Morphology, developmental stages and quality parameters of in vitro -produced equine embryos

Intracytoplasmic sperm injection (ICSI) is used to produce equine embryos in vitro. The speed of embryo development in vitro is roughly equivalent to what has been described for embryos produced in vivo. Morphological evaluations of ICSI-produced embryos are complicated by the presence of debris and the dark nature of equine embryo cytoplasm. Morulas and early blastocysts produced in vitro appear similar to those produced in vivo. However, with expansion of the blastocyst, distinct differences are observed compared with uterine embryos. In culture, embryos do not undergo full expansion and thinning of the zona pellucida (ZP) or capsule formation. Cells of the inner cell mass (ICM) are dispersed, in contrast with the differentiated trophoblast and ICM observed in embryos collected from uteri. As blastocysts expand in vitro, embryo cells often escape the ZP as organised or disorganised extrusions of cells, probably through the hole incurred during ICSI. Quality assessment of in vitro-produced early stage equine embryos is in its infancy, because limited information is available regarding the relationship between morphology and developmental competence. Early embryo development in vivo is reviewed in this paper, with comparisons made to embryo development in vitro and clinical assessments from a laboratory performing commercial ICSI for >15 years.