Stage specific changes in expression of OCT4, CDX2, NANOG and GATA6 mRNA during blastocyst formation in the horse

Abstract

During blastocyst formation two consecutive lineage segregation events give rise to the three primitive cell types: trophectoderm, primitive endoderm, and pluripotent epiblast. In the mouse, octamer binding protein 4 (OCT4) and caudal type homeobox 2 (CDX2) are essential for inner cell mass (ICM) and trophectoderm formation respectively, while differential expression of NANOG and GATA binding protein 6 (GATA6) in cells of the ICM determines which will form epiblast or hypoblast (primitive endoderm) respectively (Ralston and Rossant, 2005). However, expression of OCT4 protein in the trophectoderm and the absence of NANOG protein and gene expression during blastocyst formation in large domestic species, suggest that species-specific differences exist in the processes governing early lineage segregation (Kuijk et al., 2008). Equine blastocyst formation occurs 6–7 days after ovulation and is accompanied by an increase in cell numbers from around 150 to over 2900, where multiplication is predominantly within the trophoblast and primitive endoderm (Betteridge, 2007). Since little is known about the roles of OCT4, CDX2, NANOG and GATA6 in early lineage segregation events in the horse, we aimed to determine whether expression of these genes is consistent with lineage specification during equine blastocyst formation