An innovative approach will be followed to derive bovine embryonic stem cells (ESC). First, we will create different embryos for ESC derivation: (1) ‘giant’ blastocysts to evaluate the influence of cell mass, (2) in vivo embryos, since their cells display more pluripotency characteristics and (3) diapaused embryos which are probably better ESC donors. Besides, extrinsic factors like culture conditions will be evaluated.