology assessment, however imperfect it may be, is at present the most popular method for embryo selection prior to transfer, both in human and bovine assisted reproduction. A major difference between human and bovine embryos is the fact that in the latter, assessment of morphology is jeopardized by the opacity of the blastomeres, which is caused by lipid droplet accumulation. This opacity makes it difficult to assess nuclear and nucleolar morphology, aspects which can easily be evaluated in human zygotes or early cleaving embryos. However, recent research which focused on correlation between bovine embryo morphology and embryonic ultrastructure, gene expression and cryoresistance, has provided evidence that much more can be deduced from mere embryo morphology than previously thought. Morphological features such as colour of the blastomeres, the extent of compaction, timing of blastocyst formation and expansion and diameter of the embryo at hatching can be linked with embryo quality. On the other hand, cattle embryos of deviant chromosomal constitution or with aberrant genetic make-up cannot be selected against by means of the current morphological techniques. Possible solutions include the visualization of bovine pronuclei at the zygote stage by means of ultracentrifugation or multiphoton laser scanning microscopy, and adjustment of genetic analysis in order to reconstruct embryo genetic make-up starting from the biopsy material.